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3. British woodlands: 1977
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3.3 Site selection
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Woodlands
are in some ways the most difficult of major ecosystems to deal with in the field of conservation.
Although representing the climatic climax over much of Britain, woodland is now highly fragmented and
woodlands of native species cover only a small fraction of the total land surface. Moreover, if any
natural
woodland remains, it is of very small extent. There has, nevertheless, been a great deal of concern
about the
structure and floristic composition of natural woodland, and many conservationists believe that this
is an ideal
condition which management should aim to restore on at least some of the important woodland sites. Thus
sites
which show the closest approach to thepresumptive
original state tend to be the
most highly valued.
Woods have a dual nature;
they are assemblages of trees with value
and interest in their own right, and they are also the habitat of
other organisms, including plants of quite different life-form and
animals of many kinds. The con-servationist is interested in both
aspects of woodland, often on the same site, but sometimes
separately. Given sufficient time, the tree component of the
woodland ecosystem can be manipulated by management into almost any
desired state, but the dependent organisms are less easily
controlled at will. A desired tree structure and composition could
eventually be obtained by planting an open field with appropriate
species, but development de novo of the mature woodland with all its
associated flora and fauna is likely to take centuries rather than
decades, and is to be regarded more as a very long-term possibility.
Trees have a limited life
span and unless there is adequate
regeneration a wood will eventually die out. Woodland trees are
seldom allowed to die of old age, and are usually cut for timber
when mature. Coppicing of younger trees may prolong the life of
individuals for a very long time, but usually there comes a point in
the cycle when tree cover must be perpetuated by the growth of new
individuals. In Britain, this regeneration is usually achieved by
the planting of seedlings grown in nurseries. On nature reserves,
natural regeneration is usually prescribed as a more desirable
policy. There has thus been a tendency to rate highly those woods
which show good natural regeneration and an uneven age structure of
the dominant trees.
Areas where there has been
continuity of woodland cover right
through the Post-glacial Period are usually richer in dependent
communities and organisms than sites where tree cover has been
discontinuous during the last few centuries. The persistence of
sensitive associates dependent on tree cover may well reflect a
rotational cropping and regeneration within one woodland block,
which has ensured that parent populations of the sensitive species
were always at hand to supply new offspring to the regenerating
forest. When species have been entirely lost from a site during a
period of woodland clearance, their capacity for recolonisa- tion of
re- established forest clearly depends on their powers of spread and
establishment. Mobile creatures such as birds, mammals and some
insects most readily recolonise reestablished woodland, but this
ability varies from species to species. The crested tit is a British
forest bird showing a very limited capacity for spread from its
established Scottish haunts. Many insects, although theoretically
mobile, appear to be remarkably sedentary in distribution over a
period. Common plants are able to spread easily, and this is perhaps
especially true in general of ferns, bryophytes, lichens and fungi
with small, wind-dispersed spores. On the other hand, the rarer
members of the flora (including these cryptogams) are usually
characterised by an extremely limited capacity for spread, and once
they have disappeared from a woodland site, perhaps during the
unfavourable conditions of a clearance period, they have gone
virtually forever unless they are deliberately reintroduced, and
this may be difficult.
Because of their almost
complete inability to spread and recover
lost ground under the present, fragmented state of our woodlands,
many vascular plants, ferns, bryophytes, lichens, fungi, insects,
arachnids and molluscs are consistently absent from re-established
woodland and are therefore reliable indicators of ancient, permanent
woodland.
In general, therefore,
re-established woodlands have a lower
conservation value than permanent woodlands, in terms of richness of
components other than the tree layer, as well as in the historical
features discussed earlier. In both types, the tree layer itself may
show all degrees of change in composition or reduction in diversity
according to the conditions of management.
Woodlands have a great
diversity of interest, so that the criteria
of evaluation in assessment for conservation are necessarily
numerous. They have the most complex organisation of any ecosystem
and are the most luxuriant of all living communities; as such they
have been a favourite focus for studies of the circulation of
nutrients and energy, and of biological productivity. The economic
values of trees cannot be divorced from nature conservation and the
tree component of native woodland is commercially valuable, provides
opportunities for silvicultural research and is an important source
of genetic material. Woodlands are also important in demonstrating
ecological relationships, including the response of vegetation to
climatic change, the effects of spatial differences in soil
conditions, plant succession, and the influence of man. Trees have a
key position in the understanding of vegetational history, through
pollen analysis, subfossil wood remains, dendrochronology, and the
long-lasting effects of management practices inscribed in the living
trees. Woodlands and trees are important features of the landscape
and are of major recreational and educational value.
