8 BULLETIN '791, U. S. DEPAETMENT OF AGKICULTURE. 



THE WHEAT-GRASS CONSOCIATION. 



The wheat grasses (Agropyron) , broadly considered, constitute 

 the climax herbaceous cover. In the vegetative cover as a whole, 

 however, the wheat grasses are the subclimax type, the timber spe- 

 cies, of course, constituting the true climax. 



In its unhampered development the wheat-grass consociation occu- 

 pies all well-drained timberless or sparsely timbered areas in the sub- 

 alpine belt, where the soil is well decomposed and of at least average 

 fertility. Turf-forming wheat grasses — that is, those that reproduce 

 largely by means of rootstocks — ^usually occupy the drier hillsides 

 and exposed flats; but where slightly more than average moisture 

 prevails during the growing season, the turfed species disappear and 

 the taller and deeper-rooted wheat grasses of the bunched habit of 

 growth become conspicuous. 



Owing to the variation in the rate of soil formation and in moisture 

 conditions, intensity of grazing, and other factors, the wheat-grass 

 subclimax is often patchy, and is frequently temporarily replaced by 

 rather distinct consociations of the lower successional stages. 



Like most drought-resistant grasses, the wheat grasses thrive best 

 in full sunlight. Accordingly, they are inconspicuous or entirely 

 lacking where the herbaceous type meets the true fir-aspen cover in 

 the lower reaches of the subalpine zone and, of course, in the dense 

 spruce-fir cover of the higher elevations of the subalpine type. 



Turfed and bunched wheat grasses are seldom associated, owing 

 chiefly to the difference in the character of their root systems and the 

 difference in the distribution of the moisture content of the soils 

 which they occupy (PI. II). Small wheat grass (Agropyron 

 dasystachyum) is the most common and typical of the turfed species 

 (PI. II). Slender wheat grass {A. tenerum) and blue bunch wheat 

 grass {A. spicatum) are the most conspicuous species of the bunched 

 habit of growth, violet wheat grass {Agropyron molcweum) being 

 next in order of abundance. Small wheat grass occurs on the drier 

 hillsides, exposed flats, and on ridges where the soil is in a relatively 

 high state of productivity ; while slender wheat grass and blue bunch 

 wheat grass, which are commonly associated, are largely confined to 

 areas rather too moist for the successful development of small wheat 

 grass, but not sufficiently moist for plants appreciably less drought- 

 resistant than the wheat grasses. Thus, the well-drained areas sub- 

 ject to the full play of the high winds peculiar to the elevated sum- 

 mer range are characteristically occupied by turfed species; while 

 habitats which are reasonably well protected from the wind and 

 devoid of barriers which tend to diminish the reception of the normal 

 rainfall are occupied by bunched wheat grasses. 



