Riparian ecosystem restoration during drought in the US southwest

Will restoration of the key hydrologic processes shaping arid-region riparian ecosystems result in vegetation change at degraded sites, and at what time scales? How might multi-year drought impact these processes? We investigate these questions on the un-dammed lower San Pedro River (LSPR) in southeastern Arizona, USA. Many LSPR reaches have experienced increased depth to groundwater and decreased surface flow duration due to groundwater pumping, resulting in loss of herbaceous wetland plants and Populus-Salix forests and increases in more xeric plant associations. To reverse these shifts, The Nature Conservancy purchased several LSPR properties and terminated most alluvial groundwater pumping, thereby reducing pumping rates by ~2.7-4.3 million cubic meters per year. We described baseline floodplain geomorphology and vegetation conditions at seven restoration sites in 2002 and 2003, and tracked streamside plant species composition and cover annually thereafter. Groundwater levels were monitored quarterly at all sites. We also collected data at five LSPR reference sites. Baseline vegetation conditions at the restoration sites were typical of dry degraded areas, with low forested floodplain area (vs. shrubland, woodland, and open areas), and low absolute and relative Populus-Salix basal area (vs. Tamarix). Groundwater levels have not increased at the restoration sites during the study period (2002-2006), nor have there been large floods sufficient to drive changes in floodplain patch structure. Streamside herbaceous species composition, cover and average weighted Wetland Indicator Score have not shifted to values typical of wetter conditions. Where managers seek to restore arid-region riparian ecosystems by restoring floodplain hydrology, drought may delay the response.