20 NORTH AMERICAN FAUNA 61, FISH AND WILDLIFE SERVICE 



length of Alaska Peninsula. Spruce trees that were planted on 

 Unalaska Island grew, but did not spread. It may be significant 

 that tall willows, alders, and tall salmonberry have extended 

 westward as far as Unimak Island in heavy thicket form. Here, 

 salmonberry thickets are similar to those seen in southeastern 

 Alaska. These facts may be indicative of a territory ripe for a 

 forest. And such an advance line, based on climatic limitations 

 rather than on the present position of the forest edge, may be 

 considered to be the boundary of the Hudsonian Life Zone. The 

 biome concept, to rely strictly on the climax end product, would 

 have the same difficulty here, and published maps of the Tundra 

 Biome and the Tundra-Coniferous Forest Ecotone for this area 

 would simply substitute these terms for Arctic and Hudsonian 

 Zones. There is the same potential boundary difficulty. 



Granted that in the Boreal Zone, at least, tree growth is di- 

 rectly affected by the climatic factors usually associated with the 

 life-zone concept, to what extent is the rest of the biota affected 

 by the same influences? To what extent is it influenced by the 

 mere presence of trees? It is reasonable to believe that the woody 

 plants that comprise the understory of the Alaskan forest are, 

 to a large extent, dependent on association with trees. Some 

 forms, such as blueberries, often extend from open country into 

 scattered forest. But there is a plant association that coincides 

 with forest growth. 



Similarly, there is a fauna that has become specialized for 

 forest habitat — woodpeckers, certain grouse, certain warblers, 

 jays, squirrels, black bear, and many others. These appear to be 

 limited by the mere presence of trees. There is good reason to 

 believe that wapiti and other deer would have a much more 

 northern distribution if it were not for the physical barrier of 

 deep snow in winter. On the other hand, the red-backed mouse, 

 the hermit thrush, and the chickadee have inhabited the length 

 of Alaska Peninsula. It is possible to assume that these mobile 

 woodland forms simply would not wait for the slow-moving for- 

 est and thus have adapted themselves to more-open habitats 

 than is normal for the species. Also, this would imply a less- 

 specialized response to habitat than some of the other forest 

 species, as well as a greater sensitivity to direct climatic stimuli. 

 Birds and mammals are more or less adaptive and vary between 

 wide extremes in tolerance of adverse elements in their environ- 

 ment. However, there is a strong tendency for the majority of 

 any population to be associated with the distribution of certain 

 major vegetation types. 



