324 



ECOLOGICAL FACTORS: 



involve aerial parts of plants. Usual difficulty is 

 with excess water causing air stagnation (carbon 

 dioxide buildup and oxygen depletion) in the sub- 

 strate. Excess water limits the availability of gases to 

 plants and creates an environment that is suitable 

 only to hydrophytes. Air stagnation is a normal 

 tendency in many soils. However, air renewal can 

 occur in several ways. For example, water infiltrating 

 the soil will first replace stagnant air; when the 

 moisture either evaporates, is used by plants, or 

 percolates deeper, fresh air is drawn into the soil. 

 Other means of air replacement in soil are provided 

 by changes in wind velocity, air pressure, and tem- 

 perature. 



Soil air becomes stagnant when its oxygen is de- 

 pleted or made less available by organism respiration, 

 amount of pore space, size of pore space, and ef- 

 fectiveness of drainage. Respiration by soil or- 

 ganisms and plant roots uses oxygen and produces 

 additional carbon dioxide. Amount of pore space 

 should be directly related to amount of soil air; 

 however, heavy soils which have about 60 per cent of 

 their volume in pore space are more subject to stagna- 

 tion than are light soils which have about 40 per cent 

 pore space. This is so because heavy soils usually 

 contain more water (thus, actually have less soil air 

 and greater stagnation) and take longer to replace 

 their gases. Pore size, on the other hand, is directly 

 related to air replacement; coarse soils with larger 

 pore spaces than finer soils replace air much more 

 rapidly. Drainage is directly related to air replace- 

 ment; the better the drainage of a soil, the more 

 rapid the replacement. 



WIND 



The ecological influence of wind, direct and in- 

 direct, is important (Table 17.3). Indirectly, wind 

 can modify a local climate to the point of forming a 

 unique habitat. For example, a dry wind may re- 

 move much of the moisture and create a desert in an 

 otherwise humid area. On the other hand, winds can 

 be instrumental in developing clouds that are moved 

 into localities where they then reduce temperature 

 and water loss and create a moister environment. 

 Direct effects of wind need not be harmful. For ex- 

 ample, wind transports many plants and animals into 

 new areas. For plants wind is necessary to transport 

 the pollen and seeds of many species, and for animals 

 wind moves the eggs and adults of many species. 



The harmful consequences of wind are best seen in 

 plants of coasts, high altitudes, or flat plains. Less 

 harmful eflfects include such things as dessication, 

 dwarfing, and deformation of plants. Most serious is 

 breakage, abrasion, uprooting, and burying. A more 

 complex situation is associated with movements of 

 snow. Localized areas that are kept free of snow 

 by wind often have plants different from those 

 places free from modification or subject to accumula- 

 tion of deep snow. Finally, coastal winds subject 

 many strand plants to salt spray. Because the 

 amount of salt spray is related directly to proximity 

 to the ocean, plants nearest the sea frequently are 

 limited to salt -tolerant types. However, these coastal 

 strand types also are subject to uprooting and bury- 

 ing, perhaps the basis for the adaptations of extensive 

 roots and capabilities for growing above burying 

 sands. 



ALPINE HABITATS 



Wind is not the only powerful ecological factor in 

 high mountains, nor is it necessarily the most im- 

 portant. However, in such places as alpine areas and 

 deserts wind does reach its greatest importance as an 

 ecological factor. 



The nature of the atmosphere at high altitudes has 

 great influence on the other ecological factors. Be- 

 cause of the altitude, the atmosphere is thin. This 

 low air density means less oxygen per unit volume of 

 air, one of the few stable alpine factors. Most alpine 

 ecological factors display some of the most extreme 

 fluctuations that can be found on earth. This es- 

 pecially is the case with temperature, wind, and 

 water. The thin atmosphere allows much more in- 

 tense solar radiation than is found in lowlands, so 

 that daily temperatures may be very high and heat 

 loss rapid at night. 



Wind is important because its velocity is directly 

 related to altitude: the greater the altitude, the 

 greater the wind velocity. When wind velocity 

 doubles, the force of the wind is quadrupled. Be- 

 cause gusts of wind at high altitudes approach 

 250 m.p.h., it is not surprising that wind frequently is 

 a limiting factor in alpine areas. However, such wind 

 velocities are not found in many mountains, since ve- 

 locity is related to prevailing world winds. Only 

 mountains located within zones of strong prevailing 

 world winds can have gusts approaching the maxi- 

 mum figure — and, again, only certain areas within 



