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3. British woodlands: 1977
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3.2 Ecological variation
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ECOLOGICAL VARIATION
Habitat factors and vegetation
A classification of British
woodlands based on the dominant tree
species is the most straightforward, and that adopted here. Tree
dominance is, however, determined by climate, physical and chemical
soil conditions, and past management, so that such a classification
does not represent a simple ecological sequence. Woodland also has a
greater degree of structural diversity than any other major
ecosystem, and the separate layers of a woodland community can show
a certain amount of independence in their relationships to each
other. The orderly ecological descriptive treatment of woodland thus
poses considerable conceptual problems, and probably no single
approach is adequate. For this reason, the ecosystem will be
described in terms of structure, with the ecological relationships
of each layer considered separately; but no attempt will be made to
unify these in a complete classification of woodland types.
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THE TREE LAYER
This predominant structural
component of woodland may be defined as
the aggregation of woody species over 5 m in height and forming the
canopy. Tree seedlings or saplings are best considered as part of
whatever layer they belong to in stature. The original regional
pattern of forest cover, as measured by the dominant trees, was
determined primarily by range of climate, while the more local
variations depended on soil conditions, both physical and chemical.
In southern Britain, the temperate oceanic conditions give a western
outpost of the broad-leaved mixed deciduous forest of continental
central Europe, while in the north, the cool oceanic climate allows
the survival of a south-westerly extension of the boreal coniferous
forest of northern Europe. Although mixed woods are widespread, many
British woodlands are now dominated by single tree species.
Woodlands are obviously
much more than stands of trees, but the tree
layer exerts a powerful influence on the whole complex of other
plants and animals. Trees grow to maturity over a period measured in
decades or even centuries, but unlike many smaller plants they have
a finite life span, though this may be extended by such practices as
coppicing. Typically, in a natural stand the cycle of death and
regeneration is irregular in space and time, giving a varied
structure and age distribution of the tree layer and often a great
deal of dead and dying timber. The height and growth form of trees
are much influenced by factors such as depth of soil, exposure to
wind and density of individuals, and an examination of these
characteristics often reveals the conditions under which the tree
has grown. The age, growth form, stocking density and species
characteristics of the trees help to determine the floristic
composition of the subsidiary layers and the fauna of a woodland,
and as the one changes in time, so do the others.
Probably most present-day
British woodlands have a tree composition
resulting from a combination of factors, including planting of
seedlings and sowing of seed, natural regeneration, and selective
elimination of certain species. Even when the same native tree
species have been retained from the past, a present-day woodland
owes much of its character to the silvicultural system in use. The
stocking density of the trees, their size and shape, all depend on
the nature of management. The coppice-with-standards system has been
widely used in Britain, and gives a rather open canopy layer of
large, spreading, standard trees, with a dense coppiced scrub layer,
usually of tall shrubs (e.g. hazel) but sometimes of another tree
(e.g. hornbeam, ash and sweet chestnut). Coppice may also be
maintained without standards, and sometimes coppice of tree species
(e.g. ash, hornbeam) is allowed to grow into taller canopy woodland.
In park woodland widely scattered trees are maintained in a
grassland community and often pollarded, so that they attain a
squat, rounded and massive form, with short, thick trunks and a
cluster of large, spreading limbs; regeneration is reduced or
eliminated, the canopy kept open and the field layer changed by
grazing. At the other end of the scale is high forest with trees
which have tall, straight trunks, branching mainly in the high,
rather compact crowns. The ecological consequences of these
practices are markedly different. In coppices the field layer and
woodland margin species are encouraged, but epiphytes reduced,
whereas in park woodlands the field layer is impoverished and
epiphytic lichens and bark-dwelling or wood-boring animals are
encouraged.
The floristic composition
of the tree layer has been much influenced
by the species chosen for re-planting; these have variously been the
original species, native trees other than the original species, and
non-native species. Re-planting of the second type has especially
confused the interpretation of factors controlling the distribution
of native trees. Selective felling and other management intervention
can also obscure the original pattern of woodland composition. It is
now difficult to be sure of the details of the floristic composition
of the woodland tree layer which once characterised any one
set of site conditions.
For instance, the pollen record repeatedly
suggests that the Post-glacial forests of Britain contained a higher
proportion of elm and alder, but less ash, than at present. Yet
without knowledge of such factors as the differential pollen
production and preservation between various trees during earlier
periods, one cannot know how far inferences for present-day woodland
composition may be drawn. It may be that reduction in elm and alder
and increase in ash are partly due to man's influence. Some
ecologists believe that single-tree species dominance is a recent
and artificial condition in British woodlands, and that the original
forest was everywhere characterised by species diversity in the tree
layer. Problems such as these are not simply academic; they affect
definition of a national range of scientifically important woodland
types, and prescription for subsequent management. The matter is
open to much speculation, but the simplest hypothesis is that
consistent present relationships between habitat conditions and
woodland composition also held in the past.
The most widespread type
of British woodland composed of native
trees is that dominated by oak. Despite many exceptions, there is a
marked tendency for pedunculate oak Quercus robur to be dominant on
base- rich soils in the south and east, and for sessile oak Q.
petraea to prevail on base-poor soils in the north and west.
Pedunculate oak is, however, often the sole dominant on woods on the
less rich soils in southern England, and the high-level woods of
Dartmoor are well-known examples of western and upland pedunculate
oakwood on acidic soils. Quercus robur has been much more widely
planted and used in forestry than Q. petraea, and this bias may have
helped to obscure former, natural distribution patterns. Mixed
populations of the two oaks, with all grades of intermediate or
hybrid forms occur widely, and there is no simple pattern in their
distribution. Quercus robur is regarded as more tolerant of wet
soils than Q. petraea, and much pedunculate oakwood on heavy clays
tends towards a moist character; however, as waterlogging of the
ground becomes more severe, there is usually a change to alder or
willow-dominated wood.
As soil base-status increases,
especially in available calcium,
other tree species appear in increasing abundance until the oaks are
more or less completely replaced on highly calcareous soils. In many
parts of Britain, ash Fraxinus excelsior and wych elm Ulmus glabra
are trees which typically appear as soil base-status increases, at
first sharing dominance with oak (though wych elm is consistently
less abundant than ash), and then taking over completely on the most
calcareous sites. On calcareous soils in the south and east of
England, ash often appears abundantly after beech is cleared but the
successional relationships between the two species are not entirely
clear. Sometimes ash may persist as the climax woodland dominant,
but in some places there is evidence that beech has replaced ash;
the situation has probably been confused a good deal by management,
including planting. In the west and north, ash certainly appears to
persist as a climax woodland tree, as far north as west Ross and
Skye, but is then replaced by birch or hazel in Sutherland. Ash is
one of the most freely regenerating of our native trees.
In southern Britain, the
mixed deciduous woods on basic soils may
contain small-leaved lime Tilia cordata, but this species is
regarded as native only as far north as Lakeland. In a few
districts, such as parts of Lincolnshire, Northamptonshire and
Gloucestershire, Tilia cordata is locally in quantity, even reaching
dominance in parts of some woods. The much rarer T. platyphyllos
occurs in such woods in scattered localities, mainly in the Welsh
borders and Derbyshire. The wild service Sorbus torminalis is seldom
present as more than thinly scattered trees, but is a characteristic
species of some woods on the richer soils from Lincolnshire and the
Welsh borders southwards. Wild cherry or gean Prunus avium is a
widely distributed species in these mixed deciduous woodlands, but
is not usually abundant. English elm Ulmus procera and smooth elm U.
carpinifolia are plentiful in eastern and southern England, where
they are more usually trees of hedgerows rather than woods, though
they dominate some small woodlands. These species are believed to be
introduced; the native elm is the wych elm U. glabra. Elms are
declining severely in many southern districts because of the ravages
of a severe epidemic of Dutch Elm Disease. Another alien species now
characteristic of many mixed deciduous woods especially in south-
eastern England is sweet chestnut Castanea sativa. Field maple Acer
campestre is another typical member of mixed deciduous woods on the
better soils in southern Britain, and beech Fagus sylvatica is often
present, or sometimes abundant, in this region.
In the south and east of
England, beech locally rivals oak as the
principal woodland tree, but the relationships between the two
species are not exactly understood. Both can grow on calcareous
substrata, but it is believed that on thin soils over the Chalk and
the Jurassic limestones, beech has a competitive advantage over oak
and so becomes dominant, whereas on damper clay soils oak and ash
have the advantage. The beechwoods of the Chilterns, Cotswolds and
South Downs are regarded as particularly good examples of this
forest type, but there are all transitions to the equally fine
beechwoods on acidic sands and gravels in the New Forest, the Weald,
Epping Forest and Burnham Beeches. Some of these are ancient
beechwoods, but from historical evidence others are regarded as more
recently established plantations on the site of former oakwood or
even grassland. Beech is widespread in Britain as a planted tree, so
that the status of particular populations and the limits of their
native distribution are difficult to determine.
In the Scottish Highlands,
especially in Inverness-shire,
Aberdeenshire and Ross, Scots pine Pinus sylvestris locally replaces
oak as the climax forest dominant. The pine forests of the Spey, Dee
and Beauly catchments are probably the largest continuous areas of
native, semi-natural woodland now left in Britain. Pine flourishes
on strongly podsolised, acidic soils, and in places there are
indications that pine-wood is the climax on base-poor substrata, but
is replaced by oak and/or birch on base-rich soils; these edaphic
relationships are well illustrated around Loch Maree in west Ross.
Scots pine is widely planted in southern Britain, especially on
acidic sands and gravels, and regenerates freely in places. It is
not usually regarded as a native tree south of the Highlands, but
some of the populations on or around lowland acidic mires in
districts to the south have a natural appearance and regenerate
well, e.g. on Kirkconnell Flow, Kirkcudbrightshire.
The two species of birch,
Betula pendula (silver birch) and B.
pubescens show inter-relationships similar to those found in the
oaks. Betula pendula is more widespread and abundant in the east,
whereas B. pubescens has a more western tendency, and the subspecies
odorata is the only birch of the north-west Highlands. Intermediates
are again common in many areas. Both species can grow on quite
strongly basic soils but are more typical of base-poor substrata.
South of the Highlands, birchwood (with either species) is evidently
essentially a serai type. Birch regenerates freely and readily
colonises the sites of felled woods, lowland heaths and the peat of
drying mires. Many birchwoods become increasingly invaded by other
trees, especially oak or beech, and so eventually pass over to
climax woodland of another type. Birch is frequent in many mixed
deciduous woodlands and both species, but especially B. pubescens,
are also able to tolerate fairly wet soils, and occur in a range of
damp woods, grading to carr with alder and willows.
Within the pine forest
zone of the central Highlands, as on upper
Speyside, birch may, however, represent a climax type on the better
soils, for oak is here extremely sparse or absent as a native tree.
Oak, pine and ash do not extend to the extreme north of Scotland,
and in Sutherland, parts of Ross, and much of the Hebrides,
birchwood is clearly the climax forest type under present
conditions. Rowan Sorbus aucuparia is often abundant or even co-
dominant in some of these northern birchwoods, though this is a
widespread tree, occurring frequently in many different types of
wood all over Britain. In some upland districts, scattered growths
of rowan on elevated escarpments are all that is left to represent
the upper forest limit.
Throughout Britain, alder
Alnus glutinosa is the characteristic tree
of waterlogged soils, over a wide range of elevation. It is locally
dominant in the carrs which represent a late stage in hydroseral
development, as in the Norfolk Broads, and in this situation, alder
may share dominance with willows or birch (either species). On damp
soils where drainage impedance is less marked, ash may be abundant
in alderwoods, e.g. Carnach Wood, Argyll. Despite the apparent
abundance of the species during much of the Postglacial Period,
alderwood seldom covers large areas today, and it often occurs
merely as patches or strips within larger woods dominated by other
trees, or as a co-dominant with another species, e.g. oak in Coed
Gorswen, Caernarvonshire. In northern England and Scotland, alder
often forms small woods on wet hillsides, sometimes quite steeply
sloping but usually drift covered, though it is more typical of flat
ground at the foot of hill slopes. It also forms fringes on the
alluvial banks of hill streams.
