fluctuations have led to major changes in the range of plant communities in the
northern hemisphere with large areas of Europe and North America containing
temperate broad-leaved forest as its natural community dominated by Arctic-
liketundra. In the tropics, rainforest became more restricted and fragmented
during the drier glacial periods. There were many extinctions during these
glacial times. The spread and retreat of the glaciers affected plant distribution
throughout the north temperate, plants with good colonizing ability surviving
better than others and some tundra plants reaching their current disjunct distri-
butions, i.e. distributions consisting of several quite separate areas, through
their retreat northwards and into mountains as the glaciers retreated.
In post-glacial times (the last 10 000 years or so) we have a better record than
for any previous time from the analysis of pollen and other fossil fragments
preserved mainly in peatand lake sediments. These show that plants have
spread at different speeds north and often west as well, following the amelio-
rating climate, and that the plant communities in the northern hemisphere have
changed markedly in extent and composition throughout this time.166 Section K – Plant communities and populations