PLANT SUCCESSION AND RANGE MANAGEMENT. 45 



type of vegetation as do the new soils.^ 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 Aveed stage are usually absent. 



The most typical and abundant species of the first-weed stage are 

 goosefoot or lamb's-quarters {Chenoijodium album) ^ slender-leaved 

 collomia {CoUoviia. Im-em'-is) , tarweed (Madia gloTiverata) , Tolmie's 

 orthocarpus {Orthocarpus tolmiei), Douglas knotweed {Polygonum 

 doufflasii), and tansy mustard (Soj^hla incisa) (figs. 17 and 18). 

 Less abundant, but usually associated with the above species are 

 androsace {Androsace diffusa)^ gilia {Gilia micrantha) , ipeppergrsiss 

 (Lepidium ra?nosissi?jvu//i) , monolepis {Monolepis nuttalUana), and 

 knotweed {PolygonuTn avlculare). 



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 

 protecting the watershed from erosion. 



CONDITIONS OF GROWTH AND REPRODUCTION. 



Xo 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 



^ 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. 