The nature conservation
value of woods composed partly or wholly of
alien species is a difficult subject. The present review is
concerned essentially with assessing their value in relation to the
selection of a series of nationally important woodland sites. The
boundary between indigenous and non-indigenous species is not always
clearly defined but, in general, it is accepted that alien trees
have a lower conservation value than native species, both in
themselves and with particular reference to invertebrates, as
habitats for dependent organisms. However, well-established and
freely regenerating aliens such as sycamore are of considerable
ecological interest, and introduced poplars are important food
plants for a number of interesting moths. Scots pine, outside its
native range, is a common component of many woods dominated by
deciduous trees, and is, for example, often preferred as a nest site
by sparrowhawks. The pine rows and clumps are an artificial but
characteristic feature of the East Anglian Breckland and are here
largely responsible for the occurrence of the interesting population
of breeding crossbills. The feeling that the purity and therefore
quality of a native woodland is in some way diminished by the
presence of alien trees cannot always be substantiated in reason.
The presence of a certain proportion of non-native trees can add to
the interest of a woodland, and may
Criteria for key site assessment
and selection enhance the faunal
diversity. The undesirable state is reached when alien trees cover
such a large part of a wood that it cannot be regarded as a good
example of the native type, or when vigorously competitive aliens
are spreading at the expense of native species and creating
unfavourable habitats for associated organisms. A good example of
the second situation is the spread of rhododendron in some oak-
woods, and the resulting virtual elimination of the field and ground
layers.
Much depends on the species.
Those which cast deep shade, such as
rhododendron, sycamore and spruce tend to be regarded as
undesirable, whereas Turkey oak, common lime and sweet chestnut are
viewed with more favour. Some species are native in part of their
range and introduced elsewhere (e.g. Scots pine, beech and hornbeam)
and, while the presence of these is not regarded as objectionable,
they are not usually rated so high in conservation interest in non-
native localities. This does not always hold, as the entirely
planted woods around Stornoway Castle, Lewis, are of very
considerable regional interest. It is generally felt, however, that
woods composed largely of introduced species do not merit grade i or
2 status. This is particularly so in the case of commercially grown
species; it is pointless to give this grading to a wood of Sitka
spruce or lodgepole pine at this time, when such large areas of
these species are being planted every year.
Plantations of non-native
trees, notably conifers, are of
considerable conservation value. Especially in their younger stages,
some have proved to be important habitats for rare breeding birds
such as short-eared owl, hen harrier and Montagu's harrier (see
subsection on birds, p. 89). These non-indigenous and recently
created forests are so widespread and cover such a large total area
that they are a major wildlife habitat in Britain. Nevertheless,
most of these artificial forests, especially when mature, are so
fundamentally different from the native woodlands that they can
never represent the most important attributes of the latter or take
their place as conservation sites. Much of the artificial woodland
is re- established on ground which has had a long period without
trees, and the associated communities which develop within,
especially of plants and invertebrates, are usually of minor
interest in that they are composed of common, rapidly spreading
species, or those remaining from the treeless phase. Many conifer
forests have such a high stocking density of trees that the
subsidiary vegetational component is virtually eliminated and the
fauna becomes greatly reduced, though some woodland birds thrive.
Much can be done to improve the nature conservation value of
commercial conifer forests by modifications in management, but it
would be inappropriate to develop this theme here. Moreover, where
native deciduous woodland is cleared and replanted with alien
conifers, considerable subsequent impoverishment of the field and
ground communities and the invertebrate fauna occurs, and this
change in management involves greater conservation losses than
gains. Native woodlands in Britain are a rapidly diminishing
resource and the conservation of a representative series is a matter
of the greatest urgency.
Comparative site evaluation
With these considerations
in mind, the following attributes in
particular are weighed in the comparative evaluation of the
individual sites:
Size
Diversity
Permanence
Lack of modification
Rarity
Requirements for the national
series
Implications for conservation |
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The nature conservation
value of a woodland tends to increase with
size. Only in a large wood may it be possible to encompass the local
range of diversity or to achieve diversity by means of different
silvicultural systems or management practices. Large size is also a
safeguard against accidental destruction, e.g. by fire. Experiments
may need homogeneous conditions over an area of adequate size or
they may be concerned with ecological diversity; both requirements
are more likely to be satisfied in a large area than a small one.
