Response of velvet mesquite to groundwater decline

Mesquite Prosopis velutina bosques are groundwater-dependent riparian woodlands that were once widespread in the American Southwest. Groundwater was withdrawn from the aquifer below an ephemeral creek in the Sonoran Desert of Arizona. Temporal and spatial varation in plant water potential, leaflet size, leaflet number, canopy height, and live and dead vegetation volume all indicate that the bosque requires a shallow aquifer and that bosque traits change continuously with groundwater depth. The bosque had high water potentials, large leaflets (>7 cm²), tall stature (>12m), and large vegetation volume (>2 m³/m²) only where the water table was <5m below the surface. Summer rains and seasonal surface flow temporarily reduced water stress and increased leaflet size for some trees, but did not offset effects of groundwater decline. Trees in areas of greatest groundwater decline (18-30m) were under sublethal stress, as evidenced by low stem water potentials (<-4MPa), reduced leaflet size (<5.5 cm²), and high levels of canopy mortality (>45%). These deepest groundwater levels are in the range of those lethal to mesquite in other bosque systems. -from Authors