Functional Responses of Woody Prosopis caldenia Seedlings to Drought and Livestock Grazing in Semiarid Rangelands of Argentina

In the semiarid rangelands of Central Argentina overgrazing and drought alter the competitive balance between woody and herbaceous vegetation, favoring an increase in Prosopis caldenia. The objective of this study was to evaluate the effect of competition in perennial grasses and water stress on the relative growth rate (RGR) and related functional attributes of P. caldenia seedlings in sites with different grazing histories: a site exposed to long term (40 or more years) continuous grazing by cattle (grazed experimental site), and a long-term (around 30 years) exclosure (ungrazed experimental site). The total average RGR of the seedlings that grew in the grazed site was significantly higher, whereas the additional water supply favored those seedlings that grew in the ungrazed site. Significant relationships existed between the RGR and all the attributes measured, except in the ungrazed site where only those attributes related to leaf traits were correlated with the RGR. The above-and below-ground biomass of perennial grasses negatively affected the RGR of the P. caldenia seedlings in both experimental sites and soil moisture favored the RGR of the seedlings in the ungrazed site. Our work reveals the ability of seedlings of P. caldenia to take advantage of changes in environmental conditions, such as the release of resources due to overgrazing and to increase its RGR, which ensures its establishment.