SECTION IX 



CLASS IV. PSYCHROPHYTES. FORMATIONS 



ON COLD SOIL 



CHAPTER LXV. CLIMATIC CONDITIONS IN SUBGLACIAL 



FELL-FIELDS 



Immediately below the snow-line in Polar countries, and on high 

 mountains there develops a vegetation composed of subglacial communi- 

 ties, which though exhibiting many differences consequent on the variety 

 of the alpine and Polar conditions, yet can be grouped together because 

 they show many points of agreement. The natural conditions prevailing 

 in these situations are discussed in the succeeding paragraphs. ^ 



Temperature. The mean temperature of the warmest month is 

 low and does not rise more than a few degrees above zero centigrade ; 

 on mountains it falls by o-6 of a degree centigrade with each increase 

 in altitude of lOO metres. This fall in temperature brings in its train 

 certain limits to the distribution of species according to altitude and 

 latitude, and certain limits to the distribution of snow (local conditions, 

 such as slope and the like, play a great part in the matter-). In the 

 vegetative season sharp frosts and falls of snow occur, arresting the 

 development of plants and modifying their forms, particularly because 

 the soil is rendered cold. 



The annual and diurnal changes in temperature on mountains are not 

 essentially different from those in the plains of the same climatic zone, 

 though the temperatures are lower and their fluctuations less. Moreover, 

 the date of the average arrival of the lowest and highest temperatures 

 is later on mountains than in plains. Consequently, on high tropical 

 mountains the change of temperature is tropical in type, and there is 

 only a small difference between the temperature of the extreme months. 

 On Antisana in Ecuador the difference in temperature of the hottest 

 and coldest month is only 32 centigrade degrees. The mean temperatures 

 of all the months are above C., but frosts and falls of snow occur 

 in all months. Approaching the Poles the seasons become more sharply 

 defined, just as in the plains. Even in the cold-temperate zones, on 

 mountains temperatures almost as low as in Polar countries are met 

 with in winter. And particularly those plants that are not covered 

 with snow are exposed to very severe cold. The specific zero of species 

 must lie low. Yet the minimal temperatures in winter seem to exclude 

 certain species from only a few places, and without exercising any notable 

 influence on the character of the vegetation. The minimal temperatures 



* See Kerner, 1869; Schroter, 1904-8. ^ See Chap. XXI. 



