PLANT SUCCESSION AND KANGE MANAGEMENT. 33 
no longer bind the soil firmly. The usual increase in the available 
soil water resulting from the lowered absorption and transpiration 
power of the former cover greatly favors the germination, growth, 
and reproduction of the perennial weeds. Generally, however, where 
% the cover has been below normal in density for a number of years, 
the humus content, and, indeed, the water-holding power of the soil 
is lower than on the more densely covered lands, so that germina- 
tion and invasion of the species which immediately precede the por- 
cupine-grass-yellow-brush consociation is less vigorous than where 
the grass-brush stand has just been destroyed. 
In the degree of invasional aggressiveness, little difference has been 
observed among the three dominants; all have fairly strong seed 
habits. Once the plants are well established, however, yarrow easily 
leads in the rate of spread, the blue foxglove being the least aggres- 
sive, and the sweet sage rather intermediate between the two. This 
behavior is significant in view of the fact that the rootstocks of the 
species concerned are practically equal in length. 
As to the longevity of the dominant species few data are available. 
Owing to the relatively high palatability of the yarrow, however, 
this species is usually the first to give way under excessive grazing. 
Because of the slight difference in moisture requirements, the 
three species are often closely associated, but they seldom occur in 
equal density. When fully developed the plants form a matlike 
growth more or less pure in stand. The most luxuriant and perma- 
nent stand of blue foxglove is found on protected, moderately moist 
habitats, while yarrow and sweet sage occur in rather close associa- 
tion in somewhat drier situations. 
Regardless of the density of the cover of the foxglove-sweet-sage- 
yarrow consociation, a scattered stand representing a large number 
of secondary species occurs in association with the dominants. 
Among the grasses occasional specimens of large mountain brome 
grass (Bromus marginatus), nodding brome grass (B. porteri), 
Scribner's wheat grass (Agropyron scribneri), onion grass (Melica 
bullosa), and showy onion grass (M. spectabilis) are characteristic. 
Of the more conspicuous nongrasslike plants, the most typical are aster 
(Aster frondeus), horsemint or giant hyssop (Agastache urtici- 
folia), mountain dandelion (Crepis acuminata), geranium (Gera- 
, mum viscosissimum) , sneezeweed (Helenium hoopesii) , rubberweed 
(Hymenoxys floribunda), Sampson's mertensia (Mertensia samp- 
sonii), cinquefoil (Potentilla filipes), false cymopterus (Pseudocy- 
mopterus tidestromii), and butterweed (Senecio columbiana) . 1 
The root systems of blue foxglove, sweet sage, and yarrow are 
largely superficial in character, so that the water supply is derived 
1 Congeneric species of the plants here named occur in varying abundance. 
112655°— Bull. 791—19 3 
