Geoscience Reference
In-Depth Information
Colonization of the British Isles by vegetation
in postglacial times
KEY CONCEPT
Successions are changes in vegetation occurring over time periods of decades or at most a few hundred years.
Vegetation change can also be driven by environmental change over much longer time scales. For example,
significant climatic changes have been taking place during the postglacial Holocene epoch, since the end of the last
Ice Age at 10 ka
BP
. At the beginning of the present interglacial period the climate changed quite rapidly from the
ice and tundra severity of the Pleistocene to a more temperate climate which could support deciduous broad-leaved
forests. The natural vegetation of the British Isles is similar to the deciduous forests of the rest of north-west Europe,
except for two regions, namely the coniferous forest of Scots pine (Pinus sylvestris) in the Scottish Highlands, the
so-called Caledonian Forest, and the woodlands of south-west Ireland, which resemble the evergreen broad-leaved
woods of south-west Europe. When the ice cover was at its maximum at about 20 ka
BP
, trees and other plants
found refugiaor safe havens in continental Europe far beyond the ice limits. With the postglacial warming of climate,
different tree species spread back into the British Isles at different speeds and with different patterns. The speed
and pattern of re-invasion shown by each species depended on its biological ability to propagate and spread, and on
its particular climatic preferences.
At the glacial maximum, sea level was about 100 m lower than at present, and a land bridge attached Britain to
Europe. The treeless land surface was successively invaded quickly by birch (Betula spp.), Scots pine (Pinus
sylvestris), elm (Ulmus spp.), oak (Quercusspp.), hazel (Corylus avellana) and alder (Alnus glutinosa). The colonization
was so rapid that all but the highest mountains and north-western islands were tree-covered by 8000 years
BP
. The
next arrival was small-leaved lime (Tilia cordata), which spread more slowly but, once arrived, became a dominant
plant in the mixed deciduous forest of England. Beech (Fagus sylvatica) and hornbeam (Carpinus betulus) arrived
later, after the English Channel was formed about 5000
BP
, and both have remained restricted to southern England.
By about 4000
BP
the natural forest or wildwood appears to have reached its maximum extent, with its regional
composition influenced by regional climate, microclimate, soils and topography.
Figure 20.13
shows the five main
types of natural woodland at its maximum extent in the British Isles.
Since mid-Holocene times there have been further changes in the forest cover of the British Isles due to climatic
changes, competition between tree species, and diseases, as for example in the Elm Decline around 5000 years
BP
,
and the onset of Dutch Elm Disease in the 1970s. However, human activities have had the biggest impact through
the Neolithic Age, the Bronze Age, the Iron Age, the period of Roman occupation and through Anglo-Saxon times.
Rackham (1990) estimates that the woodland cover of England was about 15 per cent at the time of Domesday Book
in 1086, but fell to about 6 per cent by 1350 owing to population growth and farming expansion. This percentage
appears to have remained surprisingly stable until the beginning of the twentieth century, since when it has climbed
to 12 per cent for the whole United Kingdom, mainly through coniferous afforestation. There is concern about particular
types of woodland, however. The decline during the second half of the twentieth century in traditional forest practices
such as wood pasturing and coppicing has led to the neglect of ancient woodlands, and their removal for farming or
for replanting with conifers. Between 1950 and 1975 nearly half the remaining ancient woodlands were lost. Those
that remain are being vigorously protected for their conservation value.
twelve major vegetation zones.
The study of ecological successions formed an
important part of twentieth-century biogeography.
From the stimulus provided by the early work of Clements
have come classic studies by Tansley, Hutchinson,
Whittaker and Aubréville, among others. The processes
operating in primary and secondary successions are quite
(intra-specific competition). The concept of the ecologi-
cal niche was introduced by the American ecologist
Hutchinson to summarize the sum total of physical and
biological controls. The natural regions of the world are
largely climate-determined. The biome is the major large-
scale ecological unit. The scheme proposed by the German
biogeographer Walter is a basis for dividing earth into
