Seed Size, Sediment, and Spatial Heterogeneity: Post-Flood Species Coexistence in Dryland Riparian Ecosystems

Floods create landscape heterogeneity through erosion and deposition of sediment, but more information is needed on how these physical processes influence plant communities. We conducted two glasshouse experiments to determine how assemblages of riparian seeds with different traits respond to burial by sediment, and to determine whether species diversity increases in response to heterogeneous sediment deposition. In experiment #1, three sediment treatments were applied to soil collected from the riparian zone of a semi-arid river, and in experiment #2, seeds from 30 plant species were subject to three sediment and two soil moisture treatments. We found that seed mass, wetland affinity, and phylogeny all influence capacity to emerge from depth, with sediment thus acting as an environmental filter. Small-seeded wetland species were abundant in a treatment that simulated flood wetting with no sedimentation, while deposition of sediment (0.2 to 10 cm) favored large-seeded mesic and xeric species, and to a lesser extent monocots (vs. eudicots). Rapid drawdown of water further increased emergence for large, buried seeds, suggesting that deep sedimentation together with rapidly draining water can terrestrialize post-flood communities. Although spatially heterogeneous sediment depth creates patchy community assemblages, it did not increase diversity in comparison to uniform lack of sedimentation.