Of the other native trees,
hornbeam Carpinus betulus is abundant in
south-eastern England and parts of East Anglia, where it is usually
associated with oakwoods and managed as coppice, so that the species
often belongs more to the shrub than the tree layer. It grows on a
variety of soils, but is most abundant on those of intermediate base-
status. Yew Taxus baccata occurs locally as a dominant, forming
dense dark woods with very few if any other plants. Yew woods are
usually small in size, often being mere clumps of trees, and are
best developed on the Chalk formations of the North and South Downs.
Yew is common on Carboniferous Limestone, and it is also
characteristic as a subsidiary component of Lakeland oak or oak—ash
woods on the less calcareous Borrowdale Volcanic Series and
Bannisdale Slates. Holly Ilex aquifolium is most usually a member of
the woodland shrub layer, but in a few places, e.g. the New Forest,
Hampshire, can attain a considerable size and occurs in fairly pure
stands. In a few places in southern England, box Buxus sempervirens
forms woods on calcareous soils.
The willows mostly belong
to the shrub layer too, but Salix cinerea
sometimes grows tall in carr with alder and has some claim to be
considered as a woodland tree species. Salix fragilis and S. alba
are fairly large trees, but are usually planted along river-sides
and seldom occur as a wood. Of the poplars, only aspen Populus
tremula is certainly native; this species is widely distributed in
woods of various kinds, but seldom has a high cover and usually
occurs as scattered clumps, because it commonly regenerates by
suckering. Crab apple Mains sylvestris is frequent, but again
usually occurs as scattered trees within woods or along their edges,
and is equally common as a hedgerow tree.
The commonest non-indigenous
trees in Britain are sweet-chestnut,
horse-chestnut Aesculus hippocastanum, sycamore Acer pseudoplatanus,
turkey oak Quercus cerris, larches Larix spp., Norway spruce Picea
abies, Sitka spruce P. sitchensis, lodgepole pine Pinus contorta,
Corsican pine Pinus nigra var. calabrica, Douglas fir Pseudotsuga
menziesii and firs Abies spp. A sweet-chestnut facies of mixed
deciduous woodland is recognised in south-east England and East
Anglia, while sycamore has spread a great deal and dominates many
small woods in various parts of Britain. The horse-chestnut and
turkey oak Q. cerris are planted a good deal as ornamental trees but
cannot be regarded as woodland species of any importance, except
very locally. All the conifers except the Abies spp. are, by
contrast, planted on a very extensive scale in commercial forestry,
and woods of these species cover large areas, especially in western
and northern Britain.
Both the amenity planting
of alien broad-leaved species and
commercial afforestation with conifers fall outside the scope of the
Review, though the second activity is having an enormous impact on
wildlife and habitat in Britain. Most of the above species
nevertheless occur in varying abundance in semi-natural woodlands
dominated by native trees, and here they often have the effect of
enhancing ecological as well as floristic diversity. For the species
which regenerate freely, such as sycamore and larch, present
distribution is not necessarily an accurate indication of actual
planting of the trees. The associated flora and fauna of the
completely artificial woods composed of these alien species is not
sufficiently outstanding or different from that of native woodlands
to require special conservation measures at present.
In mountainous country,
tree growth is increasingly inhibited above
a certain altitude, and eventually gives way to less luxuriant life
forms. The true altitudinal sequence from forest to montane
vegetation is now very seldom found in Britain, so universally has
woodland been destroyed by man towards its upper limit. Even so, it
would seem probable that the original situation was a gradual
diminution in height of the canopy-forming trees, as exposure
increased with altitude, until a low scrubby growth marked the upper
forest limit. This scrub evidently consisted of a depauperate growth
of species such as oak, birch, ash and Scots pine, according to soil
conditions, but mixed with tall shrubs such as hazel Corylus
avellana, rowan, juniper Juniperus communis and willows. It is
likely that this upper woodland fringe passed through tall growths
of heather on acidic soils and smaller willows on basic soils as a
zone transitional to the distinctly montane vegetation of really
high ground. In many moorland areas, especially in Scotland, there
are frequent patches of willow scrub, mainly of Salix cinerea and S.
aurita, at elevations well below the tree limit, and these are often
well developed beside streams. On the higher Scottish mountains
there are still fragments of a montane willow (Salix lapponum, S.
lanatd) scrub which was probably once zoned above subalpine
birchwoods, as it is in Scandinavia at the present day.
Actual altitudinal zonation
of different woodland types is less
clear. There are places where birchwood is zoned above either
oakwood or pinewood, but as the reverse order sometimes holds, it
may well be that the altitudinal distribution of woodland in Britain
at present is more closely related to soil fertility or past
management than to elevation per se. Because of the altitudinal
descent of vegetation zones in a north-westerly direction, the
actual upper limits of woodland vary according to geographical
position in Britain. South of a line from the Humber to the Severn,
it is very doubtful if the true potential upper limit of forest is
ever reached, for there is no ground high enough. Tree growth on the
higher parts of the Dartmoor plateau may have been inhibited by
development of blanket mire since the Atlantic Period; elsewhere in
southern England, at least on all but the wettest ground, it is
likely that forest cover was once virtually continuous and that the
present absence of tree growth is due to human influence.
In Brecknock, relict fragments
of scrubby woodland still occur at
610 m and in north Wales the occurrence of scattered rowan and birch
on high cliffs suggests that woodland may once have extended almost
to this level, at least in sheltered places. In Lakeland, parallel
observations suggest a rather lower upper limit, though the top of
the Keskadale oakwood at 460 m may reflect the effects of
insufficient soil rather than a true climatic limit. The best
example of a true upper forest limit remaining in Britain is on the
spur of Creag Fhiaclach in the western Cairngorms, Inverness-shire,
where at 640 m a scrubby growth of gnarled Scots pine and jumper
passes gradually into heather moor. With distance west in the
Highlands there is a rapid descent of tree limits, and in north-west
Sutherland it is probable that the potential upper tree limit lies
at around 300 m. On the most wind-exposed western coasts, tree
growth is in places virtually extinguished altogether, and is
represented by pockets of scrub, mainly of hazel, aspen and willow,
which survive in sheltered places. The tree-less nature of the Outer
Hebrides, Orkney and Shetland may result from a combination of human
influence and unfavourable climate. The surviving fragments of
birchwood, hazel, aspen and willow scrub on the islands of some
inland lochs here may be rather depauperate and atypical examples of
former woodlands, but it is possible that the forest cover has
always been patchy in these far northern and western parts of
Scotland during the Post-glacial Period. The following
classification of major woodland types has been adopted in this work:
Oakwood
Western facies Eastern
facies
Mixed deciduous woodland
Central facies Lime facies Hornbeam facies
Sweet chestnut facies
Mixed deciduous woodland:
ancient parks and overmature
woodland
Beechwood
Ashwood
Pinewood
Birchwood
Alderwood
Other types of woodland
Holly
Yew
Juniper
Box
Rare species |
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THE SHRUB LAYER
The presence or character
of a woodland shrub layer (< 5 m) depends
on the influence of the dominant trees themselves, climate, soil
conditions, and management practice. The heavy shade cast by a pure
beechwood usually excludes undershrubs, and the heavy grazing to
which many hill woods are subjected often results in their virtual
absence, evidently by preventing regeneration. Upland woods of oak,
birch and pine are commonly devoid of a shrub layer.
Hazel is probably the most
widespread and abundant shrub layer
species in Britain. There are indications that hazel can grow on
fairly base-poor soils when grazing is absent, and may then form a
patchy shrub layer in acido-philous oakwoods. This shrub is,
however, characteristically most abundant in woods on base-rich and
calcareous soils, where it is often coppiced and then forms a fairly
dense shrub layer below or between the standard trees (usually oak).
Hazel is especially associated with northern and western ashwoods,
and sometimes remains after the dominant trees have died or been
removed. Hazel scrub with few or no trees is widespread in coastal
areas of the western Highlands and is especially characteristic of
exposed coastal ground on base-rich rocks in the Hebrides. In such
situations, hazel scrub may locally represent a climatic climax,
though it is possible that birch and perhaps ash and oak would also
be present if there were no human disturbance.
On dry, fairly basic woodland
soils, species such as hawthorn
Crataegus monogyna, blackthorn Prunus spinosa and elder Sambucus
nigra are often present in the shrub layer, and there are variable
amounts of guelder rose Viburnum opulus, gooseberry Ribes uva-
crispa, blackcurrant R. nigrum, wild roses Rosa spp., bramble Rubus
fruticosus agg. and raspberry R. idaeus. Hawthorn scrub dominated by
Crateagus monogyna is a characteristic serai scrub type on
calcareous grassland where grazing has become too light to inhibit
the growth of woody seedlings. Left to itself, this hawthorn scrub
usually becomes invaded by trees such as oak and ash and is
eventually converted to woodland. On the sheep-walks of the western
and northern hills, especially in Wales and northern England, there
are many examples of a more open and persistent hawthorn scrub which
lies within the potential forest limit on more acid soils. This
parkland type community may represent a period of recession in sheep
farming, which reduced grazing temporarily and thereby allowed the
invasion of grassland by hawthorn; subsequent restoration of sheep
flocks would prevent further invasion of the shrub. Crab apple is
sometimes represented in these hill hawthorn scrubs. Blackthorn
often occurs in thickets both within the woodland and outside, and
is found mainly on the better soils. It creates a deep shade and few
other species are able to compete in well-established blackthorn
scrub.
Scrub of gorse and broom
belongs to the acidic heath ecosystem
rather than to woodland and is not considered here. Roses and
brambles are, on the other hand, abundant shrubs of woodland. Rosa
canina is widespread, whereas R. villosa and R. arvensis are,
respectively, somewhat northern and southern species. Roses as a
whole, however, have a subsidiary role beneath the woodland canopy,
and belong more to the woodland edge and hedges. Brambles are
important low shrubs of woodland, and are often dominant almost to
the exclusion of the field layer and may form dense, tangled, masses
up to 2 m or more high. The ecology of the numerous micro-species
has not been worked out, but brambles as a whole flourish best on
soils of intermediate base- status, and in ungrazed woods, and they
are evidently favoured by certain kinds of management. They are an
important component of the woodland ecosystem for nesting birds and
insects. A high cover of brambles is especially characteristic of
oakwoods, but they can be found in association with almost any of
the common woodland tree dominants, and may well be indicators of
disturbance when dominant.
In England, the shrub layer
of woodlands on calcareous soils is rich
in species, which typically include Midland hawthorn Crataegus
oxyacanthoides, privet Ligustrum vul-gare, spindle Euonymus
europaeus, whitebeam Sorbus aria, dogwood Thelycrania sanguinea,
buckthorn Rhamnus catharticus and wayfaring tree Viburnum lantana.
Most of these calcicolous shrubs have a southern distribution and
reach their northern British limits on the limestone around the head
of Morecambe Bay. Woods on basic soils from Norfolk northwards
frequently have a good deal of bird-cherry Prunus padus, though this
is usually patchy and seldom attains a high cover; while beechwoods
on calcareous soils in southern England locally have an understorey
of yew. Rare shrubs of calcicolous woodlands, especially ash-wood on
Carboniferous Limestone, include Ribes alpinum, R. spicatum and
Daphne mesereum. D. laureola occurs more widely in calcicolous
woodlands and can be abundant locally.
On base-rich rocks, mainly
Carboniferous Limestone, in south-west
England and Wales, are a number of rare or endemic whitebeams
related to the more widespread Sorbus aria but now recognised as
distinct species. This group is of special interest as it may have
evolved since the last glacia-tion, and thus could represent a
recent example of rapid evolutionary divergence and speciation.
These species are Sorbus minima, S. bristoliensis, S. leyana, S.
anglica, S. subcuneata, S. devoniensis, S. porrigentiformis, S.
vexans, S. leptophylla, S. wilmottiana and S. eminens. A more
widespread western and northern species is S. rupicola, and there is
a local form of S. aria, S. lancastriensis, around the head of
Morecambe Bay. In Arran, there are isolated occurrences of the two
endemics, S. arranensis and S. pseudofennica, both confined to steep
stream banks on granite. Most of these whitebeams occur in open,
rocky habitats, where they are little subject to competition but
some occur in the shrub layer of woodland, and a few attain the size
of trees.
