Document: MAR-3-69-51

Effects of a fluctuating disturbance regime on a riparian wetland seed bank.

HOWE, M.M.* and M.J.C.KEARSLEY

Northern Arizona University, Flagstaff, Arizona 86011 USA ¹

Abstract:
The significance of seed bank dynamics to plant recruitment and regeneration is widely recognized, especially in areas where disturbance drives ecosystem processes. Recent management action of Glen Canyon Dam, Arizona provided a natural experiment allowing for analysis of how flooding affects wetland seed bank dynamics along the Colorado River in Grand Canyon National Park. On the Colorado River, flooding regimes vary longitudinally due to tributary input. As the frequency of natural flooding increases longitudinally in regulated river systems, the Serial Discontinuity Hypothesis suggests that lotic ecosystem processes will return to conditions resembling a pre-regulated state. Our results from monitoring the terrestrial seed bank confirm this hypothesis. By doubling the frequency of flooding with flash floods from a large tributary, we documented that species abundance and richness decreased by half in the lower two-thirds of the corridor. A depauperate seed bank, such as the one created by increased flows more closely resembles the hypothesized pre-regulation condition than the seed bank in areas with steadier flows. Species composition also changed significantly after flooding events affected the entire mainstem. Before a major flood, species composition in wetland seed banks varied significantly among sites due to greater flow variability farther downstream. Afterwards, species composition throughout the corridor was similar, indicating that flooding homogenized wetland seed banks. We conclude that managing for pre-dam conditions, which is the desired state for national parks, requires a very different flooding regime than managing for greatest biodiversity.

Keywords: wetland seed bank, flooding disturbance, riparian ecosystems, Serial Discontinuity Hypothesis, species richness

This abstract is being presented at: 10:30 AM in session:
RIPARIAN ECOLOGY