Within Britain there
is a considerable range of geographical diversity in ecosystems,
determined largely by major climatic gradients and geographical differences in
physiography and geology, which in turn shape the pattern of variation in soil
development, human activity and land use.
Climate
From south to north
there is an overall latitudinal decrease in mean temperatures,
and from east to west a general increase in oceanicity, i.e. decrease in temperature
range, radiation and insolation, increase in precipitation, atmospheric humidity and
wind speeds. These main gradients are complicated by the distribution of high land,
which is concentrated in the north and west of Britain. High ground causes an
increase in precipitation, atmospheric humidity, cloud cover and windiness, and
decrease in mean temperature, giving local gradients which reinforce the major
gradient of increasing oceanicity. The mountainous districts of western Britain,
especially in Snowdonia, Lakeland, Galloway and the western Highlands and islands
are thus extremely oceanic in terms of wetness, cloudiness and windiness.
However, the latitudinal lapse in temperature results in a south to north gradient, from
the warm oceanic climate of south-west England to the cool oceanic conditions of
the north-west Highlands, Orkney and Shetland.
The warm and sunny
south coast areas of England, especially in Dorset, Devon and
Cornwall, give the nearest approach to a Mediterranean climate in Britain, whereas
the most continental conditions are found in eastern districts which lie well away
from the sea, from East Anglia to the north-east Highlands. While there is a general
decrease in mean temperature with distance northwards, the coldest part of Britain
in winter is not in the extreme north of Scotland, but in the central and eastern
Highlands, which contain the largest mass of high mountain land. Duration of snow
cover is a factor of increasing importance with increasing latitude and altitude on our
mountains, and also attains its greatest influence in the central and eastern
Highlands.
The major climatic
gradients combine to give a general increase in severity of
conditions for plant growth in a northwesterly direction. This tendency is reflected in
the downward shift of the altitudinal zones of vegetation in upland country (forest and
scrub, dwarf-shrub heath, montane grasslands and moss-lichen heaths) in the same
direction. Over Britain as a whole, there is also a parallel between regional
differences in climate and composition of the flora and fauna. One way of describing
this is in terms of representation of the biogeographical groupings of species
associated with different climatic regimes on a continental scale; another is
according to changes in floristic composition of vegetation types, and their
associated animal communities. Regional gradients in climate within Britain are
reflected in the distribution of plant and animal species, both in presence/absence
terms, and in abundance and performance. In general, there is a decrease in the
number of species of flowering plants and animals with increasing distance
northwards and, to a lesser degree, westwards. Some groups of invertebrates,
notably those associated with sunny climates (e.g. butterflies, dragonflies) are
especially poorly represented in the north of Britain. On the other hand, the
cryptogamic flora of ferns, bryophytes and lichens is much richer in northern and
western areas than in the south and east, for these plants are adapted to cool and
humid conditions.
Geology and soils
Geological accidents
have given a prevalence of younger, softer rocks and low-lying
terrain in the south and east contrasting with older, harder rocks and mountainous
country in the north and west. Calcareous rocks are likewise erratically distributed,
consisting mainly of belts of Chalk and Jurassic limestone in the south and east, and
tracts of Carboniferous and other limestone in the north and west. Non- calcareous
rocks cover a much larger area of Britain. This is an important pedogenic factor
since most naturally occurring British soils are characterised by having calcium
(Ca2+) as the predominant exchangeable cation (base), and therefore the main
determinant of soil pH, but the nutrient quality of derived soils depends almost as
much on topography and climate as on the original composition of the parent
material. Except on local areas of acidic and porous sands and gravels, which often
have deep podsols, the soils of the lowlands are mostly base-saturated and therefore
usually fertile brown earth types with mull humus, even though free lime may not be
present. Free lime is characteristic of soils formed from chalk and limestone but true
rendzinas are very local in Britain. Whilst base-saturation and pH usually give a
measure of overall soil fertility, and affect such processes as litter-decomposition
and nitrification, some highly calcareous soils are relatively infertile through
deficiency of available nutrients such as nitrogen and phosphorus. Raw soils with
low nutrient exchange capacity occur on a variety of rocks and deposits, and in many
different situations. Peats in the lowlands are mostly of a fen type, formed under the
influence of base-rich ground water and a eutrophic vegetation. Only very locally are
there lowland raised mires and valley mires with a base-poor peat developed under
an acidophilous vegetation.
The lowlands also have
a variety of alluvial soils which are usually base-rich. Coastal
lands have moderately to strongly saline soils in a variety of situations, including not
only stabilised marine sediment but also ground above high water swept by salt
spray and solid particles. Spray and sand deposition can cause other mineral
enrichment, and many coastal dune systems have highly calcareous deposits where
there are numerous shell fragments in the sand.
Soils in northern and
western Britain, especially the uplands, show a more marked
bias towards podsolic types with mor humus, which on permanently waterlogged
ground has characteristically developed into a thick layer of acidic blanket peat. Base-
rich soils are often confined to flushed situations where there is a local downwash
and deposition of particles and nutrients from above, and the heavy rainfall results in
extensive occurrence of gley soils where lateral drainage waterlogs the surface
layers especially of glacial drift covering the lower slopes and valleys. On the higher
mountains there is a prevalence of skeletal and immature soils which reflect both
climatic severity and gravitational instability, and the effects of solifluction are often
pronounced.
In Britain, the flora
of base-rich and especially calcareous substrata is almost
invariably much richer in number of species than that of base-poor types; as the
former are so much more local, and often subjected to selectively heavy exploitation,
their conservation value is particularly high. Human influence has often been
particularly marked on and through the soil. Deforestation, draining, ploughing,
burning, grazing, peat-cutting and the addition of both natural and artificial fertilisers
have all had a profound effect, in nearly all parts of the country. Their influence
includes both reduction and enhancement of soil fertility.
