PLANT SUCCESSION AND RANGE MANAGEMENT. 6 
regular replacement of one type of plants by another is found. This 
phenomenon, known as succession, is explained on the basis of cer- 
tain more or less distinct changes that take place simultaneously in 
the substratum and may be accounted for in various ways, probably 
the most influential and universal cause being the addition of hu- 
mus. 1 The plants themselves, by adding humus to the soil through 
the decomposition of their tissues, and in this way changing the 
physical and chemical composition of the soil, prepare the way for a 
new and higher form of life, hence in a way work out their own de- 
struction. Accordingly, quite different plant types are recognized 
on soils in different stages of formation. The characteristic types 
are shown graphically in figure 1. 
Beginning with the bare, consolidated rock, the first vegetation 
consists of such inconspicuous, uneconomic forms of plant life as 
algae and crustaceous lichens. These forms mark the initial or pio- 
neer stage of development. Occasionally, amid the somewhat thick- 
ened cushion of moss growth or in the crevices of the rocks, an early- 
maturing annual herb will find its way. This consociation of lichen, 
moss, and herb is characteristic of what may be termed the "transi- 
tional" stage of development ; and so far as humus, soil moisture, and 
wide spacing of the herbaceous plants are concerned, it is not dis- 
similar to desert conditions. Like annual plants of the desert, the 
initial herbs must be able to germinate and grow to maturity in the 
shortest possible period and with the use of a minimum amount of 
moisture. 
At the advent of the first-weed stage, which, typically, is charac- 
terized by a semidecomposed soil, poor in organic matter and rela- 
tively low in available moisture content, there is a distinct predomi- 
nance of shallow-rooted, early-maturing annuals. At first widely 
scattered, the annuals gradually become more numerous, so that 
finally, as more and more soil is preempted, there is a cover well- 
nigh completely clothing the soil surface during the period of maxi- 
mum seasonal development (PL I). As soon as this vegetation 
has reached maturity, or when growth has been arrested by frost or 
other adverse conditions, the greater portion of the soil surface is at 
once exposed. It is then that the casual observer notes, possibly for 
the first time, that a few aggressive, drought-resistant, 2 short-lived 
perennial grasses and weeds have invaded the habitat. This stage of 
development affords a small amount of inferior forage if utilized at 
the proper time. 
1 This statement refers only to the evolutionary development of vegetation. The trans- 
formation from a complex to a more simple or earlier vegetational stage will be consid- 
ered later. 
2 A drought-resistant plant, as here used, implies a species which is a conservative user 
of water and which can complete its growth cycle under conditions of low available soil 
moisture content and under trying atmospheric conditions. 
