Figure 6-2. Zonation around a pond near Palmer, Alaska. 

 (1) Yellow water-lily {Nuphar polysepalum) in the deeper 

 water; (2) buckbean (Menyanthes trifoliata) and Potentilla 

 palustris in shallow water; (3) Potentilla palustris, Calama- 

 grostis sp., and Sphagnum on wet substratum; (4) hum- 

 mocky substratum with species as in (3) and willows, alder, 

 and glandular birch; and (5) zone of dead birch trees, sur- 

 rounded by the birch-white spruce forest. June. 



summer maxima temperatures, lower winter minima, 

 more rapid transitions in spring and in autumn, longer 

 growing period, and less prevalence of high winds 



(3) Snow cover: duration and depth 



(4) Chemical composition of the soil, especially bases 



(5) Soil moisture 



In the detailed analysis of vegetation the environmental ap- 

 proach has not been very successful. It is more satisfactory and 

 logical first to distinguish and describe stands by their own in- 

 trinsic qualities, particularly species composition and relative 

 abundance, and then to try to determine the causative influences. 

 The species composition can be observed and measured directly, 

 but the causes may be difficult to unravel and must often be in- 

 ferred from the occurrence of certain plants, particularly those 



182 



Classification of Comxnunities 



