PLANT SUCCESSION AND RANGE 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 {Agro-pyron scrihneri), onion grass {Mellca 

 huJhosa)^ 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- 

 nium viscosissimuTn), sneezeweed {Helenium Iwopesii), rubberweed 

 {Hymenoxys fioribtrnda) , Sampson's mertensia {Mertensia samp- 

 sonii), cinquefoil {Potentilla fllipes)^ false cymopterus {Pseudocy- 

 mopterus tidestromii) , and butterweed {Senecio columbiana) ?- 



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 



