Trees

By 12,000 B.C., Birch forests covered much of England and many parts of Western and Northern Europe. The only check for trees spreading across Europe was their rate of natural dispersal. Some trees, like birch and elm, disperse their seeds by wind. These clearly advanced more rapidly than oaks, whose seeds are dispersed by birds and other agencies such as streams and are also much slower growing. Experts believe that trees such as birch, pine, alder, and hazel could advance at a rate of 1 - 2 kilometers a year over periods of five hundred and two thousand years. A tree’s eventual range also depended on a location of the glacial refuges from which it dispersed. Pine, for example, spread from refuges on the continental shelf off western Ireland, whereas beech spread from Italy and Balkans. To this day, stands of birch predominate in eastern and central Europe in environments that further west support pine. In the absence of soil or distance constraints, plants can respond remarkably quickly to climate changes. For example, in New Zealand, the southern beech was confined to a few sheltered locations during the late Ice Age, when grassland and scrub covered most of the land. But the rapid warming at the end of the Ice Age had beeches completely replacing the open generation of earlier times within a mere three hundred years. ~ Page 66