Persistence of riparian plant communities despite human-mediated change on the Salt River, Arizona

Resilience describes the ability of dynamic systems to persist under variable conditions. Plant communities within the riparian zone are examples of dynamic systems that are expected to have high resilience because they are structured by stochastic processes. The mechanisms of resilience can facilitate recovery of degraded riparian systems as long as the driving processes are within the range of variability associated with the desired state. The Salt River in Phoenix, Arizona has been impounded, de-watered, channelized, and re-watered with urban effluent and storm drain runoff. To determine the effects of these various perturbations on riparian vegetation, the plant community and the soil seedbank were compared between three locations along the lower Salt River: above the water diversion dam, below the diversion dam at the upstream perimeter of the Phoenix metropolitan area, and below the diversion dam within the Phoenix metropolitan area at sites that had been re-watered and channelized.  The greatest differences were observed between the above-dam perennial reach and below-dam diverted reach where composition had shifted to that of a stress tolerant xero-riparian shrubland with low diversity of species and functional groups in both the seedbank and extant vegetation.  Most surprisingly, the composition, diversity and structure of the vegetation and soil seedbanks of the wet patches of the urban re-watered reach were very similar to those in the upstream perennial reach.  These results suggest that riparian plant communities have high adaptive capacity to these modified conditions which has implications for the way restoration is approached in urbanized riparian corridors.