January, 1923 ECOLOGICAL CLASSIFICATION OF PLANT COMMUNITIES 21 



a relatively immediate effect on the habitat ; the immediate effect of relatively 

 slow changes is inconsequential. In the absence of evidence to the contrary, 

 and with reference to its proximate effect on the habitat, the climate of any 

 particular area can be regarded as essentially stable. 



Other atmospheric influences are seen in the direct mechanical effect on 

 vegetation of heavy winds, of hail and sleet, of ice storms, of drifting snow 

 and the like, in the effect of irregular extremes of temperature and moisture, 

 etc. 



2. Physiographic Habitat Factors 21 



These include influences which are associated with peculiarities in the 

 form, structure, and behavior of the earth's surface. They may be subdivided 

 as follows : 



Topographic Factors. The effect of topographic conditions on the nature 

 of the habitat is directly expressed through slope ; 22 indirectly it is expressed 

 in many ways. The more favorable soil water relations along the base of a 

 hill as compared with its upper slopes, for example, and the relation of ground 

 water supplies to surface moisture conditions in general are determined very 

 largely by topography. 



Edaphic Factors. Here are included conditions due to the physical struc- 

 ture or chemical composition of the soil, or water, or whatever medium plants 

 grow in or on, other than the atmosphere. Examples are seen in the physical 

 influence of rock as compared with sand or clay, or of loose as compared with 

 compact soils, or of land as compared with water ; in the chemical influence 

 of acid as compared with basic soils, etc. 



Rhythmical Changes in Physiography. Here are included primarily the 

 changes which characterize the littoral zone along the seacoast. associated with 

 the alternate rise and fall of the tides. The annual freezing and melting of 

 the water in lakes and streams and in the surface layers of the ground, the 

 alternate rise and fall of the water level in ponds and swamps at different 

 seasons, and similar periodically recurrent physiographic phenomena might 

 also be classed as rhythmical. 



Progressive Changes in Physiography. Erosion and deposition, soil 

 leaching, emergence and subsidence, etc., are factors which in the course of 

 time may bring about progressive changes in the topographic form of the 

 land and in the physical structure and chemical composition of the substratum. 

 Such changes may be effected within a relatively brief period — a period 

 measured by years ; or they may take place so slowly that they can be esti- 



21 The term physiographic has been variously interpreted. For present purposes and 

 for want of a better word it is restricted to the application here suggested : specifically, 

 it does not include atmospheric conditions. To include both climatic and physiographic 

 factors, the term geographic is used. 



22 That is, the degree in which the surface of the land approaches or departs from 

 the level. 



