in places where they used to grow, even if the cUmate has changed so much that it 

 is no longer quite favourable to the forest in this place. The flora is, on the whole, rather 

 resistent in this respect. Thus, a floral constituent is apparently more apt to continue 

 in such a place where it has once been growing than, under the same extreme con- 



Fig. 41. Drying up forest at an altitude of about 900 m. above sea level on the south side of 



the Sayansk mountains. 



ditions, to invade as a new and extraneous element in the place concerned. But if the 

 forest under such unfavourable conditions, from some cause or other, is impaired still 

 more or entirely expelled, it has generally not strength enough to invade that place 

 unaided. Here in Norway, where at present the tree limit is sinking, the conditions are 

 quite analogous. Stumps of trees in the bogs are indications that the forest formerly 

 extended much farther up, to 2 — 400 meters higher up the mountains, above the present 

 tree limit, and also in many places here, dying forest is to be found towards the tree 

 limit. Here, as well, it is often encroachments from outside, especially reckless felling 

 of trees which has inaugurated the destruction, and has the forest so high on the moun- 

 tains disappeared, it does not return. Only in one of the valleys of Norway the area 

 of forest has, by the sinking of the tree limit within historic times, been subjected to a 



69 