Some organisms, e.g. large birds, require a fairly large minimum
area, depending on their territory size, so that a substantial area
may be needed to contain a reasonable population. |
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Diversity in woodland is
of several kinds. There is structural
diversity, including different age of the trees. Ecological
diversity involves floristic and faunal variability within any part
of a wood especially in response to varying soil conditions and
topography, and also relationships in space and time such as the
catena, serai stages and succession. |
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A site which has been continuously
occupied by woodland is usually
more valuable than one where woodland has been re-established after
a break in continuity, especially in regard to subsidiary elements
of the flora and fauna. Its value will be further enhanced if a good
historical record is available. Such woods are historical monuments,
embodying many aspects of past rural life and organisation. Climax
woodlands are to be preferred to serai types, for though examples of
the latter are valuable in demonstrating dynamic processes, they can
be more easily and rapidly created. The few examples of woodlands
which appear to approach a 'natural' state are regarded as
especially important, though they are usually fragments on islands,
landslips, cliffs and ravine sides. |
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While the tree layer usually
reflects the silvicultural system used
(and the range of system requires representation), the quality of
the shrub and herb layers is particularly dependent on the absence
of such influences as grazing by large herbivores. The permanence of
tree cover may be especially important in maintaining the floristic
richness of the subsidiary layers. |
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The presence and abundance
of rare species of trees, shrubs, herbs,
pteridophytes, bryophytes and lichens, and of animals, gives an
added value to a woodland site selected on other criteria, but
rarity is usually taken as a major criterion for selection only when
a group of such species is involved. A very few sites are selected
for the rarity of one species. |
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Selection of the national
series of key woodland sites has involved
an attempt to represent adequately the most important types within
the total field of variation. When the many directions of variation
in woodlands are considered, especially in relation to the partial
independence of the separate structural layers, the number of
possible combinations of different character, i.e. the number of
different woodland types, is extremely large. However, it is more
realistic to lump small, closely related differences, and thus to
reduce greatly the number of different woodland types required in
the national series. Certain highly modified or degenerate types of
woodland have deliberately been omitted, and a wide range of types
with high nature conservation value has been included.
Choice of sites has been
based first on the different major woodland
types as identified by the dominant tree species (oakwood, pinewood
and so on), and then on the range of diversity within the other
components of the wood and ecosystem, i.e. the other layers, flora
and fauna. Within each major woodland type the number of sites
chosen has depended on the relative geographical extent and
variability of the type. Mixed deciduous woodland is the most
widespread and variable British woodland type and so is represented
by the largest number of sites, whereas a local type such as
pinewood is represented by a much smaller number. In addition,
particular attention has been paid to representation of the
following features.
1 Major silvicultural systems
which influence the structural
character of a woodland, especially in the tree and tall-shrub
layers, e.g. high forest, coppice, coppice with standards, park
woodland. These are represented under separate ftoristic woodland
types, as relevant, e.g. oak high forest, oak-hazel coppice with
standards, hornbeam coppice.
2 Other structural variations,
related mainly to variations in
development of the tall-shrub and field layers.
3 Topographic variations.
These apply mainly in hilly country where
differences in angle of slope, aspect and rocki-ness give different
types of wood associated with plateaux, steep slopes (hanging
woods), valley bottoms and river gorges. In some districts, e.g. the
Vale of Ffestiniog, north Wales, and Borrowdale, Lakeland, the
number of sites chosen within the same area is an attempt to
represent this range of diversity, which seldom falls within the
compass of a single woodland block, although it does so in the Wye
valley, south Wales.
4 Local diversity in soil
conditions. This is often associated with
feature 3 above, especially in sites representing a catena, where
both nutrient status and wetness of soil are affected by topography.
Base-status is also strongly influenced by the nature of the parent
rock and where there are non-calcareous and calcareous rocks within
the same wood or in the slope above, the effect on woodland
floristic composition is usually very marked. Where this situation
obtains, it is often possible to have good examples of different
major woodland types represented within the same site. A classic
case is Roudsea Wood, north Lancashire.
5 Woodland succession.
Colonisation of acidic mire or heath by birch
and pine, and of base-rich fen by alder and willow, is well
represented on many peatland and heathland sites. There are also
many examples of the changes associated with the coppice cycle. The
replacement of one or more dominant tree species by others is a
rarer condition, but examples have been chosen which may throw light
on some species relationships not yet fully understood, e.g. oak-
beech, beech-ash, birch-oak, birch-pine and oak-pine.
6 Natural woodland. This
is so fragmentary that it has a
considerable scarcity value and examples are difficult to find.
Indeed, it has been said that there is no woodland in Britain
completely unmodified by man. Woods which approach most closely to
the presumptive original condition are in relatively inaccessible
situations, and in remoter parts of Britain. A range of types has
been included in the national series but none is extensive.
