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Potential for herbaceous community restoration of an impounded southwestern riparian ecosystem of North America via seed bank manipulation.
Boudell, Jere*,1, Stromberg, Juliet1, 1 Arizona State University, Tempe, Arizona
ABSTRACT- With the construction of Waddell dam in 1927, the Agua Fria River was impounded, which impeded flow to downstream reaches. Other than occasional flood releases and dam leakage, the river bed is dry. Our seed bank investigations from a nearby free-flowing, river revealed the importance of seed banks to riparian vegetation dynamics and to the maintenance of riparian biodiversity. We initiated seed bank studies of the degraded riparian ecosystem of the Agua Fria in 1998 to assess site potential for natural revegetation. 180 soil samples were collected from four dominant patch types. Seeds within soil samples were allowed to germinate in a growth chamber, programmed to mimic site conditions, over a two-year period. Species found in seed banks were placed into categories based on life-history traits, growth form, moisture tolerance and origin. Wetland ruderals dominated both Tamarix and Tamarix-Salix forest seed banks. Native wetland perennial species, such as Mimulus guttatus, were found in deeper soil layers of both of these forest seed banks. A few species (e.g., Polypogon monspeliensis) heavily dominated the seed banks of these channel-side forests, mirroring the loss of above-ground diversity; this pattern may reflect the loss of the disturbance processes that typically allow for high biodiversity in riparian ecosystems. The seed banks in the shrubland patch types had many upland individuals and thus are beginning to mirror the community shift towards xeric vegetation. However, some hydromesic riparian graminoids and forbs (e.g., Cyperus oderatus) were present in deeper soils. The presence of seed bank species with a range of life history strategies and moisture tolerances suggests to us that, if the flow regime was restored, a herbaceous riparian community would establish. The loss of riparian biodiversity in the seed bank, and shift towards upland species, suggest that reclamation may be a more probable outcome than full restoration.
Key words: riparian, biodiversity, restoration, seed bank