Propagule banks: potential contribution to restoration of an impounded and dewatered Sonoran ecosystem.

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Jere A. Boudell|J. C. Stromberg
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Abstract: The Agua Fria River, in Arizona's Sonoran Desert, was impounded and diverted over 70 years ago. Immediately below New Waddell dam there are semi-permanent pools, but water has been released into the channel only nine times since 1927, during wet years when reservoir storage capacity was exceeded. 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 forty-five soil cores from four patch types. We examined 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. Zonation occurred in the soil profile under three of the patch types (Tamarix forests and the two xerophytic shrublands), with obligate riparian species more prevalent in deep sediment and upland species more prevalent in surface soil and litter, suggesting that a riparian legacy is present in Agua Fria propagule banks. Propagule banks of three of the four patch types (Tamarix forests, Tamarix-Salix forests, and Baccharis-Bebbia shrublands) were dominated by riparian species and individuals, further indicating a riparian legacy. However, propagule banks of the widespread Hymenoclea-Bebbia shrublands were dominated by upland individuals, reflecting xerification of the riparian corridor. Given the physical barrier of the dam, continued diversion of stream flow, and only infrequent 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.
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