7 Floristic richness of
the subsidiary layers. Selection for a wide
range of tree and tall shrub species is implicit in what has been
said above (except perhaps for rare and local shrubs such as Sorbus
spp.), but this does not necessarily take equal account of variety
in the field and ground layers. Care has been taken to represent the
full range of floristic richness, especially on the calcareous rock
formations, where the herbaceous component of the woodland flora is
often very varied and rare species (e.g. certain orchids) are well
represented. In a few instances woods which are of no particular
interest for their tree and shrub component have been chosen for
their rich herbaceous flora (e.g. Conistone Old Pasture and Bastow
Wood, Yorkshire).
In western Britain many
woods, especially those with rocky floors or
stream ravines, are extremely rich in bryo-phytes, which grow in
great abundance and luxuriance. There is a strong component of
Atlantic mosses and liverworts forming an interesting
phytogeographical element for which Britain and Ireland form the
European headquarters. This Atlantic bryoflora changes somewhat in
composition latitudinally, especially in representation of southern,
thermophilous species, and there are also differences in richness
according to degree of humidity. A series of sites (especially
sessile oakwoods, since most Atlantic species are calcifuge) has
been chosen to cover the range of variation in south-west England,
west Wales, north-west England and western Scotland. Stream gorge
woodlands are well represented.
Britain has a rich Atlantic
lichen flora, in parallel with the
bryophytes, though there is a different bias in habitat preference,
and epiphytic lichen interest is only partly covered in the
selection of woods rich in Atlantic bryophytes. Selection of
additional sites for their rich lichen flora has thus paid special
attention to the park woodlands which are an optimum habitat for
some species and are so especially a feature of Britain.
8 Vertebrate interest.
Selection of woodlands for botanical features
automatically includes a wide range of variation in species of
vertebrates, especially birds. Ornithological interest has often
been used as a secondary criterion in the choice of woodland sites
primarily for botanical reasons. In some instances, however,
ornithological interest may rate more strongly. The important and
distinctive avifauna of the central Scottish pinewoods, and the
extremely diverse and large bird populations of the New Forest have
already been mentioned.
9 Invertebrate interest.
Several woodlands have been chosen
especially for the richness of their insect fauna, notably the
Lepidoptera, e.g. Monks Wood, Huntingdonshire; Waterperry Wood,
Oxfordshire; and Craigellachie Wood, Inverness-shire. This has also
been a strong supporting criterion in many other cases, e.g. the New
Forest. The other groups of woodland invertebrates have been much
less fully surveyed, and it is not known how adequate the present
selection of sites will be for the conservation of this very large
group of organisms.
10 Features of international
importance. These are mainly floristic
features associated with the strongly oceanic climate of Britain.
Particularly noteworthy woodland species and communities in Britain
are ash as a tree dominant, holly and various whitebeams in the tall-
shrub layer, bluebells and various ferns as field layer dominants,
bryophytes in general and Atlantic species in particular in the
ground layer, and lichens (especially Atlantic species) as
epiphytes. All these features are well represented in the selection
of woodland sites now to be described. |
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A major problem in the
conservation of woodlands is the high degree
of fragmentation to which this ecosystem has been subjected.
Ideally, one would wish to safeguard a large continuous block of
woodland containing the whole range of local environmental diversity
that is characteristic of the particular region. Separate blocks
would then be chosen to represent the major regional environmental
differences, which would be mainly climatic but partly geological.
This ideal situation hardly exists anywhere in Britain at present,
and it is usually necessary to select several discrete sites in
attempting to conserve the woodland diversity characteristic of any
particular district; sometimes these sites are not widely separated,
however, and there is a case for treating them as a single aggregate
site (e.g. Borrowdale Woods, Cumberland). Even the limited range of
diversity peculiar to a single local woodland type (e.g. oak-lime
woods in Lincolnshire) may not be represented in one site, and it is
necessary to include two or more adjacent sites in order to
encompass adequately this variability. The sites selected and
delimited are often only part of a more extensive block of woodland,
which also has features of ecological importance. Though every
attempt has been made to delimit precisely the area of importance,
the boundaries of some sites may, on further inspection, require
alteration. Similarly, as our knowledge of the flora, and more
particularly
the fauna, increases it
may be necessary to revise this list of key
sites.
In many cases where numerous
woods of biological importance occur in
a restricted geographical area it is perhaps inappropriate to select
a few particular sites and more appropriate to achieve conservation
through prescriptions aimed at the management of the woods as a
whole. Examples of such areas are the western Weald, New Forest,
Forest of Dean, Dartmoor woodlands, Wye valley, north Wales,
Lincolnshire, Lake District, Loch Sunart and Spey-Dee Valleys (see
also Chapter 13). Selection of separate sites in such areas is
difficult, but an attempt has been made to do this, to maintain
uniformity of treatment throughout the Review. |
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