Factors regulating the ingress and establishment of a woody plant (Prosopis glandulosa) in perennial grasslands

Typescript (photocopy).A series of experiments were conducted to quantify the extent to which dissemination and establishment of mesquite is enhanced by activities of domestic livestock. In the Rio Grande Plains of south Texas, dispersal of mesquite seeds and emergence and establishment of seedlings were substantially enhanced by cattle. Survival of mesquite seedlings on dunged microsites ranged from 75 to 95% among five cohorts initiated over two growing seasons. Based on extensive surveys, densities of mesquite seeds and seedlings were substantially higher on areas frequented by cattle. Seedling densities were highest away from adult plants, suggesting that transport of seeds away from parent plants and host-specific predators may enhance recruitment. For both the Post Oak Savanna and Rio Grand Plains resource areas, the role of herbaceous interference in limiting mesquite establishment was minimal. On both study sites survival over two years averaged over 75% during periods of normal to slightly below-normal precipitation, suggesting that mesquite seedling establishment is not episodic with regard to soil moisture. Physiological measurements in south Texas showed that within one year, mesquite seedlings had developed root systems to depths beyond those exploited by the herbaceous vegetation. Photosynthesis and conductance of one-year old seedlings was also related to moisture levels at these deeper depths. Substantial recharge of deeper soil horizons occurred after medium sized rainfall events on the south Texas site. Short-term regulation of mesquite seedling establishment by herbaceous vegetation is effective only as it affects emergence; survival was uniformly high on all vegetation manipulations in both resource areas. Threshold levels of aboveground biomass regulation of emergence were exceeded at low levels of defoliation on the protected Post Oak Savanna site. No thresholds were observed on the retrogressed stands in either the Post Oak Savanna or Rio Grande Plains site. The challenge to natural resource managers seeking to regulate encroachment of this woody plant into grasslands thus appears to lie with limiting the dispersal of the seed by domestic livestock, not in controlling herbaceous utilization