Propagule banks: Potential contribution to restoration of an impounded and dewatered riparian ecosystem

The Agua Fria River in Arizona's Sonoran Desert was impounded and diverted more than 70 years ago. Immediately below New Waddell dam there are semi-permanent pools, but water has been released into the channel only during rare wet years. To determine whether a propagule bank exists below the dam, and whether it could contribute to the revegetation of the Agua Fria riparian ecosystem should flow be restored to the dewatered reach, we collected 45 soil cores from four plant associations. We examined species in the samples in a growth chamber using the seedling emergence method. A total of 74 species (mostly herbaceous) and an abundance of individuals were present in propagule banks. The propagule banks were similar to those of a free-flowing reference river despite considerable differences in extant vegetation. Riparian species were present in propagule banks of all four associations and were the dominant type in three (Tamarix forests, Tamarix-Salix forests, and Baccharis-Bebbia shrublands). Propagule distribution varied with soil depth in three of the associations (Tamarix forests and the two xerophytic shrublands) with riparian species more prevalent in deep sediment and upland species more prevalent in surface soil and litter. Collectively these patterns suggest that a riparian legacy is present in Agua Fria propagule banks. However, riparian propagule density was low in the Hymenoclea-Bebbia shrublands, reflecting xerification of the riparian corridor. Given the physical barrier of the dam, continued diversion of stream flow, and rare flood releases, local inputs from xerophytes will dominate propagule bank dynamics in the future. Although propagule banks could contribute to redevelopment of the herbaceous component of the vegetation should stream flows be restored to this river reach, the riparian legacy likely will decline over time as riparian propagules reach the end of their lifespan while propagules of xerophytes continue to be replenished.