Forests of European silver fir (Abies alba) in temperate mountains, often with European beech (Fagus sylvatica), and Norway spruce (Picea abies) where site conditions are harsher at higher altitudes. In the southern Black Sea region and the Caucasus, the dominant species is Caucasian fir (Abies nordmanniana), often with an admixture of Oriental beech (Fagus orientalis) and Oriental spruce (Picea orientalis). At most sites, fir forests occur on acidic soils though extending on to more base-rich and mesotrophic soils where distinctive contingents of herbs augment or replace the usually heathy field layer.