Environmental Engineering Reference
In-Depth Information
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
(
Quercus spp.
), 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 1 shows the five main ty pes of
natural woodland at its maximum extent in the British Isles.
Deciphering the history of vegetation colonization in the Holocene owes much to the
technique of
palynology
or
pollen analysis
(Huntley 1988). The technique is based on
analysing peat bogs, lake sediments and acid soils where the pollen grains of flowering
plants and the spores of ferns and mosses are protected from decay. Peat can be regarded
as a time section, with the oldest layers at the bottom and the most recent at the top. The
peat deposit is sampled by taking a core with an auger, the pollen extracted in the
laboratory and mounted on a microscope slide. The pollen grains are then identified and
counted under the microscope. The data for each tree type are plotted as a percentage of
the total tree pollen at each depth on a graph or
pollen diagram
. There are complicating
factors caused by differences between trees in pollen production, in dispersal and in
preservation in the peat, and there is also a question about the exact size of area
represented in the pollen core. However, together with carbon-14 dating, the study of
archaeological remains and macro-fossils, pollen analysis provides an invaluable and
consistent type of evidence for past vegetation.
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.
