Quantifying the relationship of different grass functional groups to increasing woody plant cover is necessary to better understand the effects of woody plant encroachment on grasslands. This study explored biomass production responses of three perennial grass groups based on photosynthetic pathway and potential canopy height (C₄ short-grasses, C₃ mid-grasses and C₄ mid-grasses) to different percent canopy covers of the surrounding deciduous woody legume, honey mesquite (Prosopis glandulosa). Two methods were used to determine mesquite canopy cover, line-intercept and geospatial analysis of aerial images, and both were used to predict production of the 3 grass groups. Five years of grass production data were included in the mesquite cover/grass production regressions. Two years had extreme grass production responses, one due to drought and the other to high rainfall. Of the 3 remaining years, best-fit curves were negative linear for C₄ short-grasses and C₃ mid-grasses, and negative sigmoidal for C₄ mid-grasses using both cover determination methods, although slopes of the curves differed between cover determination methods. C₄ mid-grasses were more sensitive than the other grass groups to increasing mesquite cover. Loss of production potential when mesquite cover increased from 0–35% was 75.5, 28.7 and 23.2% for C₄ mid-grasses, C₃ mid-grasses and C₄ short-grasses, respectively. Moreover, production potential of C₄ mid-grasses under no mesquite cover was 3 and 6 times greater than C₃ mid-grasses or C₄ short-grasses, respectively. Spatial settings of the different grass groups in relation to mesquite tree size and size of intercanopy areas provided indirect evidence that the process of mesquite encroachment in the last 50 to 100 years may have negatively impacted C₄ mid-grasses more than the other grass groups. Results suggest that gains in grass production following mesquite treatment would be limited if the system has degraded to where only C₃ mid-grasses and C₄ short-grasses dominate.

Combination of data collected in the fiedl as well as analysis of aerial images of the research site.

These are all Excel spreadsheets.