ARCTIC VEGETATION. 



577 



the effect of environment is not wholly similar. The ground 

 ov'er a large part of the region is frozen to a great depth and only 

 thaws out on the surface during the summer. This makes the 

 ground-water usually very cold even during the summer. 



1065. Character of tree growth. The polar limit of tree 

 growth is conditioned by drying winds in frosty weather. There 

 is no sharp dividing-line between the forest and treeless waste, 

 but there is a gradual reduction in the number of the trees, their 

 size as well as their vigor. The formation becomes more and 

 more open, the trees are reduced more and more in size, and are 

 more and more disfigured by dead and dying branches and tops, 

 and by dwarfing and warping out of shape as one approaches 

 the true polar waste. The transition line is gradual, and the 

 two climatic formations are mixed. Where the snows are not 

 sufficient to cover the trees for several months during the winter 

 the tops of the trees die, often at quite a regular distance from 

 the ground. These trees often branch below and sprout from 

 below and thus form broad expanded low crowns. Where the 

 fall of snow is heavy the trees are often covered for several months 



Fig. 513. 



Polar limit of tree growth in Saghalin near Kamtchatka. Large trees stunted 

 and showing horizontal growth. Induced by high wind and weight of snow. 

 (After Schimper.) 



during the winter. The snow bends the tops over and the tree 

 grows more or less horizontally. The covering of snow and 



