72 



OECOLOGICAL FACTORS AND THEIR ACTION sect, i 



CHAPTER XVIII. 



THE EFFECT OF A NON-LIVING COVERING 

 OVER VEGETATION 



A NON-LIVING covering exerts an action that depends upon, inter alia, 

 its looseness or compactness ; the looser it is the greater is its action 

 in the following respects : 



1. Water is sucked in, evaporation depressed, and moisture of 



the soil increased. 



2. Radiation is lessened. 



3. Fluctuations and extremes of temperature are diminished. 



In this relation there come into play two kinds of coverings : {a) snow ; 

 (6) fallen foliage and withered grass. 



(a) Snow 



It has long been known that snow can protect vegetation very effi- 

 ciently, and that it guards winter-crops from being frozen. In high alpine 

 situations falls of snow in summer would appear sometimes to protect 

 plants from exposure to the dry, cold weather, and consequent evapora- 

 tion that often set in after such falls of snow. In arctic countries every 

 patch of surface from which the snowy covering is blown away by winter 

 storms has vegetation different from that on snow-clad depressions ; 

 on the tundras of Lapland, for instance, Lecanora tartarea dominates 

 in places exposed to wind, whereas in more sheltered places fruticose 

 lichens can exhibit dense and tall growth.^ The distribution of the 

 covering of snow determines the distribution of entire and definite commu- 

 nities : some are protected at the expense of others ; the spots that in 

 winter are covered with snow usually show in summer the greatest number 

 of species and individuals. The snow-covering is thus of oecological impor- 

 tance. Snow seizes upon the countless particles of dust in the air, purifies 

 this, and collects other small organic and inorganic particles that the wind 

 brings. When snow melts these masses of particles are deposited on the 

 ground, and there is formed gradually a fine, fertile soil, which remains 

 stationary in gentle depressions, and entertains special species of plants. 

 This ' snow-patch flora' is subsequently alluded to in Chapter LXVII.^ 



The covering of snow influences plant-form. On the one hand, we may 

 here include the influence exerted by heavy loads of snow at high alpine 

 altitudes upon the shapes of trees and shrubs, whose stems are pressed 

 down into a prostrate position on the soil and lie flat on slopes ; for 

 instance, Pinus montana assumes the habit of elfin-tree or contorted 

 shrub, while Juniperus, Alnus viridis, Fagus sylvatica, and other trees 

 dwindle to form scrub, and spruce-birch-scrub develops in South 

 Greenland.^ On the other hand in Lapland Juniperus and Picea excelsa 

 become scrub ,^ in this case because all the twigs projecting above the snow 

 regularly die off, and the individual plants acquire low, plate-hke, or 

 umbrella-like crowns. 



* Kihlman, 1890. ^ See Schroter, 1904-8. 



* Kemer, 1863, p. 512 ; Rosenvinge, 1889 ; C. Schroter, 1904-8, p. 663 ; Szabo, 

 1907- * See the illustrations in Kihlman, 1890. 



