Integrating active restoration with environmental flows to improve native riparian tree establishment in the Colorado River Delta

Drastic alterations to river hydrology, land use change, and the spread of the nonnative shrub, tamarisk (Tamarix spp.), have led to the degradation of riparian habitat in the Colorado River Delta in Mexico. Delivery of environmental flows to promote native cottonwood (Populus spp.) and willow (Salix spp.) recruitment in human-impacted riparian systems can be unsuccessful due to flow-magnitude constraints and altered abiotic–biotic feedbacks. In 2014, an experimental pulse flow of water was delivered to the Colorado River in Mexico as part of the U.S.-Mexico binational agreement, Minute 319. We conducted a field experiment to assess the effects of vegetation removal, seed augmentation, and environmental flows, separately and in combination, on germination and first-year seedling establishment of cottonwood, willow, and tamarisk at five replicate sites along 5 river km. The relatively low-magnitude flow deliveries did not substantively restore natural fluvial processes of erosion, sediment deposition, and vegetation scour, but did provide wetted surface soils, shallow groundwater, and low soil salinity. Cottonwood and willow only established in wetted, cleared treatments, and establishment was variable in these treatments due to variable site conditions and inundation duration and timing. Wetted soils, bare surface availability, soil salinity, and seed availability were significant factors contributing to successful cottonwood and willow germination, while soil salinity and texture affected seedling persistence over the growing season. Tamarisk germinated and persisted in a wider range of environmental conditions than cottonwood and willow, including in un-cleared treatment areas. Our results suggest that site management can increase cottonwood and willow recruitment success from low-magnitude environmental flow events, an approach that can be applied in other portions of the Delta and to other human-impacted riparian systems across the world with similar ecological limitations.