6 BULLETIN 791, TJ. S. DEPARTMENT OF AGRICULTURE. 
can support only vegetation characteristic of the first-weed stage; 
in still more common instances it may 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 y > 
(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. 1 
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 q 
or other adverse factors. On level areas, if they are not subject to 
severe wind 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 
upper, 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 PL 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. 
