732 



ECOLOGY 



illustrations of this habit are afforded by the spruce (Picea) and fir 

 (Abies balsamea) on the New England mountains, the white-barked 

 pine (Pinus albicaulis) on the Rocky Mountains, the foxtail pine 

 (Pinus aristata) on the San Francisco Mountains of Arizona, and 

 the mountain hemlock (Tsuga M ertensiana) on many mountains 

 from Oregon to Alaska (fig. 1053). In many alpine regions dw;irfed 

 alders, willows, and birches mingle with the conifers. The impene- 

 trability of the Krummholz is due in part to the multiplicity of scraggy 

 lateral branches, but more to the fact that these branches bend down 

 close to the ground and twist and turn in all directions. 



FIG. 1053. Alpine Krummhols, made up of gnarled and weather-beaten trees of 

 the mountain hemlock (Tsuga M ertensiana); note that the living branches bend down and 

 trail over the ground, forming a dense tangle; Mount Hood, Ore. Photograph by 

 MEYERS. 



The determining factors of the Krummholz. The factors determin- 

 ing the Krummholz probably are complex, though not as yei: ade- 

 quately tested by experiment. Probably the short, thick growth of 

 the stems is due chiefly to relatively high transpiration in proportion 

 to absorption, absorption being reduced because of the dryness and 

 the low temperature of the soil, while the transpiration often is ac- 

 celerated because of strong winds, atmospheric rarity, intense sun- 

 light, and low humidity. The multiplicity of branching is due to the 

 replacement of the terminal shoots by numerous lateral shoots, as 

 soon as the former reach a height where the excessive transpiration 

 makes further growth impossible; a prominent factor here is the depih of 

 winter snow, since branches above the snow level are exposed to trans- 

 piration for many months during which there is no absorption. Here, 



