Resilience, Restoration, and Riparian Ecosystems: Case Study of a Dryland, Urban River

Resilient systems can absorb disturbance and persist despite variability as long as the capacity of the system to adapt is not exceeded. Riparian plant communities of dryland alluvial rivers are expected to be naturally resilient systems because they persist in the highly variable floodplain. River modification has altered the flow regime on many dryland rivers, in some cases exceeding the adaptive capacity of the riparian vegetation, but these changes may be readily reversible. The Salt River in Phoenix, Arizona has been impounded, dewatered, channelized, but also re-watered with urban effluent and storm drain runoff. To determine whether riparian vegetation is resilient to these various perturbations, paired comparisons were made in the vegetation and seed bank between a non-diverted reference reach, a diverted reach, and a re-watered urban reach. In the diverted reach, composition had shifted to that of a stress tolerant xeroriparian shrubland with low diversity in both the seed bank and extant vegetation. Most surprisingly, few differences were observed in the composition and structure of the vegetation and soil seed banks between the reference reach and the urban reach, particularly in the wet patches, suggesting that hydric riparian plant communities have the capacity to adapt to these modified conditions. These results provide support for a process-oriented approach to restoration on the Salt River and other urban dryland rivers using patches of persisting vegetation as models for achievable restoration targets.