Herbaceous Biomass Dynamics and Net Primary Production following Chemical Control of Honey Mesquite

The effect of honey mesquite (Prosopis ghndulosa var. glanhlosu Torr.) control on herbnceous growth dynamics, forage production, and root and crown biomass was investigated in 1979 and 1980 on a site aerially treated with a 1:l mixture of 2,4,5-T plus picloram at 0.6 kg/ha in May 1974. Density, height, and canopy of honey mesquite trees 5 years after treatment were 248 plants/ha, 0.9 m, and 3.1%, respectively, compared to 963 plants/ha, 2.2 m, and 34.670, respectively, in the adjacent untreated control plot. Yet, there were no differences between sprayed and untreated plots after 6 and 7 growing seasons relative to species composition, growth dynamics, and production of herbaceous plants. Averaged across years and treatments, estimated aboveground net primary production was 2,525 kg/ha. Crown and root biomass in the top 10 cm of the soil profile averaged 685 and 3,837 kg/ha, respectively, with no significant treatment or year effects. Lack of treatment difference partially validates a conceptual model presently used for economic analysis of herbicide sprays for honey mesquite control. Further, it supports the hypothesis that honey mesquite trees provide critical habitat for the more productive midgrasses indigenous to this site; and that elimination of this habitat in sparse stands of the shrub subsequently limits post-treatment herbage response. Dense stands of honey mesquite often suppress herbage production with the degree of suppression primarily a function of stand density and the innate productivity potential of the treated site (Scifres and Polk 1974, Dahl et al. 1978, McDaniels et al. 1978, Authors are associate professor and former graduate research assistant, Texas Agricultural Experiment Station, Vernon; and Thomas M. O’Connor professor, Department of Range Science, Texas A&M University, College Station. Appreciation is extended to the E. Paul and Helen Buck, Waggoner Foundation, Inc. for financial support and the W.T. Waggoner Estate Ranch for study area and treatment installation. Report is published with approval of the Director, Texas Agricultural Experiment Station, as TA 20218. Manuscript accepted 9 May 1985. JOURNAL OF RANGE MANAGEMENT 39(l), January 1988 Brock et al. 1978, Jacoby et al. 1982, Bedunah and Sosebee 1984). Degree of suppression is difficult to quantify because of spatial and temporal variation in herbage response following mesquite control. Major factors affecting herbage response are initial stand density, method of control, effectiveness of control, condition and composition of understory vegetation at the time of treatment, and post-treatment climatic and livestock grazing conditions (Scifres 1980). Research in the Rolling Plains of Texas has indicated that the increase in herbaceous standing crop during the first 3 to 4 growing seasons following control of honey mesquite is generally accompanied by a shift in overall species composition toward more productive species (McDaniel et al. 1978, McDaniel et al. 1982). However, longer-term herbage responses in this region relative to seasonal growth dynamics, species composition, and aboveground net primary productivity (ANPP) have not been addressed. The objective of this study was to quantify these effects 6 and 7 growing seasons after treatment. Our central hypothesis was that control of honey mesquite would continue to enhance herbage production 6 and 7 years post-treatment. Materials and Methods Study Area The study area was located 37 km southwest of Vernon, Texas, on the W.T. Waggoner Estate Ranch (3Y’SO’N., 99O25’W.; elevation 384 m). Climate is continental and semiarid. Mean annual precipitation at Vernon is 65.2 cm. Peak rainfall months are May (11.9 cm) and October (7.7 cm). The average frost-free growing season is 224 days extending from late March to November (Koos et al. 1962). Mean annual air temperature is 17O C. Average daytime maximum and minimum temperatures range from 36O C in July to -2.F C in January (U.S. Dep. Comm. 1980). The study was conducted during the 1979 and 1980 growing