94 THE STUDY OF PLANT COMMUNITIES ■ Chapter V 



Snow water is of prime importance in those arctic and alpine 

 regions where there is practically no rain. Here the plants are 

 shallowly rooted, not uncommonly limited to the surface soil by 

 perpetual frost a few inches below. Surface water must then be 

 supplied continuously to maintain plant life. This is provided by 

 the melting snow, some of which, in depressions or other pro- 



AVERAGE ANNUAL PRECIPITATION FOR THE UNITED STATES 



EXPRESSED IN INCHES 



75 10 100 

 and owr 



FlG. 41. Average annual precipitation for the United States.— By permis- 

 sion, from Bernard, 19 in Hydrology, copyrighted 1942, McGraw-Hill Book 

 Company. 



tected places, may remain throughout the growing season. The 

 richest flora in best condition will usually be found at the margins 

 of snow patches and in drainage lines below them. Ridges and 

 raised ground are the first to be exposed at the beginning of the 

 growing season, and there growth begins almost immediately. As 

 the season progresses, more ground is exposed by melting snow, 

 and plants there begin growth. Thus, at distances of a relatively 

 few feet, may be found plants of the same species, that have 

 flowered, fruited, and dried up, and, in the moist soil beside the 

 snow, plants which have just begun their growth. 



The total annual precipitation of an area is only a rough indi- 

 cation of moisture conditions for plant growth. A light rain of 

 0.15 inches usually does not affect soil moisture, for most of it 

 will be intercepted by vegetation and will evaporate quickly. 

 That which reaches the soil will wet only the surface and like- 



