. PLANT SUCCESSION AND RANGE MANAGEMENT. if 1 
given unit of dry matter than those grown on more fertile soils. 
This accounts for the colonization of the badly depleted soil by 
shallow-rooted, early-maturing, annual vegetation, and the practical 
exclusion of the deeper-rooted, later-maturing, perennial species. 
35. The further depletion of the soil tends to decrease the density 
of the stand and the luxuriance of growth of the individual speci- 
mens. If the depletion is continued until the underlying rocks are 
exposed, the pioneer stage of plant life — algae and lichens — again 
comes into evidence. 
36. A considerable number of the ruderal-weed plants are pal- 
atable to sheep, a few are grazed by cattle, and a very few by horses. 
Douglas knotweed, the most abundant species, is fairly palatable 
to sheep, and probably furnishes as much forage as, if not more 
than all the rest of the annuals put together. The carrying capacity 
of the ruderal-weed consociation is exceedingly low, and affords 
grazing only when the herbage is green and succulent. 
37. Because of the early-maturing qualities of the vegetation and 
the fact that the first-weed-stage cover affords poor protection of the 
watershed, only the very lightest grazing should be permitted on 
this type. The safest plan is to exclude stock until the cover has 
increased appreciably. 
THE EFFECT OF GRAZING ON RANGE PLANT SUCCESSION. 
38. Grazing may cause either progressive or retrogressive succes- 
sion, depending chiefly upon the closeness with which the herbage is 
grazed annually and the time of grazing. Grazing year after year 
before seed maturity causes retrogressive succession, while grazing 
every 3 or 4 years or so after seed maturity promotes progressive 
succession. 
39. Overgrazing year after year results not only in the destruc- 
tion of the ground cover, after which erosion is apt to occur, but 
robs the soil of its fertility and producing capacity. 
40. The rate and character of the colonization of an eroded or 
otherwise depleted area is normally determined by the degree of 
soil devastation. The longer retrogressive succession continues, 
therefore, the more serious is the depletion of the soil and the longer 
is the time, required to reestablish a good ground cover. The ulti- 
mate result of continued serious erosion is the exposure of the under- 
lying rock formation. 
41. The continued use of stock driveways and bed grounds results 
practically in complete destruction of the subclimax plant cover, thus 
favoring the establishment of plants of lower successional stages. 
The species constituting the cover from time to time afford reliable 
indicators of overgrazing and inferior states of soil productivity. 