In woods on acidic soils
(especially oakwoods), holly is one of the
principal undershrubs and sometimes forms dense thickets. In the
absence of grazing, holly would probably be a common or dominant
understorey species in many hill oakwoods in western and northern
Britain; in these districts it is often abundant on cliff faces and
ravine sides. Holly is an oceanic species on the European scale, but
is widespread in Britain and well represented in many parts of the
east, though it is rare in the east Midlands from south Yorkshire to
Northamptonshire. Rowan is also present in the shrub layer of many
oakwoods on poor soils, and it often attains the size of a tree.
Northern pine and birch woods often lack a shrub layer, but locally
have an abundance of juniper Juniperus communis ssp. communis,
mainly where the canopy is not too dense. Birchwoods on basic soils
in northwest Scotland often have much hazel, which may share the
rather low canopy and, under more natural conditions, holly, rowan
and willows were probably also present. The remaining fragments of
scrub woodland on islands in lakes probably give the closest
approximation to original northern woodland, and typically show
mixtures of these species.
Where the ground is moist,
especially on clay soils, willows are
usually represented, and include Salix cinerea, S. pentandra, S.
purpurea, S. triandra, S. mminalis and S. capraea. These may reach
dominance on really waterlogged ground and are then often mixed with
alder and/or birch,
to form carrs which may
occur on their own or as patches within
other woods. Such thickets are frequently associated with fen
communities in valley mires and open water transition/flood-plain
mires, and may overlie deep peat. Alder buckthorn Frangula alnus, is
locally abundant in carrs, whilst tall bog myrtle Myrica gale
sometimes occurs in the more open swamp woods and may then be
regarded as part of the scrub. Carr is characteristically a late
stage in hydro-serai development, and may persist indefinitely or
change to woodland, depending on the final relationship between
water table and ground surface obtaining on a particular site.
Three of the four woody
climbers, honeysuckle Lonicera periclymenum,
ivy Hedera helix and woody nightshade Solatium dulcamara, belong in
many respects to the field layer rather than the shrub layer. Both
honeysuckle and ivy form low tangles or dense carpets on the ground
as well as ascending trees. Both species occur most commonly on soil
in the medium fertility range but honeysuckle can grow on fairly
acid mor soils. Both are kept in check by grazing and so are often
more characteristic of lowland than of upland woods. Ivy is an
oceanic plant in Europe but is very characteristic of trees and
woods of such dry districts as East Anglia in Britain. Woody
nightshade is mainly a plant of swamp woods and carrs especially in
the south. The fourth woody climber, old man's beard Clematis
vitalba, is an indicator of calcareous soils and occurs mainly in
southern England as a climber of scrub and woodland edges; it has
increased since myxomatosis reduced rabbit populations.
In many woods, species
such as Mahonia aquifolium and Rhododendron
ponticum have been planted or are established as escapes.
Rhododendron often becomes a pest from the ecological viewpoint as
it spreads rapidly and creates such dense shade that the field and
ground layer species beneath are killed. Dominance of certain
shrubs, e.g. elder and perhaps holly, is sometimes the result of a
wood having been used as a colonial roost or nesting place by berry-
feeding birds.
Scrub is a serai development
in other formations, such as grassland,
heath or mire, so it is considered also in Chapter 6, and, more
briefly, in Chapter 8. |
|
THE FIELD LAYER
A woodland floor is typically
covered by a dense growth of small
shrubs, herbs and ferns usually less than I m in height, and termed
the field layer. Some field communities of woodland occur with
little variation from the extreme north to the extreme south of
Britain and are thus of small value in identifying regional woodland
types. Others contain groups of species with distinctive
geographical distribution patterns which are of value in regional
characterisation. The field layer varies in relation to chemical and
physical soil differences, and variations in management, especially
herbivore grazing. Field layer floristics are useful in
characterising edaphic and biotic diversity within woodlands. The
chief directions of variation in regard to soil conditions are base-
status and degree of wetness. On the whole, field layer species have
a need for the shade and humidity conferred by the tree and shrub
layers, as is evidenced by the occurrence of many of the most
characteristic species in the deep, vertical crevices of treeless
limestone pavements, or on the moist ledges of elevated mountain
cliffs. An important ecological separation of the woodland field
layer is between grazed and ungrazed types.
Upland woods, especially
of the hanging type, are typically unfenced
above and are mostly heavily grazed, as they are often used as
wintering places for the sheep pastured on the adjoining open
hillsides. Only where enclosed farmland occurs above the woods, as
in parts of Wales, are the woods fenced against the intrusion of
grazing stock. On the other hand, most lowland woods are surrounded
by agricultural land and are fenced, so that they are usually
ungrazed, except sometimes by cattle or, less often, pigs. Some
differences in the field layers between upland and lowland woods
which were once attributed to the more obvious climatic differences
between such sites, have been shown to result in part from this
divergence in management.
The effect of heavy grazing
by large herbivores in woodlands is to
promote an increase of grass species at the expense of
dicotyledonous herbs. A heavily grazed woodland thus has a generally
grassy appearance, though the actual species vary according to soil
conditions. On the dry, base-poor brown earths of upland oak and
birchwoods, Agrostis tennis, A. canina, Anthoxanthum odoratum and
Deschampsiaflexuosa are among the abundant grasses, whilst Molinia
caerulea is often dominant in wetter situations. Bracken Pteridium
aquilinum is often locally dominant in these upland woods,
especially where the tree canopy is not dense, and may here be
encouraged by grazing. Royal fern Osmunda regalis was once
widespread in a variety of acidic woodland habitats, but has been
much reduced by collecting and is now local and found especially in
rocky situations, as in wooded ravines, or on islands in lochs.
The ungrazed counterparts
of these acidic oak and birch-woods are
much scarcer, and have field communities which are usually
distinguished by a dominance of bilberry Vaccinium myrtillus, Luzula
sylvatica or ferns such as Dryopteris dilatata, D. filix-mas, D.
borreri and Thelypteris limbosperma. Bracken also flourishes in many
ungrazed woods and evidently has an original niche in the woodland
field layer; it is abundant or even dominant in such a wide range of
woodland field communities that there are grounds for regarding it
as a separate layer on its own, between the shrub and field layers
in stature. Smaller ferns such as Thelypteris dryopteris and T.
phegopteris also tend to be more abundant or luxuriant in ungrazed
woods. Lonicera peri-clymenum is abundant in many of these ungrazed
woods on poor soils, sometimes forming an important component of the
field layer, and there is often an abundance of Corydalis
claviculata in rocky places. Where the light intensity is high,
Calluna and Erica cinerea may occur as members of the field layer in
oak and birch woods. Whether grazed or ungrazed, the flora is
usually poor in species and typical associates consist of Potentilla
erecta, Galium saxatik, Melampyrum pratense, Succisa pratensis,
Digitalis purpurea, Luzula pilosa, Blechnum spicant, Dactylorchis
maculata and Solidago virgaurea. Where soil conditions are rather
less acidic and base- deficient, the field layer of oak and birch
woods shows a greater variety of herbs. Holcus mollis, Endymion non-
scriptus, Anemone nemorosa and Oxalis acetosella are locally
dominant, and Teucrium scorodonia, Viola riviniana, Veronica
ojficinalis, Hypericum pulchrum, Lathyrus montanus, Stellaria
holostea, Silene dioica, Galium aparine, Conopodium majus, Holcus
lanatus, Poa trivialis and P. nemoralis are characteristic of these
less heavily podsolised soils. Some of these plants appear to thrive
under grazing, whereas others do not, but there is always the
tendency to a higher cover of grasses in heavily grazed woods. The
various species of bramble may be regarded as belonging to either
the field or shrub layers, or both, and flourish mainly on soils of
intermediate fertility.
Oak, oak-ash or ash woods
on base-rich soils have grasses such as
Brochypodium sylvaticum, Melica uniflora and Dactylis glomerata on
drier ground, and Deschampsia cespitosa in damper places. Forbs are
abundant and typically include Mercurialis perennis, Geranium
robertianum, Primula vulgaris, Sanicula europaea, Fragaria vesca,
Potentilla sterilis, Prunella vulgaris, Ajuga reptans, Ranunculus
ficaria, Gk-choma hederacea, Arum maculatum, Veronica chamaedrys and
Circaea lutetiana. Taller species include Geum rivale, G. urbanum,
Filipendula ulmaria, Allium ursinum, Urtica dioica, Valeriana
officinalis, Listera ovata and Stachys sylvatica. Athyrium filix-
femina and Carex sylvatica are also usually present. Widespread but
less constant forbs of these richer soils include Mycelis muralis,
Paris quadrifolia, Epipactis helleborine, Scrophularia nodosa, Carex
laevigata and the ferns Phyllitis scolopendrium and Polystichum
aculeatum. Woods on basic substrata have many local or rare species,
and those characteristic of the strongly calcareous formations
include Lithospermum purpurocaeruleum, Actaea spicata, Helleborus
viridis, H. foetidus, Iris foetidissima, Polygonatum multiflorum,
Atropa belladonna, Gagea lutea, Cephalanthera rubra, Orchis purpurea
and Cypripedium cakeolus. Most of these rare and local basiphilous
species occur in small quantity, but a few, e.g. oxlip Primula
elatior in some Cambridgeshire woods, attain local dominance.
Heavily grazed woods on
base-rich soils usually show dominance of
grasses, with an abundance of grazed-down forbs. Ungrazed woodland
field communities on basic soils have a lesser abundance of grasses
and a higher cover of forbs and ferns, which attain a greater
luxuriance and often have a larger number of species than in grazed
woods. Tall forbs are especially sensitive to grazing, and ungrazed
woods tend to have a higher representation of species than those
which are heavily grazed.
As well as the range from
acidic to basic and grazed to ungrazed,
the woodland field communities show a gradient from dry to wet. The
wet acidic type shows an approach to certain types of oligotrophic
soligenous mire communities; there is typically an abundance of
sedges such as Carex echinata, C. nigra and C. rostrata, and of
other species such as Juncus effusus, J. acutiflorus, Molinia
caerulea, Agrostis stolonifera and Viola palustris. Slightly richer
wet soils have Ranunculus repens, Chrysospknium oppositifolium and
Carex remota, whilst the strongly basic wet mulls approach rich-fen,
with Carex paniculata, Caltha palustris, Cardamine amara, Oenanthe
crocata, Senecio aquaticus, Crepis paludosa, Iris pseudacorus,
Eupatorium cannabinum, Equisetum tel-mateia and Thelypteris
palustris. Grazing animals tend to avoid these wet woodland soils,
so that there is usually less differentiation here between grazed
and ungrazed communities. Since many of the basic woodland soils are
clayey, some of the typical basiphilous field layer plants are
species with a moderate moisture requirement and also occur in fen,
e.g. Filipendula ulmaria, Valeriana ojficinalis and Deschampsia
cespitosa.
The field communities described
so far show only a limited degree of
association with particular woodland tree dominants. The wettest
types, both base-rich and base-poor, are usually found in alderwood,
but oak, ash and birch all tolerate moderately wet soils and some of
the relatively hydrophilous field communities can be associated with
these trees. The dry acidophilous types occur mainly in association
with oak and birch, while the dry basiphilous communities are found
with these trees as well as with ash. The field layer of beechwood
is usually a shade-impoverished derivative of one of the above
communities, within the range of acidophilous to basiphilous dry
types as beech does not grow on wet soils. Beechwoods often have a
very poorly developed field layer, with little but saprophytes such
as Neottia nidus-avis and Monotropa hypopitys, but they are the
habitat of a few rare or very local orchids such as Epipogium
aphyllum, Cephalanthera rubra, C. damasonium and Epipactis purpurata.
The woodland field layer
may show little relationship to the
particular tree species overhead, but its composition is greatly
influenced by the intensity of shade cast by the canopy.
