PLA:tTT SUCCESSION- AND RANGE MAFAGEMEFT. 71 



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 — algse 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 fertilitj^ 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 drivewa,ys 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. 



