304 



ECOLOGICAL FACTORS: 



areas had to be interpreted in reference to departures 

 from the world norm. The world norm, then, would 

 be the spectrum of any locality where chance alone 

 was operating to determine the species present, or 

 where there was little ecological restriction of the 

 flora. 



CLISERAL MOVEMENTS 



The more important climatic aspects as related to 

 vegetation, heat, and moisture, are not static. Cli- 

 mate displayed very remarkable changes in the past 

 and fluctuations are taking place today. As a reflec- 

 tion of these climatic variations, vegetation has been 

 and is evolving. Although evolution of plant species 

 is of primary importance, another significant conse- 

 quence of climatic change is the murement of vegeta- 

 tion. In the northern hemisphere, with cooling there 

 is a general southward displacement of vegetation; 

 with warming, there is a general northward one (Fig- 

 ures 19.12 through 19.20). At the present time, there 

 is a tendency for climates to become warmer and 

 drier, with a resultant northward plant movement. 

 Such shifts of plants and their associated animals are 

 chseral movements, a term that refers to changing cli- 

 mate (hence vegetation) in local situations. The 

 sequence of communities at a particular locality 

 during a single climatic trend constitute a clisere. 



TEMPERATURE 



The extremes of air temperature on our planet are 

 about —95° and 140°F. However, surface soil tem- 

 peratures in deserts are said to exceed 200°F. Even in 

 a single locality temperature displays many associa- 

 tions. There are daily and seasonal fluctuations re- 

 lated to the presence and angle of the sun's rays. In 

 addition, the closer a locality is to one of the poles, 

 the greater is the solar rays' angle from the zenith, 

 hence the colder the daily or seasonal temperature 

 range. Finally, at a given latitude an increase in alti- 

 tude often entails a reduction in temperature — ap- 

 proximately a 3°F. loss for every lOOO-foot rise in 

 elevation, equivalent to moving about 600 miles 

 poleward. 



TEMPERATURE VARIATIONS 



In addition to daily and seasonal temperature vari- 

 ations, temperature is related to many physical and 



biological factors in local areas (Table 17.2). First, 

 dark-colored surfaces absorb more heat than do light- 

 colored ones, because light surfaces reflect more ra- 

 diation. Second, coarse soils acquire heat more 

 rapidly than do fine soils, because fine soils are wetter 

 and water warms more slowly than do other soil ma- 

 terials. Third, temperature extremes usually are less 

 in areas covered by plants than in open areas. In the 

 daytime plants interrupt solar radiation and reduce 

 insolation; at night plant cover inhibits the loss of 

 surface heat. Seasonally, plant cover tends to keep 

 winter temperature minima higher and summer 

 maximum lower and to retard spring warming and 

 winter cooling of the underlying soil. 



Fourth, surface cover such as snow or organic mat- 

 ter usually is more effective than plant cover alone in 

 preventing temperature extremes. 



J ABLE 17.2 SOME TeMPEKATURE-ENVIRONMlNT 

 RELATIONSHIPS 



Temperature Conditions 



Environmental Features 







Day 



Night 



Summer 



Winter 



Low latitude or altitude 



High latitude or altitude 



Dark-colored objects 



Light-colored objects 



Coarse soils 



Fine soils 



South slopes 



North slopes 



Wide valleys 



Narrow valleys 



Remote from water 



Near water 



Hot winds 



Cold winds 



Open areas 



Vegetation or surface cover 



Dry, clear, or calm air 



Moist, dusty, cloudy, or foggy 



X X 



XX X 



X X 



X 



XXX 



XXX 



X X 



X X 



XXX 



XXX 

 XXX 



XXX 

 X 



X 



X X 



X X 

 X 



^Actually, day temperatures normally are higher than night 

 temperatures. This temperature condition is recognized because it 

 contrasts with temperature conditions of adjacent environmental 

 types. 



