The goal of the proposed research is to experimentally evaluate the mechanisms contributing to the conversion of grasslands to shrubland. Woody species in the southwestern USA become established in disturbed grassland and act as foci for the recruitment of other woody species. The reasons postulated for the conversion are overgrazing, reduced grass competition, lack of fires, changes in climate, spread of seeds by livestock, or a combination of factors. Although many of these successional sequences have been described, and mechanisms postulated, experimental evidence supporting ther mechanisms are lacking. Historically, woody plants were thought to invade shrub-free productive grasslands; but more recently have been thought to be colonizers of highly disturbed grasslands. To partially test this postulate, a series of field experiments will be conducted which deal with competition between a woody increaser, Prosopis glandulosa (honey mesquite) and native grasses of the semi-arid savanna of southwestern North America. The hypothesis to be tested is that after woody plant establishment, the woody plants and the grasses obtain resources from different root zones, and competition between the two is significantly reduced. A competion experiment between P. glandulosa and a native grass, grown in different depth pots will be conducted. Statistically, the H0 would be that the reduction of P. glandulosa biomass would be equal across all treatments (depths), while the Ha would be that the reduction is unequal. If the reduction in P. glandulosa biomass is inversely correlated to depth, it would suggest that competition is more intense during the establishment stage of the woody plant, when both the grasses and woody plants have roots in the same zone. Once the woody plant root extends below the grass root zone, competition then decreases. Although this experiment is very restricted due to pot experiments, if results are only extrapolated to P. glandulosa establishment rather than competitive interactions after establishment, the experiment could offer worthwhile information.
