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 Trevezel (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. Conversely, in protected valleys and on richer soil we find more southerly communities to the North of their main area. In the Maas valley between Maaseik and Maastricht a type of loss, rich in minerals and with a mild local climate supports a type of Calluna vegetation 1.50 m high with as co-dominant species Sarothamnus scoparius. This is a subatlantic community with Southern and Central European tendencies, mainly developed in low mountains: Eifel, Black Forest, and in a modified composition on the Plateau Central (De Smidt, 1966). Often it is the interaction between local climate and soil conditions that provides a suitable habitat for such “extra-zonal” communities, as is the case with this Calluna-Sarothamnus heath. On Roc Trevezel the local climate is the main factor. An “extra-zonal” heath due to soil conditions is the poor Calluna heath with Hypnum cupressiforme var. ericetorum, Festuca ovina, Carex pilulifera on deep sandy deposits at Revingehed, East of Lund (South Sweden). This type is common in Northern Belgium, the Netherlands, and Northwest Germany with its northern border in South Jutland. A striking phenomenon is the absence of boreal species that are common in the heath of the region: Vaccinium myrtillus, V. vitis-idaea, Cornus suecica, Trientalis europaea, Lycopodium clavatum.