H] WOODLAND ASSOCIATIONS 53 



Variation of Vegetation in the Oak Woods 



The vegetation of the oak woods varies greatly from place 

 to place. The variation in the vegetation is due to the varia- 

 tion of the various ecological factors. It is impossible, in the 

 present state of knowledge, to give anything like a complete 

 account of these factors ; and it is still more difficult to state 

 the action of the various factors either on the vegetation as a 

 whole or upon the individual plants. However, some operating 

 factors may be recognised ; and doubtless future work by plant 

 physiologists will suggest what are the effects of these factors 

 on the vegetation and on the individual plants. 



In the woods of Quercus sessiliflora, important ecological 

 factors are the water-content of the soil, the kind and quantity 

 of humus present, and the amount of light which penetrates 

 the leaf-canopy of the trees and shrubs. These factors are cor- 

 related in the most complex manner, and can perhaps best be 

 illustrated by considering various woodland habitats where any 

 one of them becomes pronounced. It must be remembered, 

 however, that the habitats chosen are connected by all possible 

 intermediate stages ; and it is the sum of these minor habitats 

 which constitute the more general habitat of the association 

 as a whole. 



(1) Marshy places. Where springs arise and by the 

 sides of the various rills and streams, wet and marshy places 

 occur where the soil is well aerated; and consequently any 

 humus that is present is mild (not alkaline) humus and not 

 acidic humus. In such places, the oak {Quercus sessiliflora) 

 tends to become very rare, and the alder (Alnus glutinosa) 

 and the ash {Fraxinus excelsior) to become correspondingly 

 more abundant. Birches (Betula pubescens, and B. pubescens 

 var. parvifolia) may however remain. At the lower altitudes, 

 the crack willow (Salix fragilis) is sometimes found ; and at 

 the higher altitudes, where, however, the soil-water may be 

 more or les^ acid, Salix aurita is locally abundant. S. cinerea 

 is usually an abundant species ; and where the two last are 

 found together, hybrids {S. aurita x cinerea) occur. The bird 



