1907] 



YOUNG— FOREST FORMATIONS OF COLORADO 



351 



U 



While 



Murraya 

 scopulor 



2400 to 2850 

 1650 to 2400 



m 



difficult to separate them in determining what factors exercise the 



influence in limitin 



these factors to two in answering the question ^' Why are zones present 

 in mountain forests?" The factors of light and humidity may be 

 rejected at once, since their variation at different altitudes is practically 



Fig. 12, — The Populus occidentalis formation in the valley of Boulder Creek, six 

 miles east of Boulder; approximate altitude 1590"^. 





negligible. The obvious answer is that mountain zonation is due 

 entirely to temperature. We have already seen that the diifercnce in 

 temperature between timber-line and the base of the mountain is 

 very marked and the same is true, even to a greater extent, of the 

 length of the growing-season. That water content may play as great 

 if not a greater role than temperature, however, is at least possible. 

 I have already noted the fact that the rainfall hi the high mountains 



about 25"^^ 



annu 



The part played, moreover, by the development of the formations 

 and their relation to the glacial history of the country may be very 