Considerable changes in relative abundance of species follow
thinning, coppicing or clearance of both the tree and shrub layers,
and there are serai developments in the field layer as the overhead
canopy is re- established. Many species spread rapidly, reach
dominance in ' societies', and flower profusely when woodland is
cleared. Some of these are wide amplitude species occurring commonly
in treeless habitats, e.g. Silene dioica, Filipendula ulmaria,
Glechoma hederacea and Deschampsia cespitosa, but others are
typically woodland plants which flourish for a period under the
abnormally high light intensities, e.g. oxlip. An abundance of
Digitalis purpurea and Chamaenerion angusti-folium is especially
associated with burning of the brushwood. There are also important
seasonal changes in floristic composition of the field layer between
early spring and the middle of summer, which corresponds with
decreasing light intensity as the canopy develops. Species such as
Endymion non-scriptus, Ranunculus ficaria, Anemone nemorosa and
Allium ursinum put on rapid vegetative growth and flower early in
the year, then die down and almost disappear above ground. They are
followed by later flowering species such as Conopodium majus,
Circaea lutetiana and Geum urbanum. Some species flower early but
persist vegetatively right through the summer, e.g. dog's mercury
Mercurialis perennis. A few plants, such as the bluebell and the
primrose, are found mainly in woodland in the south and east, but
are equally characteristic of tree-less habitats in the north and
west.
The Scottish pinewoods
have fairly distinctive field communities of
two main types. The first, where the canopy is open and light
intensity high, is a Callunetum which differs in only minor respects
from that of the open moorland, while the second is a Vaccinium
heath which resembles that of ungrazed oak and birchwoods but has
typically an abundance of V. vitis-idaea as well as V. myrtillus.
There is also a distinctive floral element of the pinewood
communities which includes the northern species Goodyera repens,
Linnaea borealis, Pyrola minor, Orthilia secunda, Moneses uniflora,
Listera cordata and Trientalis europaea. Some of these plants are
found in birchwood, and there is a good deal of overlap in the field
layers of pine and birch wood in the Highlands. The abundance of
Molinia in some wetter pine- woods may be partly a grazing effect.
Besides the examples mentioned
under pinewood, certain floristic
differences may be found in comparing southern with northern woods.
On acidic soils, Luzula forsteri is a characteristic southern
woodland species, whilst Thelypteris phegopteris, T. dryopteris and
Melampyrum sylvaticum (rare) belong to the northern woods. Field
layer composition shows greater divergence on basic soils, with
Lithospermum offi-cinale, Galeobdolon luteum, Campanula trachelium,
Cephalanthera damasonium, Epipactis purpurata, Ophrys insectifera,
Arum maculatum, Carex strigosa, C. pendula and Polystichum setiferum
as geographically diagnostic (though seldom constant) species in the
south, and Stellaria nemorum, Rubus saxatilis, Crepis paludosa,
Trollius europaeus, Cirsium heterophyllum, Geranium sylvaticum,
Campanula latifolia, Festuca altissima and Melica nutans in the
north. Oceanic species particularly characteristic of woods along
the Atlantic seaboard include Dryopteris aemula, Polystichum
setiferum and Hypericum androsaemum, though these are also well
represented in woods of the extreme south of England, especially
Sussex and Kent.
Where rock outcrops and
block-litters occur within woods, they
often, or locally, have species of small shrub, herb and fern which
are not regarded as woodland species, e.g. Umbilicus rupestris,
Sedum anglicum and Dryopteris abbreviata on acidic rocks; and
Helianthemum chamaecistus, Sedum forsteranum, Saxifraga aizoides,
Alchemilla alpina, Geranium lucidum, Arabis hirsuta, Pimpinella
saxifraga, Orchis mascula, Carex flacca, Asplenium trichomanes, A.
viride and A. adiantum-nigrum on basic rocks. In addition, outcrops
with big ledges and the precipitous sides of ravines often carry
good examples of unmodified field communities which have been
protected from grazing, and in some hill woods certain species of
shrub, herb and fern may be confined to these habitats.
The definition of phytosociological
units within the woodland field
layer is difficult. Many species show a patchy abundance or
dominance which makes for floristic heterogeneity, and the concept
of the 'society' is particularly applicable to analysis of the
pattern of variation. It therefore seems best to adhere to a small
number of broadly-defined community units, within which separate
societies can be recognised.
Some herbaceous plants
of the field layer show a particular
association with tree cover and are thus reliable indicators of
permanent woodland, e.g. Moneses uniflora, Primula elatior and Carex
strigosa. This should in theory be true of species with mycorrhizal
and saprophytic associations with trees, such as Goodyera repens,
Neottia nidus-avis and Lathraea squamaria, but it is always possible
that these species may be transplanted along with tree seedlings and
thus appear in recent plantations. |
|
THE GROUND LAYER
The floor of a woodland
may be covered largely with the litter of
fallen leaves when canopy shade is intense, as in closed beech or
yew woods, but where light intensity is higher, there is typically a
carpet of bryophytes, and a lesser but varying abundance of lichens.
On acidic soils, a characteristic group of species occurs with
little variation over the whole country and includes Hypnum
cupressiforme, Pleuro-zium schreberi, Hylocomium splendens,
Plagiothecium undula- tum, Dicranum scoparium, Polytrichum formosum,
Mnium hornum, Leucobryum glaucum, Campylopus flexuosus, Lopho-colea
bidentata, Calypogeia muellerana, Lepidozia reptans and Diplophyllum
albicans. On the more shaded and dry soils, this list may be only
partly represented. The moss carpet may occur below a layer of
species such as bilberry or in mixture with grasses, and it tends to
be favoured by grazing, which reduces or suppresses the competition
from vascular plants. In the western and northern woods, species
such as Rhytidiadelphus loreus, Dicranum majus, Isothecium myo-
suroides and Thuidium delicatulum are often abundant.
Many of the western and
northern woods are situated on the lower
mountain slopes and frequently have a block-strewn floor and rock
outcrops of varying size, including the walls of deep stream-cut
ravines. The sides and crowns of larger blocks and sloping outcrops
usually have carpets of the above-named bryophytes, with few
vascular plants, and in the extreme west there is a strong
representation of large and conspicuous bryophytes which have a
markedly oceanic or Atlantic distribution in Europe. These include
Dicrano-dontium denudatum, Hylocomium umbratum, Sphagnum
quinquefarium, Scapania gracilis, Plagiochila spinulosa, Mylia
taylori, Saccogyna viticulosa, Bazzania trilobata, Lepidozia pinnata
and Adelanthus decipiens. These plants require a permanently humid
atmosphere and thus flourish within the shade and shelter of
woodlands in heavy rainfall areas of Britain. On acidic, exposed
rocks the constant and abundant bryophytes include species such as
Sphagnum tenellum, Andreaea rupestris, A. rothii, Cynodontium brun-
tonii, Rhacomitrium aquaticum, R. heterostichum, Campylopus
atrovirens, Hyocomium flagellare, Heterocladium heterop-terum,
Marsupella emarginata, Scapania undulata, Mylia taylori and
Diplophyllum albicans. These bare, rocky situations afford suitable
habitats for a large number of other oceanic and Atlantic
bryophytes, many of them small and rupestral, including many
competition-intolerant species, and ranging from those which need
permanently wet sites to others which favour dry, but shady rocks.
The ravine habitats in particular provide a diversity of conditions
and are particularly rich in these plants. In the south- west, the
Atlantic bryophyte flora contains a higher proportion of
thermophilous species with tropical and Macaronesian connections,
while in the north-west, montane oceanic species adapted to cool,
humid conditions are correspondingly better represented.
Among the more notable
of these rupestral oceanic and Atlantic
bryophytes of acidic rocks are the mosses Semato-phyllum
novae—caesareae, S. demissum, Daltonia splachnoides, Rhabdoweisia
crenulata, Dicranum scottianum, Hypnum callichroum and Grimmia
hartmanii; and the liverworts Cephaloziella pearsonii, Acrobolbus
zvilsonii, Radula carring-tonii, R. aquilegia, Colura calyptrifolia,
Aphanolejeunea microscopica, Drepanolejeunea hamatifolia,
Harpalejeunea ovata, Lejeunea lamacerina, Frullania germana, F.
micro- phylla, Lophocolea fragrans, Metzgeria hamata,
Plagiochila tridenticulata, P. punctata, P. atlantica, Saccogyna
viticulosa, Harpanthus scutatus, Jamesoniella autumnalis and
Tritomaria exsecta. Species associated with rocks in streams or
water trickles include the mosses Fissidenspolyphyllus, F.
serrulatus, Isothecium holtii and Eurhynchium alopecuroides; and the
liverworts Jubula hutchinsiae, Porella pinnata, Radula valuta and
Riccardia sinuata. The two filmy ferns Hymenophyllum tunbrigense and
H. wilsonii behave as bryophytes and grow on shaded mossy rocks and
tree bases in the western woods and luxuriate on the walls of damp
wooded ravines. A much rarer relative, the Killarney fern
Trichomanes speciosum occurs in its few stations in Britain mainly
in shady, damp or dripping caves and rock crannies in wooded ravines
in the extreme west.
The Atlantic bryophyte
flora consists mainly of calcifuge or
indifferent species, and it is thus represented mainly in oak,
oak—ash and birch woods on poor to moderately rich rocks and soils.
The ground layer of woods on strongly basic soils has another
distinctive group of bryophytes which includes Rhytidiadelphus
triquetrus, Eurhynchium striatum, E. praelongum, Brachythecium
rutabulum, Ctenidium mollus-cum, Hylocomium brevirostre, Mnium
undulatum, Atrichum undulatum, Fissidens taxifolius and Plagiochila
asplenioides var. major. Thuidium tamariscinum is usually abundant
on soils of moderate base-status, and H. splendens is often more
plentiful on these than on highly podsolised soils. Basic outcrops
in woods and wooded ravines also have a characteristic bryophyte
flora which includes Neckera crispa, Tortella tortuosa, Grimmia
apocarpa, Gymnostomum recur-virostrum, G. aeruginosum, Anoectangium
aestivum, Ortho-thecium intricatum, Breutelia chrysocoma, Campylium
pro-tensum, Fissidens cristatus, F. osmundoides, Anomodon
viticulosus, Distichium capillaceum, Brachythecium plumosum,
Thamnium alopecurum, Ditrichum fiexicaule, Bartramia hallerana,
Preissia quadrata, Scapania aspera, Radula com-planata, R.
lindbergiana, Cololejeunea calcarea, Metzgeria pubescens, Leiocoka
muelleri and L. turbinata. Basiphilous Atlantic bryophytes include
Marchesinia mackaii and the rare Lejeunea mandonii.
As drainage impedance increases
at the acidic end of the scale,
Bryalean mosses are replaced by Sphagnum spp., including S.
palustre, S. recurvum, S. fimbriatum and S. capillaceum, though
Polytrichum commune often has a high cover in these damp woodlands,
and Acrocladium cordi-folium may be plentiful. Woodlands with wet
soils of intermediate base-status may have other Sphagnum spp., such
as S. squarrosum, S. contortum, S. plumulosum and S. warn-
storfianum, but the richer wet woodland soils tend to be dominated
by vascular plants with a poor development of a bryophyte carpet.
Species such as Cratoneuron commutatum, Pellia endiviifolia and
Acrocladium giganteum occur in these wet situations.
The ground layer also contains
a number of lichens growing on soil,
litter or rock surfaces, including common species of heaths and
grasslands such as Cladonia impexa, C. gracilis, C. cervicornis, C.
rangiformis, Peltigera canina, P. polydactyla, P. horizontalis,
Peltidea aphthosa, Parmelia saxatilis, P. omphalodes, Sphaerophoms
fragilis and Stereo-caulon vesuvianum, and rarer, somewhat Atlantic
species such as Sphaerophorus melanocarpus and Nephromium
lusitanicum. Basidiomycete fungi are an important component of the
woodland flora, but have not been studied in the Review.
