276 Eduard Riibel 



8. Aestisilvae: Summer green forests (deciduous forests). — The 

 dominant trees become leafless regularly in the cold season; 

 their buds are always protected. A warm moist summer allows 

 them a very high water balance. Aestisilvae are the widely ex- 

 tended forests of middle Europe, eastern Asia, and eastern 

 America. The beech woods of Europe are being closely studied 

 at present in many countries. The preeminent forest of the east- 

 ern United States and southeastern Canada is the beech-maple 

 forest; toward the drier Middle West it changes through a transi- 

 tion of maple-elm forest into extensive oak forests. Deciduous 

 oak forests occupy the stretches of moderately continental cli- 

 mate and struggle for area with the more continental prairie. The 

 ratio of precipitation to evaporation is 80 to 100 per cent, and 

 in beech forests it is more than 100 per cent. 



In southern Switzerland, below the beech belt on calcareous 

 soil are oak forests (Quercus sessili flora) ; on siliceous soil, chest- 

 nut forests {Castanea vescd). Wet or moist forests in running 

 water are the "Auenwalder" of Europe, forests of Alnus incana 

 with willows, many lianas such as hops (Humulus lupulus) 

 and Clematis vitalba. The equalizing action of water permits the 

 Populus communities to follow the rivers far into the African 

 desert, and Populus fremonti and Fraxinus tomei in the river 

 bed at Tucson into the Arizona desert. In stagnant water, Alnus 

 glutinosa dominates the English "Carr" and the "Bruchwald" 

 of the European continent. Near these may be ranged the Taxo- 

 dium distichum community of the Great Dismal Swamp in 

 Virginia and North Carolina. 



g. Aestifruticeta: Summer green scrub. — Summer scrub is 

 biotically and edaphically conditioned; a few of its constituents 

 semiclimatically. Where the climate permits closed deciduous- 

 leaved summer vegetation, this suffices, as a rule, for tree 



