The vegetation type and the aggregations of plant species 

 found in an area are determined by a combination of factors: 



1. Biotic factors: includes the presence and effects of man, 

 other animals, and other plants. 



2. Soils: physical and chemical composition. 



3. Physiography: slope, terrain, etc. 



4. Climate: temperature, radiation, wind, water, fire. 



Biotic Influences on Vegetation 



In the past migrations and distributions of various major 

 genera and famiHes, certain continents or large geographical areas 

 were "left out". In other areas (such as the Arctic) the stringent 

 climate prevents certain life forms (such as tall trees) from being 

 present, so obviously no forest vegetation can develop. 



An important influence on the composition of vegetation is 

 the effects plants have on each other and the effects animals or 

 micro-organisms have on plants. 



In the redwood forest, as well as other coniferous forest types, 

 the intense shade cast by the large evergreen trees undoubtedly 

 is a prime influence in preventing the estabHshment of a number 

 of plant species under these trees. Even the light shade cast by 

 a deciduous tree standing in a field may have an important 

 effect on determining what plants can grow under the tree. 



A second effect that plants have on each other is expressed 

 via competition for water. Sometimes this is on a "first come, 

 first served" basis, that is, the oldest individuals in a community 

 may have well-established and pervasive root systems that prevent 

 the establishment of either their own seedlings or seedlings of 

 other species because these systems are so efficient in taking up 

 water. This effect is particularly important in arid regions where 

 water supplies are generally at a premium. It is probable that the 

 wide, orchardlike spacing of many desert shrubs is a by-product 

 of competition for water. 



The presence and activities of fungi and bacteria may also 

 have an effect on the nature of the vegetation. In warm, wet 

 regions decomposition of leaves that fall from trees is very rapid; 

 in cooler or drier areas this decomposition - which is the result 

 of bacterial and fungal action - may be slowed down so con- 



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