ALASKA FORESTS 237 



above sea level, and again at the head of Cook Inlet, 

 where the traders state that birch canoes are used by 

 the Indians on Knik River, without its apparent exist- 

 ence in intermediate localities. This distribution would 

 indicate a species from the interior that has crossed the 



range. 



The associated occurrence of the eastern Populus bal- 

 samifera with its western congener, Populus trichocarpa 

 (if indeed the two species can be separated), reported by 

 the late Dr. G. M. Dawson from the Pelly and Lewes 

 Rivers, and observed by us on Kadiak Island, is an 

 example of this interchange, and Pinus contorta (or 

 murrayana) reported by Dall at the confluence of the 

 above named rivers, furnishes another instance of wan- 

 dering. 



The greatest interest in regard to this approach or inter- 

 change of the two floras would center in the region around 

 Iliamna Lake, at the base of the Alaska Peninsula. Here 

 the Pacific coast flora finds its western terminus, and the 

 interior or Atlantic forest flora descends along the Mul- 

 chatna and Nushagak Rivers almost to the very shores of 

 Bering Sea, while the low passes between Cook Inlet 

 and Lakes Clark and Iliamna should favor transmigration 

 of the two floras, unless other impediments bar their 

 progress. 



While in a general way temperature and moisture con- 

 ditions are certainly the most influential factors deter- 

 mining the distribution of life groups, it must be evident 

 that with tree growth, combinations of these with factors 

 other than those which determine the distribution of an- 

 nual and perennial low growths, must be potent. The 

 winter rest in the seed, and the short cycle of develop- 

 ment in a single season, characteristic of the annuals; 

 the partial death in winter, and the low stature of the 

 perennials, warmly covered by the winter snows and pro- 



