About 10,000 years ago, the last Ice Age came to an end. As the climate improved, animals and plants moved back into Ireland across the land bridges which connected this country to Britain and Britain to Continental Europe. The gradual rise in sea level as more and more of the ice caps melted eventually submerged these land connections but not for the indigenous plant and animal species present today had recolonized the country. By examination of fossilized pollen grains trapped in lake mud and in bogs, it is possible to build up a picture of how the vegetation developed as the climate improved.
From a tundra-like vegetation, similar to that in Lapland or Northern Canada today, the first response of the flora to warming was the development of rich meadows of grass. These were quickly invaded by juniper and juniper scrub soon took over. About 9,500 years ago, willows began to overshadow the junipers and these in turn gave way to birch trees. Gradually, Hazel and pine began to appear and to shade out these earlier pioneer tree species. Hazel, in its turn, was replaced by Oak and Elm on the richer soils while Pine retained its foothold on the poorer sandy soil. The development of the high forest of Oak and Elm brought about a relatively stable condition which was to last from about 8,500 years ago to 7,000 years ago. For this period, therefore, Ireland was covered by endless sheets of trees, broken only by the waters of lakes and river channels.
About 7,000 years ago, the climate became wetter and this allowed Alder to form fen-woods on the damper soils. Thus the forest mosaic was made of Alder woods on the wetter soils, Oak and Elm forests on the better, drier soils and Birch and Pine holding their own on the upper lands. About 5,000 years ago, there is evidence in the fossil records of a sudden decrease in Elm pollen. This suggests a severe attack by disease, similar to the Dutch Elm disease outbreak of recent years. Around this time also, Neolithic farmers started to reach Ireland and with their polished stones axes, opened up clearings in the forests to grow cereals and grass for their cattle. The exploitation of the woodlands had started!


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