lo] ENVIRONMENTAL FACTORS 293 



or chinook winds that, sweeping down from mountains, can raise 

 the temperature of the air locally by as much as 30" C. in a very 

 short time. For such reasons a good deal can be inferred about 

 the wind-climate of a habitat by direct obser^•ation of the vegetation, 

 etc. Wind velocitv is measured by an anemometer, but its effect 

 is included in observations obtained from stationary hygrometers 

 and atmometers. 



As we saw in Chapter IV, wind is an important agent of dispersal. 

 It may also be of significance phytogeographically in determining 

 the local distribution of species or communities of plants, some 

 types being markedlv more wind-resistant than others, and many 

 being unable to flourish or even exist in exposed situations. Thus 

 the tree-limit on the sides of mountains is apt to be due largely to 

 winds, as mav be seen bv the frequent persistence higher up of 

 groups of trees in sheltered pockets ; such trees further reduce 

 exposure verv locally and enable tender herbs to grow in their 

 company. 



Ocean currents, such as the warm Gulf Stream or the cold East 

 Greenland Current, may have a considerable effect on temperature 

 both locally and on land at a distance — especially when the winds 

 are predominantly on-shore. By bringing in fresh materials, such 

 currents may also alter the conditions with regard to nutrient salts, 

 etc. Moreover, as we saw in Chapter IV, water currents can be 

 an important aid in plant dispersal. 



Physiographic 



The physiographic factors are those introduced by the structure, 

 conformity, and behaviour of the earth's surface— e.^. by topographic 

 features such as elevation and slope, by geodynamic processes such 

 as silting and erosion, and consequently by local geology. Other 

 causes of physiographic change from place to place include the 

 blow^ing of sand or dust which in time or special circumstances can 

 assume vast proportions. It is in such connections that the various 

 landforms described in Chapter XVII tend to be most significant 

 to students of plant geography and ecology. 



Physiographic factors act on local vegetation largely through 

 climatic or edaphic features which they engender. They are 

 consequently sometimes classed with these other groups. Yet, 

 ecological factors being so widely interdependent in any case, it 

 seems more conducive to clear understanding to consider the 



