The water economy, the mineral content of the soil, and human influence are the principal ecological factors governing the variation of the heath vegetation of a limited region. Sloping of the surface is also an important factor. In hilly country it is of a twofold nature: on the one hand the difference between high and low altitudes, based on the water economy, on the other hand differences in (micro-) climate. If the hills are higher, this results in greater climatic differences. In extremely oceanic and in boreal regions a rise in altitude of 100 m is sufficient for creating a noticeable decrease in temperature and an increase in precipitation, aerial moisture, and wind force. This results in the occurrence on the hills of heath communities that have their main distribution more to the North. The same observation was made by Gimingham (1961). On Slieve League on the Donegal coast (Ireland) Salix herbacea and Lycopodium selago occur in the heath at an altitude of 600 m, near Tongue on the Scottish north coast Dryas octopetala, Saxifraga oppositifolia, Alchemilla alpina and Thalictrum alpinum at an altitude of 60 m. West of Apeldoorn in the Netherlands are found extensive stretches of heath with abundant Vaccinium myrtillus and V. vitis-idaea at an elevation of 60-80 m, even on south-facing slopes. This is an area with high precipitation due to ascending air west of the hill ridge of the Eastern-Veluwe. Here the Vacciniums, elsewhere requiring the protection of the forest, can tolerate the habitat of the open heath (Stoutjesdijk, 1959; De Smidt, 1966). Higher elevation combined with north-facing slopes creates extreme conditions e.g. on Roc Trévézel (300—360 m) in Brittany, with Vaccinium myrtillus, Melampyrum pratense, Hymenophyllum wilsonii and Rhytidiadelphus loreus. These species are virtually lacking in the surrounding plains where the heath consists of such South Atlantic species as Erica cinerea, E. ciliaris, Ulex gallii, Lobelia urens, Lithospermum prostratum and Symethis planifolia.