PLANT SUCCESSION AND RANGE MANAGEMENT. 45 
type of vegetation as do the new soils. 1 The cover consists essentially 
of ruderal or annual plants, mostly weeds. This colonization of the 
early- weed stage continues until enough organic matter has accumu- 
lated in the soil to favor the invasion and establishment of the sec- 
ond or late weed stage. 
The density of the cover varies greatly according to the character 
of the soil, the seed crop available for germination, and the growth 
condition of the current season; the cover may be quite dense one 
year and relatively sparse the next. When one good growing season 
follows another the cover is particularly dense and the individual 
plants are large ; also the pioneer species of the second or late weed 
stage usually begin to appear. A succession of dry years, on the 
other hand, brings forth a sparse stand of the first, or early weed 
stage, the plants of which are small; also the pioneer species of the 
second weed stage are usually absent. 
The most typical and abundant species of the first- weed stage are 
goosefoot or lamb's-quarters {Chenopodium album), slender-leaved 
collomia {Collomia linearis), tarweed {Madia glomerata), Tolmie's 
orthocarpus {Orthocarpus tolmiei), Douglas knotweed {Polygonum 
douglasii), and tansy mustard {Sophia incisa) (figs. 17 and 18). 
Less abundant, but usually associated with the above species are 
androsace {Androsaoe diffusa), gilia {Gilia micrantha) , peppergrass 
(Lepidium ramosissimuia), monolepis {Monolepis nuttalliana) , and 
knotweed {Polygonum aviculare). 
In general the first-weed-stage cover reaches maturity earlier than 
any other. Because of the shallow roots and the lack of conspicuous 
laterals, this cover is of little value in checking erosion or otherwise 
preserving the watershed. The root systems are essentially of the 
specialized or tap character, and are confined almost entirely to the 
upper foot of soil — indeed the roots of the majority of the species 
do not penetrate deeper than about 8 inches. Therefore, where the 
topography, soil, climatic conditions, and other factors favor torren- 
tial runoff, the ruderal-weed stage is of the least value of any in 
j)rotecting the watershed from erosion. 
CONDITIONS OF GROWTH AND REPRODUCTION. 
No group of perennial plants has as strong seed habits and is sub- 
ject to as few failures in seed production as the first- weed-stage spe- 
cies. While no systematic study has been made to ascertain the size 
and viability of the seed crop, observations and repeated germination 
tests indicate that the normal seed crop is unusually large and fertile. 
Germination tests of well-developed seed of Douglas knotweed and 
1 Soils heavily packed, which often takes place when stock (especially sheep) trample 
denuded or sparsely vegetated areas excessively, also commonly support only ruderal 
or annual plants. 
