ABSTRACT
During the Pleistocene glaciations large areas of land in lowlands fringing the major ice sheets and on the lower slopes of mountains fringing local ice caps had too rigorous a climate to permit the persistence of closed forest. Here herbaceous plants and dwarf shrubs were able to become dominant. The origin of these plants is still open to some speculation; it is possible, however, that many of them evolved from less hardy and less specialised species in order to inhabit the bleak mountain tops which came into existence during the alpine orogeny of mid-Tertiary times. They are adapted to an open environment with high light intensities and were therefore annihilated in mid-latitude lowland areas when the re-advance of forests took place in the wake of the retreating Würm ice caps. It was only on cliffs and very steep slopes, where absence of soil prevented the establishment of forest cover, that a few were able to survive. This probably accounts for their presence today in crannies in some steep limestone crags in the Pennines. In Teesdale the alpine saxifrage (Saxifraga nivalis) and spring gentian (Gentiana verna) are found, the dwarf willow (Salix herbacea) appears locally in Craven while mountain avens (Dryas octopetala) and a few other alpine and tundra species are found in both these areas [51]. The Pleistocene migrations also account for the fact that, in both Eurasia and North America, the arctic tundra vegetation is very similar to the vegetation of mountain slopes above the forest limits, not only in life-form but also in species-composition. It is clear that the Pleistocene tundra communities, originally occupying large, continuous areas, were able to survive only by migrating in diverse directions; there was a northward retreat along a continuous front and an up-slope retreat wherever tundra communities found themselves cut off on mountain flanks. 1 The present-day boreal and sub-alpine forests thus 96give way ultimately to formations of low-growing plants which, thoug not identical, are very similar.
