6 BULLETIN 791, TJ. &. DEPAKTMElSTT OF AGRICULTUEE. 



can support only vegetation characteristic of the first- weed stage; 

 in still more common instances it ma}^ support an admixture of 

 annual and perennial weeds of the first and second vegetational 

 stages. 



While changes in the ground cover from a more or less permanent 

 (subclimax) type of high forage value to an unstable or temporary 

 one of low forage value, may be brought about in many ways, over- 

 grazing or other faulty management is usually accountable for the 

 retrogression in the vegetation on range lands as a whole/ 



The grazing of live stock may either appreciably change the orig- 

 inal palatable vegetation, for instance, transforming a pure grass 

 cover to a mixed grass and weed consociation; or it may cause an 

 entirely new plant cover to come in, as is almost invariably the case 

 on denuded grazing lands. The character of the vegetation follow- 

 ing denudation is largely determined by the topographic features 

 and the seriousness of the depletion of the soil as a result of erosion 

 or other adverse factors. On level areas, if they are not subject to 

 severe wiTid or sheet erosion, the climax vegetation is sometimes 

 destroyed without appreciably changing the fertility of the soil or its 

 available water content. Where the fertility of the soil is not appre- 

 ciably lowered, the higher type of vegetation reappears without the 

 more primitive forerunners, or the intervening successional stages 

 are short-lived and more or less intermixed with the climax species. 

 But on the hillsides or other exposed, readily drained lands, where the 

 Tipper, fertile layer of soil has been much depleted and its water- 

 holding capacity greatly decreased, and a large proportion of the 

 soluble salts and other plant foods carried with the water down the 

 drainage channels, the plant cover is thrown back to shallow-rooted, 

 early-maturing annual herbs, similar to those characteristic of the 

 first- weed stage (fig. 1 and PI. I). 



The time required for thorough revegetation of lands where retro- 

 gressive succession has taken place is approximately in direct propor- 

 tion to the degree of depletion of the soil, hence to the stage of vege- 

 tation which the soil is capable of supporting, so long as the climatic 

 conditions, topographic features, and type of soil remain the same. 

 On range lands the rate of progressive development, or revegetation, 

 may be greatly expedited' by cropping the herbage in such a manner 

 as to interfere as little as possible with the life history and growth 

 requirements peculiar to the different successional plant stages. Ac- 

 cordingly, the best results in promoting progressive succession are 

 obtained where the season of grazing is determined on the basis of 



1 Factors such as the formation of a road or trail, the colonization of a prairie dog 

 town, and the like, may greatly change or even destroy the vegetative cover, but the 

 effect of such factors is seldom far-reaching economically as compared with faulty man- 

 agement of live stock. 



