GRAZING TYPES. 283 



Poa is not a true serai consocies, such as the annual Avena and Bromus. 

 Among other such consocies of importance are Plantago patagonica, Portu- 

 laca oleracea, Boerhavia torreyana, and Polygonum aviculare. These are all 

 indicators of disturbance, particularly overgrazing, but in the green condition 

 they also have more or less value as indicators of an available weed type. 

 Other indicators of disturbance are represented by such plants as Hilaria 

 mutica, Scleropogon brevifolius, Franseria, and Bulbilis. These occur in playas 

 or "swags" which are subject to flooding and in which a thin annual layer of 

 silt is often deposited as well. The first two are commonly associated, partly 

 owing to the fact that the disturbance of the Hilaria consocies by trampling 

 and overgrazing favors the spread of Scleropogon. Tobosa swags are typical 

 serai areas in the desert scrub as well as in the zone of savannah which lies 

 between this and the desert plains. In the latter particularly, Hilaria is a 

 characteristic subclimax, in which Scleropogon is usually an indicator of 

 grazing disturbance, frequently with a similar associate, Sporobolus auri- 

 culatus. Hilaria is an indicator of summer grazing, while the other two are 

 rarely grazed except under drought conditions. The playas of the southern 

 Great Plains are marked by a similar subsere, in which Franseria is the im- 

 portant early stage and Bulbilis the subclimax. Both of these are grazing 

 indicators, though the value of the Franseria is relatively small (plate 69). 



Fire indicators and grazing — The typical indicators of fire are trees and 

 shrubs, and they may have a direct or indirect relation to grazing. The 

 indicators may themselves be browsed, or they may be associated with layers 

 of herbs or shrubs which furnish feed. Grasses and other herbs may indicate 

 fire, but are usually associated with woody indicators or their relics. The 

 most important "burn" communities are pine forest, aspen woodland, chap- 

 arral, and savannah. In addition, there are grass and sagebrush parks which 

 also represent subseres initiated by fire. Savannah has already been con- 

 sidered, while the grazing value of grass parks is obvious. Sagebrush and 

 chaparral are primarily browse types, though they contain a larger or smaller 

 amount of grass or herbs as well. When young, aspen woodland furnishes a 

 large amount of browse, but it is chiefly valuable for the more or less luxuriant 

 ground cover. This changes with the course of succession from firegrass, 

 fireweed, and other pioneers to the characteristic mixed layer communities 

 of the mature aspen subclimax. The latter exhibits three chief grazing types, 

 herb, grass, and shrub, of which the first is the most common and the second 

 the most valuable. The pine communities which regularly indicate burns are 

 lodgepole and knobcone forests. The subclimax of lodgepole, Pinus contorta, 

 is much the most extensive and important, occurring in both the montane 

 and subalpine zones of the Petran and Sierran regions. The community of 

 knobcone pine, Pinus attenuata, is a similar fire subclimax, but it is confined 

 to southern Oregon and California. In the Rocky Mountains, the mature 

 lodgepole forest is almost completely without a ground cover, and hence pos- 

 sesses almost no grazing value. In its earlier stages, herb and grass associes 

 are well-developed, and for a time aspen scrub may form a typical stage. In 

 the Coast forest, Pseudotsuga and Larix are fire indicators and their commu- 

 nities exhibit herb and shrub layers in the early stages especially. 



