Draw 23-30" PZ

Description

The Midgrass Prairie Community (1.1) was an open prairie community dominated by midgrasses. Trees such as hackberry, western soapberry, American elm and Osage orange grew along the drainage ways and areas protected from fires. The most common midgrasses are thought to have been sideoats grama, vine mesquite, cane or silver bluestem, meadow dropseed, Arizona cottontop, and white tridens. Buffalograss and blue grama were common shortgrasses with lesser amounts of plains bristlegrass and sand dropseed. Texas wintergrass, Canada wildrye and western wheatgrass were important parts of the midgrass and cool-season grass component. Alkali sacaton was common where soil salt accumulations were high and sedges were present in most depressions. Forbs included Engelmann daisy, Maximilian sunflower, Illinois bundleflower, bush sunflower, and annual forbs. Shrubs and vines were scarce but probably included fire resistant species such as catclaw acacia, bumelia, ephedra, greenbriar and lotebush. (See plant community composition table below for complete listing of probable climax species.) The Midgrass Prairie Community (1.1) produced as much as 5000 pounds herbage in good moisture years and as little as 2000 pounds in dry years. Grasses and forbs contributed up to 90 percent of the total annual production in reference conditions.

The Mixed-grass Prairie Community (1.2) is a grassland community being invaded by woody species that had previously been held at low densities by repeated fires, infrequent droughts and competition from a vigorous grass component. With continuous heavy grazing the more palatable plants are consumed more readily than less palatable ones. When they are subjected to repeated over-utilization they lose their competitive advantage and are replaced by less palatable or more grazing resistant species. Western wheatgrass and Canada wildrye generally decline with overgrazing. Most of the perennial forbs remain in this plant community, although in lesser amounts. Saline conditions may occur in overgrazed areas, resulting in increased amounts of alkali sacaton. Salt cedar can invade this site where saline conditions develop. The encroaching woody species are generally less than four feet tall and subject to control by prescribed burning in conjunction with proper stocking and grazing management. The woody canopy varies between five and 15 percent depending on length and severity of grazing, timing and frequency of fires and seed availability of invading species. Typically, mesquite, pricklypear, lotebush, wolfberry and bumelia were early and persistent encroaching woody species. Annual primary production is reduced slightly relative to the reference community, ranging from 2000 to 4000 pounds per acre.

Submodel

Description

The Shortgrass/Mixed-brush Community (2.1) is comprised of woody species producing 15 to 35 percent of the annual production. The diversity of the grassland component has declined while unpalatable woody plants and forbs increase. All, but the more palatable woody species, have increased in size and density. The typical woody plant cover is a mesquite dominant overstory with pricklypear, lotebush, ephedra, and lotebush as common understory shrubs. Tobosa dominates the herbaceous layer in the interspaces between trees and shrubs in the western portion of the MLRA where it was present originally. Buffalograss becomes dominant elsewhere. Remnants of reference community grasses and forbs and less palatable species such as alkali sacaton, vine-mesquite, meadow dropseed, threeawn, silver bluestem, white tridens and annuals occur in the woody plant interspaces. Some forbs are found in this community. Cool-season grasses, such as Texas wintergrass and Canada wildrye, are persistent under and around woody plants where shading occurs and they are protected from grazing. Annual herbage production ranges from approximately 1800 to 3750 pounds per acre.

The Mixed-brush/Shortgrass/Annuals Community (2.2) is a usually a mesquite-dominated shrubland with hackberry and American elm trees along the stream line. Other trees such as western soapberry, Osage orange or oaks may be scattered throughout the drainage. Salt cedar is often invasive in wetter places. Junipers may also invade the site. Remnants of the climax grassland vegetation, mostly tobosa (in western portion) and/or buffalograss, cool-season grasses and annuals, occupy the shrub interspaces. Alkali sacaton is found where saline conditions have developed. As this plant community nears maturity, trees and shrubs provide 75 percent or more canopy cover and dominate biomass production. Annual primary production is approximately 1800 to 4000 pounds per acre.

Submodel

Description

Site is converted into cropland or pastureland using native or introduced grass species. Land is also abandoned and let go back into a brush invaded community.

Submodel