TWO RACES OF ASPEX 



Bv F. S. Baker 



U. S. Forest Service, Ogden, Utah 



In the mountain ranges running north and south through the State 

 of Utah, apsen (Populns trcmnloides) is an exceptionally common 

 tree at middle elevations, covering large areas with solid bodies of 

 pure type, particularly in the Wasatch Mountains and the Wasatch 

 and Fishlake Plateaus. 



In the spring when the leaves are unfolding it is commonly noted 

 that the leafing time of adjacent bodies is quite dissimilar. Not in- 

 frequently a line can be traced for upwards of a mile through the 

 aspen where on one side the leaves are half developed and on the 

 other the buds are just bursting. This line is almost invariably clear 

 cut and distinct with little intermingling of the two forms. The 

 boundary is apparently not determined by topography or soil conditions, 

 as it may run up and down slopes or across them, or perhaps more 

 frequently it will pursue a serpentine course entirely independent of 

 topograph}-. 



While spring leafing is the most striking feature in which these two 

 forms of aspen dififer, there are also other characteristics which are 

 apparent. The leaves on the trees which come out first last longest in 

 the fall, as a rule, although there is less consistency in this manner 

 than spring development, due apparently to the very uneven action 

 of frosts in a mountainous country. 



Furthermore the trees that leaf out earlier than their neighbors 

 are characterized by a somewhat yellowish or brownish tinged bark 

 in contrast to the pure white or light greenish tinged bark of those 

 that leaf out later. The difiference is least pronounced at lower ele- 

 vations where it may be almost imperceptible, but higher in the moun- 

 tains the difference is much more marked, the "yellow" irees taking 

 on a much deeper cast. The "white" trees seem to be m.ore widely 

 distributed at low elevations, while the highest bodies that run up 

 almost to 10,000 feet as thickets of scrubby trees, are almost invariably 

 "yellow." As a rule, when seen side by side the "yellow" trees appear 

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