CLIMATIC FACTORS 107 



The extremes of light intensity in cloudless arid regions are not 

 very great. The plants of deserts and steppes are not injured either by 

 too much or by too little light. Shade vegetation plays here a negli- 

 gible part. 



On account of the great transparency of the air on mountains, total 

 light increases with altitude while diffuse light decreases. The greatest 

 light intensity ever observed — 2,083 Bunsen units — was measured by 

 Wiesner (1907) at Old Faithful in Yellowstone National Park (over 

 2,000 m.) under a cloudless sky (altitude of sun 50° 47'). 



Snow by reflection considerably increases the light supply to 

 exposed vegetation. From frozen snow as much as 89 per cent of the 

 incident hght may be reflected (Riibel, 1912, 1928) . On account of the 

 cloudiness of mountain regions, plants receive more diffuse than direct 

 light, in spite of the fact that direct light increases and diffuse light 

 decreases with altitude. 



Significance of the Light Factor for Plant Sociology.— The indi- 

 vidual plant performs its life processes within a certain range of light 

 intensities. Similarly, plant communities have their definite light 

 requirements, depending on locahty (latitude, altitude, exposure, slope, 

 covering), on their position in relation to other communities or layers, 

 on length of growing season, and on duration of the snowless period. 



Just as there are heliophytes (light plants) and sciophytes (shade 

 plants) corresponding to extremes of light, so also light- and shade- 

 loving communities are recognizable. All one-layered communities and 

 the uppermost portions of many-layered communities in regions with 

 low or medium cloudiness are termed heliophilous. Their optimum 

 hght requirements are full sunlight. Submerged aquatic vegetation, 

 many communities of steep northern slopes, of rock crevices, and of 

 caverns; the late-appearing communities of arctic and alpine snow-land; 

 many epiphytic communities; and in general the lower layers of 

 stratified communities, whose light requirement decreases from upper 

 to lower, are designated as sciophilous. Even leafless upper layers 

 may intercept large amounts of light. On INIar. 27, at noon, Wiesner 

 measured 0.166 Bunsen unit in the shade of the leafless trees of a mixed 

 deciduous forest, near Vienna, whereas the total light in the open 

 measured 0.712 unit. The role of light in the forest was critically 

 studied by Cieslar (1904) and by Park (1931). Malta (1921) has 

 emphasized the importance of light in the control of associations of 

 bryophytes. 



Studying the vegetation of Lapland, Lippmaa (1929) noted the 

 influence of light on the formation of red pigments in different groups 

 of plants. He constructed a phytochromatic spectrum and demon- 



