Figure 1-1. Climatic, topographic, and biotic factors all play 

 a part in determining the various kinds of subalpine and 

 alpine vegetation. Several sharp community boundaries are 

 evident here. Sheep are searching for the more tender parts 

 of plants in the high Sawtooth Valley in Idaho. September, 

 1958. (U.S.D.A. Soil Conservation Service.) 



because of its importance it may well be classified separately. 



Some of these factors, particularly water, light, humidity, heat 

 and cold, mineral nutrients, and gases, affect plants directly. 

 Wind, soil texture and structure, and precipitation usually influ- 

 ence plants indirectly through the direct factors. Physiographic 

 factors are more remote, for they affect the indirect factors which 

 in turn influence the direct factors; for example, an increase in 

 elevation may cause greater precipitation which in turn increases 

 the soil moisture. The response of plants is often related to perio- 

 dicity, such as summer and winter periods of precipitation, and 

 to the ranges and fluctuations in various phases, rather than to 

 average values. 



Each environmental factor has two or sometimes three phases: 

 intensity, duration, and quality. High intensity of heat accom- 



Ecological CHaaracteristics of Species &. Popmalatioxms 



