SPRING PRECIPITATION AND HEIGHT GROWTH 689 



tality is usually very high. There is reason for believing that what has 

 been discovered in regard to western yellow pine will apply to other 

 tree species and to other forms of vegetation in this region, wherever 

 moisture is a critical factor. That moisture conditions during the 

 arid fore-summer exercise a profound influence upon vegetation is 

 shown by the way in which various plants have adjusted their growth 

 habits with respect to this period. A few perennials complete their 

 seasonal growth and mature their fruit before June i ; the great major- 

 ity, however, remain more or less dormant during the dry fore-summer 

 and do not begin active growth until the advent of the summer rains 

 in July. The same is true of annuals, many of which do not germinate 

 their seed until the summr rainy period. Deep-rooted perennials, in- 

 cluding forest trees, grow during the dry period of May and June, 

 although the seeds as a rule germinate only during the warm, humid 

 period of July and August. 



There are sites within the range of western yellow pine in this region 

 on which height growth apparently is independent of precipitation. 

 An investigation of these sites would probably show that the moisture 

 supply is normally abundant and that heat is the controlling factor. It 

 will probably also be found that the control does not lie in mean annual 

 air temperature. Possibly the relation will be found in the air tem- 

 perature for a relatively short period of the year ; more likely, however, 

 it will not be found in air temperature at all, but in a direct measure of 

 the heat absorbed by the leaves. 



