Long-Term Grass Yields following Chemical Control of Honey Mesquite

Abstract

Long-term herbaceous response data following herbicidal treatment of honey mesquite (Prosopis glandulosa Torr.) are needed to develop more accurate projections regarding economic feasibility of these treatments and to model ecological interactions between woody and herbaceous plants in rangeland systems. Our objective was to measure herbaceous yield and mesquite regrowth 10 or 20 years after mesquite was aerially sprayed with either mesquite top-killing or root-killing herbicides. Treatments evaluated included mesquite top-killing herbicides at 10-12 years (T10) and 19-21 years (T20) post-treatment, mesquite root-killing herbicides at 10-12 years (R10) and 19-21 years (R20) post-treatment, and an untreated control where mesquite were >30 years old (C30). Treatments were applied in the late 1970's or late 1980's. Grass yields, measured annually from 1998 through 2000, were quantified within patches of 3 perennial grass functional groups: cool-season mid-grasses, warm-season mid-grasses, or warm-season short-grasses. Coolseason annual grass yields were also quantified within these perennial grass patches. By 1998, mesquite canopy cover was 55, 47, 36, 24, and 12% in C30, T20, T10, R20, and R10 treatments, respectively. Warm-season mid-grass yields were most sensitive to differences in mesquite cover in all 3 years and declined sharply when mesquite cover exceeded 30%. Cool-season midgrass yields declined slightly with increasing mesquite cover. Warm-season short-grass and cool-season annual grass yields were not related to mesquite cover, except in 2000 when warmseason short-grass yield beneath mesquite canopies increased with increasing mesquite cover. Results suggest that herbicide treatment life (defined by increased perennial grass yield in response to mesquite treatments) was at least 20 years for the root-killing herbicide, but no longer than 10 years for the topkilling herbicide.

Utility of Satellite and Aerial Images for Quantification of Canopy Cover and Infilling Rates of the Invasive Woody Species Honey Mesquite (Prosopis Glandulosa) on Rangeland

“…Therefore landscape scale management decisions could be significantly compromised if they were based solely on mesquite cover and grass production estimates from the 30-m images. Similar errors are possible when assessing other management variables, such as amount and extent of mesquite cover for wildlife habitat, amount of open grassland for grassland bird habitat, or woody based carbon or bioenergy stocks [11,17].…”

“…For example, mesquite at 10% cover-as determined for the entire study region by the 1-m image-does not negatively affect grass production significantly, but it could significantly reduce grass production at 19% cover, as determined by the 30-m image. If the 1-m cover value was slightly higher (e.g., 15-20%), then these comparisons would be much more dramatic as [11] has shown there is a rapid drop off in C 4 mid-grass production when mesquite cover exceeds 25%. Therefore landscape scale management decisions could be significantly compromised if they were based solely on mesquite cover and grass production estimates from the 30-m images.…”

“…In only a very small portion of the mesquite cover spectrum, 10-20% cover, did we find a reasonable match in cover between the 1-m and 30-m images. Of greatest concern was that the greatest deviation of cover estimates from the 30-m image compared to the 1-m image occurred in the critical mesquite cover range of 20-50% ( Figure 5(c)) This is the range of mesquite cover where mesquite begins to negatively impact grass production [11,15] and thus any remote sensing technology that could be of benefit for detecting potential problem areas from a brush invasion and brush/grass competition viewpoint must be able to detect changes in cover within this range of covers. The 30-m images provided by Landsat 5 were unable to do that.…”

“…In addition, woodland thickets may have utility for ecosystem carbon storage and bioenergy uses [16][17][18]. Because these potentially detrimental and beneficial effects often become significant only at large spatial scales (e.g., impacting livestock grazing, watershed function, wildlife habitat) and at certain density and canopy cover levels [10,11], accurate and detailed mapping of woody distribution at such scales is essential for determining extant conditions and invasion rates and developing long-term management plans for these regions.…”

“…High densities of mesquite can negatively affect these lands by reducing grass growth and livestock production [7,10,11], altering nutrient and hydrologic cycling [12,13] and increasing potential for soil erosion [9]. In contrast, the presence of mesquite either in savannas or dense woodlands may be beneficial for wildlife habitat and herbaceous diversity [14,15].…”