Hydrologic Characteristics of Vegetation Types as Affected by Livestock Grazing Systems, Edwards Plateau, Texas

Infiltration rate and sediment production were assessed in oak, bunchgrass and sodgrass vegetation types in moderate continuous (MCG), heavy continuous (HCG), and intensive rotation (shortduration, SDG) grazing systems and in a livestock exclosure (LEX). Infiltration rate was related to the total organic cover and bulk density characteristics of the site (RJ = 36). The amount of cover was more important than type, indicating that protection of soil structure from direct raindrop impact was the primary function of cover on infiltration. The SDG and HCG pastures had lower total organic cover with correspondingly lower infihration rates compared to the MCG and LEX pastures. Bulk density, an indicator of soil structure, was significantly lower in oak mottes than in the grass interspace, but there was no significant difference between pastures. Sediment production was related to the total aboveground biomass and the bunchgrass cover of the site (P q .79). Obstruction to overland sediment transport and protection from the disaggregating effect of direct raindrop impact were the primary functions of the total aboveground biomass and bunchgrass cover. Total aboveground biomass was greatest in the oak motte and least in the sodgrass interspace, consequently the sodgrass interspace had the greatest amount of sediment production and the oak mottes had the least sediment production. Midgrass cover and total aboveground biomass in the MCG and LEX pastures was significantly greater than in the SDG and HCG pastures; thus sediment production from the MCG and LEX pastures was significantly lower than from the SDG and HCG pastures. The hydrologic condition of a range site is the result of complex interactions of soil and vegetation factors. Infiltration rate and sediment production integrate these factors and are good indicators of hydrologic condition. The type of livestock grazing system and stocking rate differentially impact soil structure and vegetation growth in different plant communities. Successional trends in plant communities were directly proportional to grazing intensity, with the most severe changes occurring under heavy grazing (Ellison 1960). Palatable species decline as grazing pressure increases and are replaced by shrubs or other vegetation which are less preferred by livestock and more resistant to grazing (Dyksterhuis 1949). Perennial bunchgrasses are especially important indicators of rangeland condition in the Edwards Plateau of Texas. Many bunchgrass species are palatable and nutritious for livestock and provide good soil stabilization. However, many bunchgrasses have aboveground apical meristems and are not tolerant of repeated heavy grazing. Rich and Reynolds (1963) found that perennial bunchgrass basal cover was reduced by heavy grazing but was unaffected by moderate grazing. Rhoades et al. (1964) Sharp et al. (1964) and Dunford and Weitzman (1955) found that heavily grazed pastures were dominated by sod-forming grasses whereas bunchgrasses dominated moderate and non-grazed pastures. The authors are presently range research scientist, P.O. Box 2954, Mogadishu, Somalia; professor of watershed management, Department of Range Science, Texas A&M University, College Station 77843; and superintendent of the Texas Agricultural Experiment Station at Sonora 76950. Send reprint requests to second author. At the time of research, the senior author was graduate researchassistant, Department of Range Science, Texas A&M University. The authors would like to thank the personnel of the Texas Agricultural Research Station at Sonora and Ronnie Anson for their help during this study. This study was a cooperative project between U.S. Department of Agriculture and Texas Agricultural Expenment Station. Financial support of the Soil Conservation Service RCA Special Study and a Science/ Education Grant is acknowledged. Published with the approval of the Director, Texas Agricultural Experiment Station as TA 20746. Manuscript accepted 29 May 1986. JOURNAL OF RANGE MANAGEMENT 39(6), November 1966 Infiltration rates are generally observed to be highest under trees and shrubs, followed in decreasing order by bunchgrasses and sodgrasses (Smith and Leopold 1941, Woodward 1943, Reed and Peterson 1961, Box 1961, Blackburn 1975, Wood and Blackburn 1981, Knight et al. 1984). Each life form responds differently to grazing pressure and range improvement practices. The extent to which these rangeland uses alter the vegetation composition is a prime factor determining the effects on the soil structure and hydrologic condition of the site. The objective of this research was to study oak, bunchgrass, and sodgrass plant communities to determine how their infiltration rate and sediment production differ from each other. Also, the infiltration rate and sediment production of each of the 3 plant communities was studied to determine how the effects of livestock grazing systems differ for each plant community under moderate continuous, heavy continuous, and intensive rotation grazing systerns and livestock.