The major and interacting
environmental factors of climate and soil together
determine the fundamental and original diversity of ecosystems which this country
contains and which has to be provided for in our national series of sites requiring
conservation. The present pattern of variation in ecosystems has, however, to be
seen as the result of gross climatic changes during the period since the last advance
of the Quaternary ice, modified increasingly during the last 2000 years by the impact
of man. The general picture was of a slowly warming climate allowing the gradual
spread of major plant formations and their associated fauna in a wave-like sequence
to the north and west across Britain, as types with increasing warmth requirements
and luxuriance successively replaced those adapted to cooler conditions but
possessing less competitive power. The open fjaeldmark and tundra which first
colonised ground freed from the ice were invaded by birch, juniper and willow scrub
of the taiga type. Pine forest spread and took over from birch, but was itself later
replaced by mixed deciduous forest, mainly of oak, ash, elm and hazel, but locally
dominated by beech, alder, lime, hornbeam and yew. By migrating northwards and
upwards, the earlier plant formations each survived where their optimum conditions
remained, so that the time sequence of Late-glacial and Post-glacial vegetation types
became zoned latitudinally and altitudinally within Britain.
The fjaeldmark and
tundra complex became restricted to high levels of the
mountains of northern and western Britain, though with lower-lying outposts in
situations where closed woodland could not develop, as on exposed coasts and
areas of bare rock and unstable, shallow soils; its limits are taken to define montane
conditions. Below this on mountains were zoned submontane scrub and woodland,
and the lowlands of the south and east were evidently covered with great tracts of
forest. During the whole of the Post-glacial Period, woodland has thus been the
climatic climax formation over most of Britain, and limited in its extent mainly by high
altitude, severe wind exposure and waterlogging of the ground. Primitive man
probably kept limited areas clear or thinly covered with trees, but populations were for
a long time too sparse to have much effect in reducing forest cover. Woodland
floristic composition varied according to geographical position and soil type but in
general there was probably a greater diversity in age and size of the trees, and a
more constant development of a shrub layer, than is usual in British woodlands of the
present.
Around 5500 B.C. the
climate became markedly wetter, and the widespread
occurrence of buried tree stumps at the base of peat deposits suggests that, in the
higher rainfall districts of western and northern Britain, the extent of forest became
substantially reduced by the spread of mire vegetation over ground where drainage
was poor. Yet, during this Atlantic Period, thermophilous trees and other plants
probably reached their maximum extension and abundance in Britain. About this
time, the rising sea-level cut the land bridge formerly connecting Britain with the rest
of Europe, so that the immigration of new species was much curtailed for many
groups of organisms.
Later still, around
500 B.C., the climate became cooler again, and this was followed
by a slight downward and southward shift in the main climax vegetation zones.
During this Sub-Atlantic Period, which has continued, with minor climatic fluctuation,
up to the present day, human activity probably first began significantly to alter the
general pattern of vegetation cover in Britain. Forests were destroyed on an
increasing scale, to provide land for cultivation and grazing of domestic animals, and
for fuel, including charcoal for smelting of iron. Woodland clearance, particularly rapid
in Norman times, has only been compensated by extensive re-afforestation during
the last 50 years. Animals dependent on the forest were either incidentally or
deliberately eradicated or reduced, though new ecosystems, especially grasslands,
were created, and some reached a fair measure of stability under the existing
management practices. New species of plant and animal were introduced by man,
either casually or purposely, and some of these arrived here so long ago that the
concept of a native species is of doubtful validity.