The woodland bryophyte
and lichen communities and flora are
particularly sensitive to management effects. Many of the
bryophytes, especially Atlantic species, need the shade and high
atmospheric humidity conferred by an overhead canopy, and the more
sensitive species rapidly die out when the tree or scrub cover is
lost. For common and rapidly spreading species there is little or no
problem - even after clear-felling of a wood, spores from other
populations elsewhere in the district will recolonise the site,
provided trees grow up again. However, for the rarer species,
capacity for spread appears at present to be so limited that re-
colonisation after complete clearance does not occur, and it is only
when parent populations are allowed to persist continuously on the
site (i.e. through avoidance of clear-felling) that the species in
question survive. For similar reasons, re-established woods usually
show an even greater scarcity of the kind of plants discussed above
than those which have regenerated after clear-felling. |
|
EPIPHYTES
A vascular epiphytic tree
flora is very poorly represented in
Britain. Ivy and honeysuckle have been noted as common climbing
shrubs which also belong to the shrub and field layers, and
mistletoe Viscum album is a local parasite of deciduous trees in
southern Britain, but most commonly in orchards. In western Britain,
oaks in particular often have vigorous growths of polypody
Polypodium vulgare, but the bulk of the epiphytic flora consists of
bryophytes and lichens. These increase in variety and luxuriance
towards the west, and are best developed as communities under
strongly oceanic conditions and where atmospheric pollution is
least. Some genera of mosses, notably Ulota and Orthotrichum are
largely arboreal, but in the western woods a large number of
bryophyte species, including many with rock habitats, may be found
growing on trees. Some of the
larger species described
as forming communities over stable block
litters in western woods also grow over the surface roots and bases
of trunks of larger trees and in especially shady, humid situations
the filmy ferns behave similarly.
The most abundant epiphytic
bryophytes in these woods are varieties
of Hypnum cupressiforme, Isothecium myo-suroides, Dicranum
scoparium, Ulota crispa and Frullania tamarisci. Strongly Atlantic
species especially characteristic of trees include Ulota vittata
(mainly on hazel), Frullania germana, Plagiochila punctata and Mylia
cuneifolia. A considerable number of arboreal bryophytes are
characteristic of dry and even continental conditions, but many of
these are local and some appear to be decreasing. They are usually
associated more with single or scattered trees than with closed
woodland, e.g. Leucodon sciuroides, Tortula laevipila, T. tirescens,
Pylaisia polyantha, Dicranum montanum, D. flagellare, D. strictum,
several Orthotrichum spp. and Ptilidium pulcherrimum. Some epiphytic
bryophytes appear to grow mainly on trees on acidic soils, whereas
others have an association with trees on basic soils.
Some bryophytes, mainly
liverworts, are especially characteristic of
dead fallen tree trunks which have been left to decay in situ; these
include the conspicuous red Nowellia curvifolia, Lophocolea
cuspidata, L. heterophylla, several species of Cephalozia and
Cephaloziella, and rarities such as Sphenolobus helleranus and
Calypogeia suecica.
In districts farthest from
sources of atmospheric pollution, there
is also a rich epiphytic lichen flora. Such communities are
especially well developed in the west and include a number of
species which are strongly Atlantic and others which flourish most
abundantly in the west but are much less markedly Atlantic. These
distinctive lichens include large foliose types such as Lobaria
pulmonaria, L, laete- virens, L. laciniata, Lobarina scrobiculata,
Stictina sylvatica, S. limbata and S. fuliginosa, and smaller
species such as Pannaria rubiginosa, Parmelia laevigata, Parmeliella
plumbea and Normandina pulchetta. Rich lichen floras containing
oceanic species are, however, by no means confined to the west, and
numerous important outposts occur in eastern and southern England
where rainfall is low. More widespread corticolous lichens include
Evernia prunastri, Hypogymnia physodes, Parmelia sulcata, P.
caperata, P. saxatilis, Cetraria glauca, Ramalina farinacea,
Alectoria spp., Usnea spp. and Physcia spp. Many of these epiphytic
lichens are associated with large, old trees, especially in the
drier, eastern districts, and here they may be indicators of
permanent woodland, or at least open tree growth. Epiphytic lichens
are less tolerant of deep shade than many woodland bryophytes, and
in England some of the richest areas for these lichens are park
woodlands with their scattered growth of trees. Their connection
with big, old trees is evidently related to a low capacity for
spread, at least under present conditions. In western Scotland,
however, some of the larger Lobaria spp. and Sticta spp. grow on a
much wider variety of species and age classes of tree than in less
oceanic districts. Conversely, some epiphytic species appear unable
to withstand the high humidity of the extreme west and are absent
from woods in the wettest areas.
The dependence of the less
common arboreal bryophytes and lichens on
continuity in time of tree cover is obvious. Some species need the
shade and high humidity associated with a more or less closed
canopy, but others grow in sun-exposed situations where it would
seem that the permanence of the right substratum, i.e. tree bark, is
more important than the micro-climate conferred by tree cover. The
rarer species are again therefore good indicators of permanent
woodland, or at least continuity of open tree growth.
Epiphytic bryophytes and
lichens are believed to have declined
considerably in districts subject to urban-industrial atmospheric
pollution, and are very poorly represented, or even absent, within
and around the larger cities. They increase in a fairly consistent
manner with distance from these major sources of atmospheric
pollution, so that concentric lichen zones can be distinguished, but
it is possible that there may be adverse effects over considerable
distances. Probably the outstanding richness of the western
Highlands for lichens is a measure not only of favourable climate
but also of remoteness from significant atmospheric pollution. The
restriction of many species to older trees in some districts may
reflect the inability of sporelings to establish on new host trees
since pollution became significant at the beginning of the
Industrial Revolution. |
|
Flora
The flora of woodlands
can be considered in different ways. In the
previous subsection, woodland vegetation has been analysed into
different layers, and some account given of the dominant, constant
and characteristic species of each layer; to this extent, the flora
of woodlands has been partially examined already. This description
has, however, dealt adequately only with the rather limited range of
species in the tree and tall-shrub layers, and the present
subsection will deal explicitly with the very much larger number of
species in the field and low-to-medium shrub layers. The field layer
contains very many herbaceous species, both dicotyledons and
monocotyledons, and is of especial interest to many botanists.
The woodland flora may
be regarded as consisting of four main groups:
1 Species found exclusively
or mainly in woodland.
2 Species of wider ecological
amplitude, but with a strong
representation in woodland.
3 Species belonging essentially
to other habitats which sometimes
occur within woodland, e.g. mires.
4 Species of communities
such as grassland, heath and scrub which
are serai to woodland and do not persist long when closed canopy has
become established. The woodland edge and rides are included in this
category.
Because of the very large
number of vascular species involved in all
four of these classes, only the first two will be dealt with here.
The placing of species in the different classes has been a somewhat
arbitrary procedure, based on the experience of a few people, and
may be improved in the light of further knowledge. Table 5 (p. 82)
lists all native vascular species of the field and low-medium shrub
layers which are judged to belong to categories i and 2 above; and
the ecological and geographical distribution of these plants is
indicated. This gives a basis for discussing various features of the
woodland flora, though the tree and tall-shrub species will not be
considered further.
Very few species or groups
of species belong to a particular
woodland type, as characterised by tree dominants. The main
exception to this is the group of northern pinewood species already
mentioned on p. 77 and including Goodyera repens and Moneses
uniflora. Most of the other plants which are specific to a
particular woodland type belong to a rather specialised habitat
characterised also by a limited range of tree species, e.g.
Equisetum telmateia, Thelypteris palustris, Corallorhiza trifida and
Pyrola rotundifolia are associated with alder-willow woodland
because they need moist conditions, and Cypripedium calceolus, Carex
digitata, Poly-gonatum odoratum and Polemonium caemleum tend to
occur in ash wood because this is the usual type on the rocky
limestone ground which they require.
The number of exclusively
or even mainly woodland species in Table 5
is small - 76 out of a total of 236 species. Most of these are
presumed to have a need for shade conditions in some degree, but for
some of the mycorrhizal, saprophytic, semi-parasitic or parasitic
species there is a more definite association with woody species or
their litter, e.g. Goodyera repens, Monotropa hypopitys, Neottia
nidus-avis, Melampyrum pratense, Lathraea squamaria, Hedera helix
and Viscum album. Species certainly needing shade are the three
members of the Hymenophyllaceae and many of the mosses and
liverworts which form such a significant component of many western
British woods. Nearly all these hygrophilous species can, however,
grow in other shady habitats, as amongst rocks on north to east
aspects where trees are absent.
Only 66 of the 236 species
are associated with acidic, base-poor
soils, and of these 15 are ferns and nine are grasses. It therefore
follows that the richest woods floristically are those on base-rich
or calcareous soils. Most of the basi-philous species grow over a
wide range of base-rich substrata, but some are found mainly on
highly calcareous rocks such as chalk and limestone, e.g. Actaea
spicata, Daphne mezereum, Polemonium caeruleum, Lithospermum
purpurocaeruleum, Atropa belladonna, Cardamine impatiens, Phyteuma
spicatum, Polygonatum odoratum, Cypripedium calceolus, Orchis
purpurea, O. militaris, Ophrys insectifera, Hetteborus viridis, H.
foetidus and Cephalanthera rubra. The northern calcicoles are often
associated with limestone pavement and some species are able to grow
in the deep clefts or ' grikes' even where there are no trees. Some
woodland species have a preference for heavy, moist and base-rich
clay soils, e.g. Equisetum telmateia, Phyllitis scolopendrium,
Polystichum setiferum, P. aculeatum, Ranunculus repens, Stellaria
nemorum, Primula elatior, Glechoma hederacea, Ajuga reptans,
Eupatorium cannabinum, Allium ursinum, Carex sylvatica and C.
pendula. Other species need soils rich in humus, either in the mull
type with high base-status (e.g. Crepis paludosa, Impatiens noli-
tangere, Cardamine amara) or with a mor surface horizon with low
base- status (e.g. Goodyera repens, Trientalis europaea,
Pyrola minor). A few woodland species have an alternative habitat in
calcareous dune slacks, e.g. Monotropa hypopitys, Corallo-rhiza
trifida, Pyrola rotundifolia, Epipactis leptochila and E.
phyllanthes.
There is a predominance
of base-rich soils in the south and base-
poor soils in the north, and this trend is paralleled by the
tendency for southern species to be basiphilous and northern species
acidophilous or indifferent to base-status. The number of woodland
species is greater in the southern half of England than in the north
(Table 5). This may be an expression of the greater extent of base-
rich soils (with their richer flora) in the south, but it may also
reflect the originally greater total area of woodland in this
region. Species associated with rock habitats in woodland are absent
from East Anglia and much of south-eastern England, as these
habitats are missing (except in the Weald).
Another group (column 11
of Table 5) consists of species which show
a wide distribution in Britain, except in the lowlands of central
and eastern England. With some of these the scarcity or absence
extends to the Welsh Borders, Gloucestershire, Hertfordshire, East
Anglia and the East Riding of Yorkshire (cf. distribution maps of
Vacdnium myrtillus and Erica tetralix). Most of these species are
calcifuges and their scarcity matches the very limited occurrence of
acidic soils in this part of Britain. They are not northern species,
for they appear again in abundance on areas with a prevalence of
base- deficient soils in the extreme south of England. Many species
are necessarily scarce or absent in the Fenlands simply because
there is so little woodland of any kind in this district.
Many species are widespread
in Britain except in the north of
Scotland (Table 5, column 10) but their scarcity or absence in this
region is probably due to more than one factor. First, there is a
general scarcity of woodland, and much of the far north is now
virtually treeless moorland and mountain, so that extent of suitable
habitat is limiting. In addition, there is the general decrease in
floristic richness with distance north in British woodlands, and
some of the species concerned may be at or near their northern
limits in the Highlands.
Judged by British distribution,
there are only eight markedly
oceanic vascular species amongst the list in Table 5, and six of
these are ferns. This is rather surprising in view of the large
number of Atlantic bryophytes associated with British woodlands, but
probably many of the vascular species with pronounced southern
tendencies are oceanic in the sense of being thermophilous. J. R.
Matthews (1937) lists 10 of the species (in Table 5) other than
ferns in his oceanic southern and oceanic west European elements.
Physospermum cornubiense is the only Mediterranean woodland species
in Britain. Most of the 22 species which belong to the southern half
of England fall within Matthews' continental, continental southern,
oceanic southern and oceanic west European elements.
Many widespread and northern
woodland herbs grow on basic cliff
ledges up to 910-1070 m in the Scottish mountains, far above even
the potential tree limit. In part, these herbaceous communities
represent an upward extension of the field layer once characteristic
of submontane woods on the better soils but now largely eradicated
or severely modified by the heavy grazing which has affected all but
a few of these hill woodlands. Communities of this type are well
developed in the subalpine birchwoods of Scandinavia. The species
commonly found in the tall-herb ledge communities include Cirsium
heterophyllum, Trollius europaeus, Geranium sylvaticum, Angelica
sylvestris, Crepis paludosa, Silene dioica, Valeriana officinalis,
Succisa pratensis, Filipendula ulmaria, Rubus saxatilis, Geum
rivale, Luzula sylvatica and Des-champsia cespitosa. Similar
communities are also represented in ungrazed hay meadows in northern
England and Scotland, and are dealt with in Chapter 6.
