10 



BULLETIN 791, U. S. DEPARTMENT OF AGEICULTUEE. 



2.20 

 2.00 



/.eo 



«o /.60 



• 5 /.20 



I /-^^ 

 Q .60 



has become well established the percolation of moisture is exceedingly 

 slow; and, except after prolonged and heavy rainstorms, a 

 surprisingly small proportion of the moisture passes beyond the 

 densely matted soil stratum. Accordingly, the difficulty which other 

 species encounter in gaining a foothold and their practical failure to 

 compete successfully with the grass for the moisture essential to their 

 proper development and perpetuation account chiefly for the char- 

 acteristically pure stand of small wheat grass where its development 

 is undisturbed. 



The roots of the bunched species, slender wheat grass and blue 

 bunch wheat grass (fig. 2), extend approximately 3| times as deep 



into the soil as those of the small 

 wheat grass, the average maximum 

 depth being about 40 inches. Hence 

 a large proportion of the root- 

 absorbing surface of the bunch 

 grasses is well below the average 

 maximum depth of that of the 

 turfed species. There is no appre- 

 ciable diiference in the root charac- 

 teristics of the two bunch grasses 

 under consideration. The distance 

 between the bunches varies from a 

 few inches to several feet, depend- 

 ing upon the moisture and other 

 physical conditions. However, re- 

 gardless of the distance between 

 the bunches, provided the tj'pe is 

 fully developed, there is relatively 

 little diiference in the character, 

 density, and luxuriance of the other species which inhabit the inter- 

 vening space, the normal stand of which is usually sparse. 



CONDITIONS OF GROWTH AND REPRODUCTION. 



While the wheat grasses thrive under a considerable range of con- 

 ditions, their oj^timum development is reached only where the soil is 

 reasonably well decomposed and in a fairly high state of productivity 

 and where sufficient moisture is available to supply vigorous plants 

 during the first half of the growing season. 



In the area under observation there is usually ample precipita- 

 tion early in the spring of the year to saturate the soil (see Table 1 

 and fig. 3). Occasionally, however, the rainfall in June and in 

 the first half of July is so light that the soil contains insufficient 

 moisture for the promotion of vigorous growth.^ 



.$. .eo 



.20 

 O 





































































1 











1 











i~ 











1 





















1 







!~ 



June 



Ju/y 



Auq. 



Sept. 



Fig. 3. — Average monthly precipitation 

 in the wheat-grass formation during 

 the growing season, 1914-1917, in- 

 clusive. 



1 Growth in the subalpine zone begins about the last week of June. 



