8 BULLETIN 791, tf. S. DEPARTMENT OF AGRICtfLTtrRE. 
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 
(PL 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 violareum) 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. 