The acidophilous northern
species are best represented in the
pinewoods of the eastern Highlands, though some of the species
concerned grow more widely in both pine and birch woods. A few
species, notably Actaea spicata, Ribes alpinum and Cypripedium
calceolus are in Britain confined to northern England, and Impatiens
noli-tangere is found also in north Wales.
The 23 rare woodland species
include one, Euphorbia pilosa, which is
evidently extinct in Britain. Of the other 22, 12 are plants of
basic soils in the southern half of England, and the remainder vary
widely in distribution and soil requirements. Some rare woodland
species may have become relict through destruction of the habitat
and loss of many former localities, but probably others are rare
because of an inability to spread from their few existing colonies,
which became established largely through chance during favourable
conditions.
Matthews' classification
of geographical elements in the British
flora includes only 68 flowering plants of the 236 woodland species
in Table 5, the others being unclassified (presumably they are
either widespread or irregularly distributed in Europe). Seven
species of fern have also been classified using Matthews' system.
The numbers of species in the different elements are as follows (the
additional figures in parentheses are for tall shrubs and trees):
Total number of classified
species 75 (15)
Mediterranean Oceanic southern
Oceanic west European Continental
southern Continental Continental northern Northern montane Arctic-
alpine
i
7
10
IS
12
22
7
I
(o) (i) (i) (3) (6) (4)
(o) (o)
Some taxonomic groups are
well represented in woodlands, especially
ferns (19 species), and the families Ranun-culaceae (10 species),
Rosaceae (12 species), Scrophulari-aceae (8 species), Liliaceae (10
species), Orchidaceae (20 species), Cyperaceae (9 species) and
Gramineae (26 species). On the other hand, the Cruciferae,
Caryophyllaceae, Chenopodiaceae, Papilionaceae, Umbelliferae and
Compo-sitae are poorly represented in proportion to their size.
Notes
Categories Amplitude
1 = exclusively or mainly
in woodland.
2 = wider ecological range,
but often in woodland. Acidic: soil pH <
4.8, <3o mg exchangeable calcium/ioo g. Medium base: soil pH 4.8-
6.0, 30-300 mg exchangeable calcium/ioo g. Calcareous: soil pH 6.0,
300 mg exchangeable calcium/ioo g. Moist—wet: ~]
Shade loving: y
Subjectively assessed. Brackets indicate sometimes
found under these conditions. Especially rock habitats: J
Widespread common: throughout
Britain, in nearly all suitable
habitats.
Widespread local: widely
distributed in Britain but absent from some
areas and suitable habitats. Widespread except in northern Scotland:
discussed in text. ' Widespread except in east-central England:
discussed in text.
Scattered very local: widely
but discontinuously distributed, with
many absences from suitable habitats. Oceanic: mainly in southern
and western coastal districts. England and Wales: southern tendency
in distribution but reaching to northern England and sometimes
Scotland (very
sparse and mainly in south).
Southern: confined to England
south of grid northing 3/ooo. \
Northern: mainly or entirely in northern England and Scotland. Rare
species: recorded in only 1-15 lo-km squares since 1960. European
distribution: M = Mediterranean; OW = Oceanic west European; C =
Continental; NM = Northern montane;
OS = Oceanic southern;
CS = Continental southern; CN = Continental
northern; AA = Arctic-alpine. |
|
Fauna
MAMMALS, REPTILES AND AMPHIBIANS
INSECTS
ODONATA
SPIDERS |
|
Many of the British mammals
are associated with woodland in some
degree, but few of our species are exclusively forest dwellers. All
the British deer frequent woodland, although the native herds of red
deer in England and Scotland have become adapted to treeless upland
country and are mostly to be found! within woodland only when the
weather is severe. The introduced or escaped red deer of the English
lowlands have, however, resumed the role of forest animals and in
areas such as the Breckland they are seldom seen outside the woods.
The roe deer Capreolus capreolus is more particularly a woodland
species, favouring especially open areas, rides and wood margins,
and has shown a general increase and expansion of range during
recent decades, coincident with extensive re-afforestation of many
parts of Britain. After suffering great restriction of range during
the main period of forest clearance, it has now become a widespread
species again in England and Scotland, but not Wales. Roe deer range
over more open country around their forest haunts and often feed on
grasslands, heaths and moorlands.
Feral herds of the introduced
sika deer Cervus nippon are widespread
but very local in England and Scotland, and live mainly in woodland,
both broad-leaved and coniferous, favouring those where there is
dense undergrowth. Fallow deer Dama dama are widespread but local
and are either descended from possibly native stock (as in the
ancient forests of Epping, Rockingham, Cannock Chase and the New
Forest) or represent feral populations founded on escapes from deer
parks. They frequent woodlands, mainly broad-leaved or mixed and
preferably with thick shrub and field layer, and feed at night
around the edges or on adjoining fields. The Chinese muntjac
Muntiacus reeved is another introduced species which is now
widespread as a feral animal in the southern half of England, where,
however, severe winters depress numbers periodically. Muntjac
inhabit woods with dense shrub and field layers, and are relatively
elusive animals whose presence may go undetected.
Deer can cause considerable
damage in woodlands by nibbling or
uprooting seedlings and rubbing or stripping the bark from saplings
and young trees. Newly afforested ground may have to be enclosed
with deer-proof fencing and efforts made to control the population
in older forests by selective shooting. Uncontrolled heavy grazing
by deer may also restrict or prevent natural regeneration of
woodland. Locally, deer also cause damage on arable crops on
farmland. Their activities may nevertheless have value to wildlife
in checking the growth of some vigorous, competitive plants and in
helping to prevent complete continuity of tree or shrub cover.
Carnivorous mammals are
well represented in woodland. Badgers Meles
meles are perhaps the most typical forest dwellers among the British
species, and the majority of breeding places (setts) are in woods or
copses, from which the animals forage over the surrounding
countryside. The species is omnivorous and able to subsist in a wide
variety of habitats; it is widespread in the British Isles and in
the north often lives in treeless country. In some districts the
badger is persecuted a good deal, usually needlessly, but the
Forestry Commission encourage this animal and provide 'badger gates'
in their rabbit-proof fences. The fox includes woodland among its
haunts, and in the lowlands many 'earths' are inside woods, though
these are simply refuges from which the animals range and feed over
a wide area of surrounding country. This predator is ubiquitous in
mainland Britain but absent from the Scottish islands except Skye.
The strongly carnivorous habits of the fox bring it into general
disrepute, but despite control measures, its numbers seem to be
maintained remarkably well in most districts.
The wild cat Felis silvestris
is partly a forest dweller within its
range in the Scottish Highlands. Dens are in both broad-leaved and
coniferous woods, but especially mountain woods where block litters
provide safe refuges and breeding places. Wild cats have recently
spread into coniferous plantations in the Campsie Fells in
Stirlingshire, and the species seems to be increasing locally or
holding its own elsewhere, despite local persecution by gamekeepers.
The pine marten Martes martes is another predator of mountain woods
in north Wales, Lakeland and the Highlands (more especially the
north- west) but it can also live and breed in
treeless
country. Dens are usually deep in block litters or other holes, but
occasionally disused tree nests of the larger birds are used. The
pine marten is now rare outside the western Highlands, but does not
appear to be much molested nowadays. It is an elusive and largely
nocturnal animal and may easily escape notice.
The polecat was formerly
widespread, but persecution reduced its
range; some survivors hung on mainly in secluded parts of central
Wales (a situation closely parallel to that of the red kite).
Favoured situations for dens are in block litters, tree roots or
rabbit holes within woods, including plantations. The polecat is
increasing in Wales and quite numerous in some districts, but it is
still destroyed where there is much rearing of poultry or preserving
of game. The smaller mustelids, the stoat and weasel, both have a
niche in woodlands where they feed mainly on small rodents or birds
and their eggs and young. These are widespread species in Britain
except in the Scottish islands, though a different subspecies of
stoat Mustela erminea ricinae is recognised on Islay and Jura.
The wood mouse Apodemus
sylvaticus is the most characteristic small
rodent of woodland and lives in runs in and below the leaf litter.
It is common over much of Britain, including many of the Scottish
islands. The yellow-necked mouse A.flavicollis is another woodland
species widespread in England and Wales, but apparently much less
numerous than A. sylvaticus. The bank vole Clethrionomys glareolus
is most abundant in woodland and scrub, and is widespread though
absent from most of the Scottish islands. The short-tailed field
vole occurs less commonly in woods than in more open habitats, but
it is sometimes plentiful in clearings within woods or uncultivated
rough ground around woodland edges. It often builds up to very high
densities in recently afforested ground which has been fenced
against sheep and deer and thus develops a rank growth of grasses,
other herbs and dwarf shrubs, before the trees are tall enough to
suppress this. The dormouse Muscardinus avellanarius is a local
species found mainly in the south of England, where it is less
numerous than formerly. It prefers dense woodland and scrub with an
abundance of trees which produce edible seeds, e.g. beech, hazel,
sweet chestnut.
The small rodents of woodland
form an important food source for
certain predators, including most of the mammalian species already
mentioned, and certain birds, notably the owls. While the
characteristic woodland rodents do not show the violent population
fluctuations typical of Microtus, there are some changes in numbers
according to season and year, and these may affect the breeding
output of predators such as the tawny owl. These small animals may
seriously reduce or prevent natural regeneration of native trees by
eating the seeds and seedlings, and bank voles sometimes 'bark'
small trees.
The mole is a widespread
and common woodland dweller, although in
upland districts it is found mainly in woods on the more base-rich
soils, except where these are too shallow for burrowing. The common
shrew Sorex araneus is also widespread in woodland and scrub (but
absent from some of the Scottish islands): the pygmy shrew S. minutus
and water shrew Neomys fodiens
are, however, less numerous in these habitats, although they are equally widespread. In fact, the pygmy
shrew,
along with the wood mouse, is the most widely distributed mammal in Britain, and occurs in most of the
larger
Scottish islands. These four small mammals also form the prey of various woodland predators, notably
the tawny
owl.
The
hedgehog occurs in woods over most of the British mainland, and on some of the Scottish islands where
it has
probably been introduced. It tends to avoid really dense wood and is most numerous in open woodland
and scrub,
though the nests are usually well hidden in undergrowth or burrows. The rabbit and brown hare include
woodland
among their range of habitats, though the rabbit is much sparser almost everywhere since myxomatosis
appeared
in 1954. Both species sometimes do considerable damage in young plantations by eating seedlings and
'barking'
saplings. Woodland predators such as the buzzard, goshawk, fox, badger, stoat and weasel take both rabbits
and
brown hares (more especially the young) as prey.
Of
all the British mammals, the red squirrel Sciurus vul-garis has the strongest association with
woodland, and is
more or less confined to this habitat. This animal has generally declined in numbers and retreated northwards
during recent decades. It is locally plentiful in East Anglia, but is otherwise found mainly in northern
and western
Britain. The favourite habitat is coniferous woodland, but it also occurs widely in mixed and broad-leaved
woodland. The dreys are usually placed high in trees or occasionally in tall hedges bounding a wood.
The
introduced grey squirrel S. carolinensis has largely replaced the red squirrel over much of southern
Britain;
sometimes there appears to have been competition but the red squirrel has also declined seriously in
areas where
the other species has not appeared. The grey squirrel is more a species of broad-leaved woodland, where
its dreys
are placed high in the trees and are conspicuous in winter. Both squirrels can do considerable damage
in woodland,
by eating seedlings and young shoots, and by 'barking' trees. The grey species is particularly harmful
in this
respect. They are also predators of birds' eggs and young and so may harm game interests.
Many
species of bat have some association with woodland and roost and hibernate in hollow trees, and hunt
insects over the canopy or along rides and edges. Some species appear to use hollow trees only during
summer.
