Streamside herbaceous vegetation response to hydrologic restoration on the San Pedro River, Arizona

Recovery of physical conditions and processes is increasingly emphasized in ecosystem restoration. We examined a restoration project aimed at recovery of groundwater levels and base flows on an undammed desert river. We sampled streamside plant communities and hydrology annually (2003-2008) at six restoration sites at two farms (Three Links Farm and H&E Farm) after groundwater pumping was curtailed for restoration purposes, and at six reference sites. Vascular plant cover was recorded, and species were classified into functional groups based on water needs and life history. Synthetic vegetation metrics and community composition were compared between restoration sites and perennial-flow reference sites representing target conditions. Perennial sites had higher cover, species richness, relative cover of hydric perennials and hydric annuals, and lower wetland indicator scores than non-perennial sites, but did not differ in relative cover of non-native species. Perennial sites had distinct species composition and high species constancy among years. Streamside vegetation was similar at Three Links Farm and perennial reference sites, indicating restoration success; streamside vegetation at H&E Farm differed from target conditions according to most measures. Hydrology is a key factor shaping desert streamside plant communities. However, single river sites may respond differently to hydrologic restoration. Contrasts in site response were likely due to differences in hydrogeomorphic context, exacerbated by drought. Understanding the constraints on restoration response is critical for setting realistic restoration goals and anticipating time frames of ecosystem change.