| Abstract: | Abstract Vegetation structure in semi-arid regions of northern Mexico and the southwestern USA has changed dramatically over the last century; shrubs such as mesquite (Prosopis sp.) have expanded into, and have become dominant in, ecosystems that once supported semi-arid grassland. The upper San Pedro River watershed, which extends from northern Sonora (Mexico) to southeastern Arizona (USA), highlights these changes. Between 1973 and 1992, grasslands decreased by 18% and the total area dominated by mesquite increased during the same period by 412%. In the areas where the density of mesquite shrub is medium to high, grass has completely disappeared under the shrub canopy and has been replaced by bare soil. Undoubtedly these cover changes have affected surface—atmosphere interactions by introducing a different partitioning of net available energy into sensible and latent heat flux. At the same time, different root system and root density and plant physiology have introduced a change to the surface water balance, reducing inflitration and increasing runoff. Our study addressed the sources of water used by mesquite (Prosopis velutina) along a natural gradient from an area of high mesquite cover with no herbaceous grass cover, to open savanna with high grass cover and widely spaced mesquite. The presented results from the SALSA 1997 monsoon-season campaign (July—October) show that mesquite shrubs that have access to both ground-water and surface water use surface water when available, potentially competing with grass for surface water and out-competing grass during successive periods of drought when only groundwater is available. |