Where there are no trees with suitable cavities, bats may use woodlands for feeding only. Most British
species use
hollow trees if available. They include the widespread whiskered bat Myotis mystatinus,Daubenton's
bat M.
daubentoni, noctule Nyctalus noctula, pipistrelle Pipistrellus pipistrellus, and long-eared
bat Plecotus auritus; the
more local (mainly England) barbastelle Barbastella barbastellus (also Wales), Natterer's bat Myotis
nattereri (also
Wales), serotine Eptesicus serotinus and Leisler's bat Nyctalus leisleri; and the rare
Bechstein's bat Myotis
bechsteiniof southern England. |
|
Woodland
is not a particularly important habitat for
reptiles
and amphibia. Adders often lie up in the shade of woods during hot weather and may sometimes hibernate
within woodland. This snake, with the grass snake Natrix natrix and smooth snake occur in open
places and rides
in woods and amongst open scrub. Common frogs Rana temporaria and toads Bufo bufo also
frequent the more
open and damper parts of woods and the more open habitats within woodland. |
|
BIRDS
The
breeding birds of woodland have been divided into two groups. The first consists of species which nest
in
closed woodland, and includes those whose major niche is non-arboreal as well as those which are confined
to
woodland. The second group consists of species which show some association with trees or tall shrubs
but not
with closed mature woodland. Data on habitat, distribution and population size for these two groups
are
presented in Tables 6 and 7.
The
total list of woodland birds thus defined is quite long, with 92 species, a larger number than for any
other
major habitat group. Of these, only 35 species have an invariable association with woodland, trees or
tall shrubs
during the breeding season, but 23 other species belong mainly to these habitats. The passerines are
especially well
represented in woodland, and of the British breeding birds of prey, only two (the marsh harrier and
peregrine)
have no association with trees. The data for regional distribution do not indicate relative abundance
within each
region, but when taken in conjunction with the overall abundance class, they indicate which widespread
species are
common (e.g. woodpigeon) and which are local or rare (e.g. crossbill).
The
preferences of birds for broad-leaved or coniferous woods are indicated and it will be clear from this
that the
first type tends to have the larger number of species. It follows that mixed woods containing both types
often
have the richest avifauna. However, much depends on other aspects of diversity, such as woodland structure
and
representation of transitional habitats, or on age of the trees, so that generalisations are often better
avoided. It
may nevertheless be said that maturing uniform conifer forests grown in dense canopy and often lacking
any
subsidiary layer (even bryophytes) are generally notable only for the relative poverty of their breeding
bird fauna.
There
is perhaps only one well-defined ecological-geographical group of woodland birds in Britain, namely
that of
the pine forests of the central Highlands, notably on Speyside. This group consists of the capercaillie,
crossbill,
siskin and crested tit, with a very few pairs of golden eagle, osprey, goshawk and, in 1969, wryneck.
More
widespread species showing a preference for coniferous woods or trees include long- eared owl, collared
dove, coal
tit, redpoll (in Wales and south-west England) and goldcrest. Birds especially associated with broad-leaved
woodland are the three species of woodpecker, marsh tit, willow tit, nuthatch, tree creeper, redstart,
nightingale,
blackcap, garden warbler, willow warbler, chiffchaff, wood warbler, pied flycatcher, hawfinch and greenfinch.
The
nightingale, blackcap, garden warbler, whitethroat, lesser whitethroat, wren, chiffchaff and long-tailed
tit
prefer woods and woodland clearings with a good deal of under growth, especially tangles of bramble,
honeysuckle
or tall herbs, in which they breed. By contrast, the wood warbler, and to a lesser extent the willow
warbler, favour
woodland with a rather bare floor. Other ground nesters such as the woodcock, mallard, capercaillie
and robin, nest
most often where there is moderate cover of low shrubs and herbs. The remaining species are tree nesters
and the
degree of cover in the field layer usually has little influence on these.
Species
which breed within the tall shrub layer (e.g. hazel, blackthorn, hawthorn, holly, rhododendron, willows)
below the main tree canopy include woodpigeon, turtle dove, collared dove, jay, song thrush, blackbird,
hawfinch,
greenfinch, redpoll and chaffinch. When mature woodland is available, many species nest by preference
at a
considerable height above the ground (i.e. more than 9 m) and are often in the canopy; these include
all the
predatory species, raven, carrion crow, hooded crow, rook, magpie, siskin and crossbill. Where only
low trees are
available, however, most of these species are sufficiently plastic in their requirements to breed much
closer to the
ground. Sparrowhawks, crows and magpies frequently nest in birch and willow scrub where taller trees
are absent,
and in the Highlands herons breed in low scrub on islands in lakes. The kestrel, hobby, merlin, tawny
owl and long-
eared owl use the old nests of other species, mainly crows, magpie and squirrels, and are limited by
the choice of
the actual nest builders.
Tree-hole
breeders may be divided into those which utilise natural holes and those which excavate their own. The
former group include goosander, kestrel, tawny owl, barn owl, little owl, stock dove, wryneck, jackdaw,
great tit,
blue tit, marsh tit, redstart, pied flycatcher, starling and tree sparrow, while the excavators are
the three
woodpeckers, crested tit and willow tit. The nuthatch takes over a natural or excavated hole, which
it plasters over
to its own size, and the tree creeper is a semi-hole nester, with its favourite nest site behind partially
detached
bark. The natural-hole nesters mostly need fairly mature timber in which cavities have had time to develop.
Woodpeckers need dead or at least moribund trees, although the green woodpecker can dig holes in trees
which
appear healthy from the outside. The crested tit and willow tit depend on very soft, rotten wood or
stumps in
which to excavate their nests. Trees with holes and dying or dead trees are likely to be removed in
normal forestry
practice but the situation can be retrieved to some extent by the substitution of artificial nest boxes.
In an area of
the Forest of Dean, and in Yarner Wood, Devon, the populations of pied flycatchers have been boosted
to very
high levels by this means. Some tree- hole nesters also breed in holes in rock faces or masonry.
While
the number of species breeding within a wood tends to increase with size of the wood, this is partly
because
diversity also often tends to increase with size. The honey buzzard probably needs large woods but may
be
limited mainly by the abundance of its main food source, wasps' nests. The large raptors often choose
tree sites
with
a
good look-out, and it may be for this reason that species such as the golden eagle and osprey tend to
nest where
trees are rather scattered or clumped and not in dense woodland.
In
eastern Europe where the peregrine nests in trees, it is said often to choose the sharp cut edge to
a tall wood,
where the wall of trees is equivalent to a cliff face. A high proportion of sparrowhawk nests in large
woods are
placed close to the wood edge, a ride or some other break in continuity of the tree cover. This species
is, however,
one of the very few tree nesters which needs a wood as such and seldom nests in rows of trees or scattered
trees.
Of the birds classed as woodland species, some (such as the carrion crow, magpie, mistle thrush and
crossbill) are
locally found nesting more numerously in small clumps or rows of trees and in isolated trees (especially
in
hedgerows) than in large woods. The heron, raven, kestrel and hobby sometimes favour small woods and
copses as
nesting places, but are equally at home in large woods. The rook establishes its colonies in woods of
all sizes or in
groups of trees around houses, but is less inclined than some of its relatives to nest in low trees.
Among
the species listed in Table 7 are those which have a decided preference for scattered trees, e.g. stock
dove,
barn owl, little owl, tree sparrow and merlin (insofar as this species nests in trees at all); these
may nest also along
a woodland edge, but seldom far inside a wood. A few birds
o '
from
both groups often nest in orchards (which represent a rather open kind of woodland), e.g. magpie, jay,
chaffinch, hawfinch, goldfinch and bullfinch, and, when old trees are present, various hole nesters
such as the
woodpeckers, wryneck and tits. Probably the majority of species which breed in hedges were originally
scrubland
and/or woodland breeders. The red-backed shrike, linnet and cirl bunting are birds of scrubland only,
the first
species in fairly tall scrub and the others in low-medium scrub.
Several
birds belong to the ephemeral transitional stage between the clearance of a wood and its regeneration,
or
between the afforestation of unwooded ground and the formation of a more or less closed tree growth.
The
nightjar, which favours the barest ground, is the first to appear and disappear as the trees grow up.
When the field
layer becomes dense, often within a year or two of tree planting and fencing against stock (and sometimes
rabbits),
the pheasant, grasshopper warbler, whinchat, yellow-hammer and (more locally) stonechat often appear,
along
with birds such as the mallard, whitethroat and willow warbler. In the north, young conifer plantations
often
develop high densities of field voles and this usually draws short-eared owls to breed. These young
plantations
have been the refuge of the hen harrier, probably contributing significantly to the post World War II
recovery and
spread of this raptor. Black game also flourish in this habitat, and have recovered their numbers in
many areas
where they had declined. In the south, the Montagu's harrier has also colonised young conifer plantations
in places.
These
young conifer plantations support high densities of breeding birds, and for two species at least, the
grasshopper warbler and Montagu's harrier, they probably represent the main habitat in Britain
at the present time. For all the species named, except the black
grouse which feeds on conifer shoots, the trees are incidental, and
it is the luxuriant grass or low-shrub dominated vegetation which is
the attraction, either in its food supply or cover for nesting. Some
of the passerines use the trees as song or display flight posts, and
two other species, the tree pipit and woodlark, have this particular
association with trees and bushes in their grassland and heathland
nesting haunts.
Careful management of woods
can obtain and maintain a generally rich
avifauna, certain species individually or a group of species.
Richness of species, however, also depends on geographical position
and despite the special interest of the Highland pinewood birds,
there is a distinct tendency for the number of woodland species to
decrease with distance north. The New Forest is probably the most
important woodland area for birds in Britain, in variety of species
and size of their populations - a reflection of the diversity of
habitat and large extent of this woodland. At least 75 of the
species in Tables 6 and 7 breed in this district. By contrast, at
the opposite end of Britain, the birchwoods of west Sutherland can
together muster a total of only about 15 species.
Of the rare woodland birds,
the red kite is a true relict species,
confined to a limited area of central Wales, where it nests mainly
in hanging oakwoods but hunts a great deal over open hillsides,
moorlands and valley pastures. The rest may be regarded as fringe
species which only just reach Britain from their European centres of
distribution; this is true of the goshawk, honey buzzard, osprey,
hobby, wryneck, redwing and firecrest. Four other sporadic woodland
breeders in Britain, the hoopoe Upupa epops, golden oriole Oriolus
oriolus, fieldfare Turdus pilaris and brambling Fringilla
montifringilla, are also very much fringe species here. Conditions
in this country may be marginal for some species such as the hobby,
but some fringe birds may be in
The symbol X denotes strong
representation of a species, according
to both habitat and regional distribution; + denotes scanty
representation in these respects.
Column 4 includes orchards,
park woodland, hedgerow and field trees
and dead/dying trees.
Column 17, abundance class,
uses the notation for Table 6.
the process of increasing
and spreading. The appearance of the
redwing has been associated with the recent trend towards colder
springs, and the return of the osprey evidently reflects an overflow
of population from southern Fennoscandia. Honey buzzard, red kite
and goshawk were all formerly more widespread in Britain and may
well be limited by persecution. This is clearly the reason for the
common buzzard's absence from woods in eastern England. On the other
hand, the sparrowhawk population withstood a great deal of
destruction by gamekeepers, and this was an almost ubiquitous
woodland species until the organo-chlorine pesticides caused its
disappearance from much of southern and eastern England.
During autumn and winter,
the bird fauna of woodland becomes
impoverished by the loss of summer visitors, and these migrants
contain a high proportion of insectivorous species. The numbers of a
few species, such as the woodcock, are locally boosted by
immigration, and flocks of starlings (which often roost in woodland)
may reach enormous size. Many of the residents show a tendency to
flock together for feeding, and roving bands of the smaller
passerines such as finches, tits and thrushes are characteristic of
woodlands in winter. Some of the corvids, especially rooks, jackdaws
and carrion crows, roost communally in woodland during winter, and
wood pigeons are usually found in flocks both for feeding and
roosting. Many of the remaining species retain a normal dispersion
in pairs, though in some the pairs break up and individuals lead a
more or less solitary existence.
The woods which attract
birds in autumn and winter are those with
the most prolific food supply. For the vegetarians this means trees
and shrubs which produce edible fruits and seeds (especially
berries), e.g. oak, beech, alder, hazel, Scots pine, rowan,
hawthorn, blackthorn, holly, yew, elder, cherries, crab apple,
spindle, juniper, guelder rose, briars, brambles, currants and
bilberry. The most attractive woodlands to birds at this time of
year are thus those with a well-developed layer of underscrub, or
edges, rides and clearings with an abundance of these shrubs; the
least attractive are heavily grazed woods in which shrubs are poorly
represented or absent. In good seed years, however,
even oak and beech woods
with few shrubs become attractive to some
species. It is interesting that the jay, which hides acorns in the
ground, may be instrumental in promoting the regeneration of oak,
sometimes at some distance from the parent trees. Crossbills follow
the coniferous woods almost exclusively, and their numbers are
influenced by the cone crop.
The insectivorous woodland
birds, such as the tits and woodpeckers,
are less dependent on a shrub layer, but need trees which provide a
plentiful supply of insects, such as old trees with rough bark and
dying or dead timber of all kinds. The predators mostly remain in
their woodland haunts throughout the year, though some, such as the
sparrowhawk and kestrel, do much of their hunting outside woods. |
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LEPIDOPTERA
Although not all directly
associated with tree species, more
lepidoptera occur in woodland than in any other major habitat.
Probably more than half the British species are to be found in
woodland. The trees themselves support a large proportion of these;
for example, more than 100 species feed on oak, sallow, birch and
hawthorn. Because of this abundance, the selection of species
included in Table 8 is somewhat arbitrary, although all the woodland
butterflies have been included.
Whilst many species are
widely distributed geographically within a
given woodland type, some are much more restricted in range. The
butterflies with extremely restricted ranges include the heath
fritillary, one of our rarest species, which is confined to a few
woods in the extreme south-east and south-west of England; the black
hairstreak - associated with old, large, blackthorn thickets - which
is restricted to woods on the boulder clay in Huntingdonshire,
Lincolnshire, Northamptonshire, Buckinghamshire and Oxfordshire ;
and the chequered skipper, which is exceedingly local, only
occurring in a few woods in the east Midlands and the west
Highlands. It is possible that two distinct subspecies are involved
in this bicentric distribution of the chequered skipper. The purple
emperor, a butterfly of oak and beech woods (the larva feeds on
sallow), is probably not as rare as is supposed though it has
declined during this century.
The two most widely distributed
woodland butterflies are probably
the purple hairstreak, an oak-feeding species, and the green-veined
white, which is a common species in all woodlands although it occurs
in most other habitat groups.
A post World War II feature
of the woodlands in eastern England has
been the decline in the Fzo/a-feeding fritil-laries. As the food
plants have not shown a comparable decline, the decrease in these
butterflies is at present unaccountable. There is some evidence that
the fritillaries are recovering slowly in Yorkshire and at Monks
Wood, Huntingdonshire.
Of the moths, some are
also exceedingly local, e.g. the Kentish
glory, a birch-feeding species only occurring in Wyre Forest in
England and in apparently declining numbers in the eastern
Highlands; the scarce hook-tip, with its larva feeding on the small-
leaved lime and confined to the Forest of Dean area; and the netted
carpet, restricted to a few places in the Lake District and north
Wales where its very local larval food plant Impatiens noli-tangere
occurs in quantity. This plant has special habitat requirements and
needs moist, base-rich soils with deep shade and high rainfall,
while the pupal stage of the moth itself appears to need flushed
soils and a heavy rainfall as a protection against desiccation. Some
species such as the Rannoch sprawler and the Saxon are Arctic-
alpine, being only found in the extreme north of Britain.
Widely distributed moths
include the poplar hawk, common lutestring,
and mottled umber which occur throughout Britain in broad-leaved
woodlands; and the pine beauty, pine carpet and bordered white are
equally widely distributed in coniferous woodlands containing Pinus
spp.
Generally speaking the
lepidopterous fauna of the deciduous
woodlands is much richer and quite distinct from that of coniferous
woodlands. A few species, however, such as the black arches, the
green-veined white and the pearl-bordered fritillary, occur in both
where good rides exist.
The richness of a woodland
lepidopterous fauna depends greatly on
the structural and floristic diversity of this ecosystem, and thus
on the management practices involved. Closed canopy woodland is much
less rich in species than open woodland. Many species, especially of
butterfly, are associated mainly with the transient, serai types of
vegetation which form part of a woodland complex, i.e. the rides,
edges, clearings and recent coppice, because the larval food plant
of some species flourishes better under these more open conditions
(e.g. Viola spp. for fritillaries), and shrubs and herbs with
entomophilous flowers visited by the adults are similarly most
abundant. Modern land-use methods accentuate the sharp boundaries
between woodland and arable land in the agricultural lowlands.
Butterflies and moths are most numerous in species and numbers where
the woodland edge has a graded series of habitats, with tall shrubs,
a rich field layer and an outer marginal strip of grassland with
abundant dicotyledons. Woodland which changes suddenly to arable
crop land is thus less valuable, but the more diverse conditions can
be provided within the wood, e.g. along broad rides and on recently
cleared or coppiced ground. Scrub, with its flora and fauna, is
discussed more fully in Chapter 6, but it may be noted here that
careful management is needed to maintain the entomological interest
of scrub and other open habitats associated with woodland.
Well-developed hedgerows
with herbaceous verges and scattered trees
maintain some woodland butterflies and moths, but this habitat is
being reduced at a considerable rate in the southern and eastern
counties of England. The conservation of the right type of woodland
in the right condition therefore becomes even more imperative for
the maintenance of entomological interest. The extensive replacement
of broad-leaved woodland by coniferous plantations has a
considerable influence on the insect fauna and some woodland
Lepidoptera are declining in consequence. However, if the conifer
forests contain broad rides and do not develop too dense a canopy,
many lepidotera can survive, and the planting of amenity fringes of
broad-leaved trees and shrubs is also beneficial.
Certain woodland moths,
whose larvae feed on the leaves of trees,
can cause severe defoliation. Such moths include the oak roller moth
(on oak), the mottled umber moth and the winter moth (on oak and
other broad-leaved species), the northern winter moth (on birch) and
the bordered white or pine looper (on pine). |
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Except where they contain
aquatic habitats, such as streams, ponds
and mires, woodlands are not the breeding place of dragonflies.
Nevertheless, woodlands are much frequented by some species, both as
feeding and resting places. The edges and rides are most favoured,
and on warm, sunny days large species such as Brachytron pratense,
Aeshna cyanea and A.juncea hawk up and down these situations and
rest at times on the trees. In the south and east of England the
migratory A. mixta is probably more common along woodland edges and
rides than in any other habitat, and in the Highlands, the northern
A. caerulea tends to frequent the open pine and birch woodland
habitats adjoining its peatland breeding haunts. Among the smaller
species, the darter dragonflies of the genus Sympetrum are also
often associated with woodland edges. Where woods are close to their
breeding places, most of the dragonfly species will be found at some
time to resort to the trees, if only for resting, but there seems to
be no species in Britain which is unable to exist without trees. |
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Table 9 lists 59 species
which are found exclusively in woodland or
nearly so, and a further 46 species which are not so rigidly
confined to woodland but are more plentiful there than in any other
habitat. Many other species are also common in woodlands but may
also occur abundantly elsewhere. Owing to the large numbers of
species, and the fact that the spiders of woodlands have not been
widely studied, it is not possible to describe them in so much
detail as some other groups.
Woodland
spiders fall into two broad categories, those which live in the litter layer and those which live on
the
undergrowth or on the trees. These categories overlap to some extent as some species which live high
up on trees
in the summer move down to the litter in winter. Some species are also specialised for living on or
under bark or
living on dead trees and these are also indicated in the table. The most widespread and characteristic
bark species
are Clubiona brevipes, Moebilia penicillata, Syedrula innotabilis, Labulla thoradca, Drapetisca socialis,
Clubiona
corticalis, Theridion mystaceum and Araneus umbraticus. Notably rare bark species are Tetrilus
macrophthalmus,
Tuberta maerens, which is known only from Bloxworth and Wytham, Dipoena torva,which is mentioned
in
Chapter 9, and Zygiella stroemi, known only from bark at the Black Wood of Rannoch and at Wytham,
and in a
hut at Leckford in Hampshire. Most species are found mainly in deciduous woodland but some are confined
to, or
show a marked preference for, coniferous woodland (usually pine). We cannot be sure that any species
is confined
to a particular type of woodland, but some species show a distinct preference for a certain type. Hyptiotes
paradoxus is apparently confined to yew and box, and is known only from Box Hill, Bagley Wood, and
several
sites in the New Forest and elsewhere in Hampshire. Similarly, Hybocoptus decollatus is only
known from yew in
a few woods, including Box Hill and Kingley Vale, but has been found also on gorse in two sites on the
south
coast. Other more widespread species which show a preference for yew or box are Diaea dorsata
and Theridion
tinctum.
More
species are characteristic of pinewoods. These include some found only in upland woods, namely H.
sorenseni, C. subsultans, D. torva, R. scoticus, D. elevatus, P. elongata and L. expunctus
(all also described in
Chapter 9) which may be the only species strictly confined to pine-woods. The most widespread and abundant
pinewood species are Moebilia penicillata, Scotina celans, Hahnia helveola, Walckenaera cucullata
and Araneus
sturmi. With the possible exception of the first named, all of these also occur less commonly in
other habitats.
Other characteristic pinewood species of more restricted range are Philodromus collinus, known
only from a few
sites in East Anglia and at Box Hill, P. emarginatus, P. margaritatus, Salticus zebraneus, Cyclosa
conica and also
Araneus displicatus known only from two sites in Surrey.
Among
the predominantly deciduous woodland species, two small subgroups may be recognised - those which
occur mainly in association with oak or beech. The most characteristic oakwood species are Clubiona
brevipes,
Micrommata virescens, Xysticus lanio, Ballus depressus and Anelosimus vittatus, and species
found mainly in
beech- woods are Araneus bituberculatus, which is known only from Burnham Beeches and one old
record from
Essex, Centro-merus albidus,recorded from three sites including Box Hill, and C. cavernarum.
The only species
which is characteristic of birch is Araneus marmoreus, which occurs in a few sites in eastern
and northern
England, including Skipwith Common and Woodwalton Fen. Helophora insignis and Linyphia
Criteria
for key site assessment and selection 101
hortensis
are unusual in that they occur commonly in a wide variety of deciduous woodland, but nearly always in
the field layer on Mercurialis perennis.
Among
the most widespread and abundant species of deciduous woodland in general may be mentioned Clubiona
compta, Anyphaena accentuata, Pardosa lugubris, Theridion pollens, Gonatium rubellum, Microneta viaria,
Linyphia peltata, Clubiona pallidula, Philodromus dispar, Araneus cucurbitinus, Erigonidium graminicola,
Hypomma cornutum, Diplocephalus latifrons and Linyphia montana.
Some
species appear to be almost equally common in both deciduous and coniferous woodland. These include
Tapino-cyba pattens (only in the north), Syedrula innotabilis, Drapetisca socialis, Theridion
mystaceum, Araneus
umbraticus, Minyriolus pusillus, Monocephalus fuscipes, Centromerus dilutus, Macrargus rufus, Lepthyphantes
minutus, L. alacris, L. flavipes and L. tenebricola.
Rarities
not already mentioned are Micaria subopaca, known only from a few sites in Surrey and Sussex,
including
Ashdown Forest, Achaearanea simulans,recorded from three areas including Box Hill and the New
Forest,
Araneus inconspicuus and A. alpicus(records again include the New Forest and Box Hill), Trematocephalus
cristatus, known from four sites in Surrey and Sussex including Box Hill, Centromerus capucinus,
which has
occurred in a few scattered localities including beechwoods in the Chilterns, Lepthyphantes earn,
which has been
recorded only from Sherwood Forest and Windsor Forest, and Walckenaera mitrata,which has recently
been
recorded for the first time in this country from Blean Wood.
Many
of these woodland species occur also on or under isolated trees or clumps of trees or bushes and in
hedgerows, and such habitats must be important for their conservation. Some species, especially those
which are
found mainly in the field layer or on low bushes, require fairly open conditions such as occur in rides
or at the
margins of woods. An example is Micrommata virescens, a local species which occurs on young oak
trees or on
Molinia tussocks below young oaks in damp areas. |
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