Tolerance of four wetland plant species to flooding and sediment deposition

A greenhouse assessment of the physiological responses of four common Pacific Northwest wetland plant species ( Carex rostrata and Carex stipata (sedges) and the flood-tolerant trees Alnus rubra (red alder) and Fraxinus latifolia (Oregon ash)) to flooding and sediment deposition was conducted. Experiments simulated two ecosystem perturbations which occur when watersheds are urbanized: (1) alteration of hydroperiod and (2) deposition of sediment along channels and in wetlands. Sedges were subjected to alternating flooding and drying cycles and to sediment deposition with different flooding levels. The trees were subjected to static flooding, cycled flooding and drying, and sediment deposition. C. rostrata and C. stipata were resilient to cycles of flooding and drying, but sediment deposits resulted in decreased biomass which was diminished further by high water levels. Static flooding to or above the soil surface killed saplings of A. rubra and F. latifolia in less than a week. A. rubra saplings exhibited decreased photosynthesis and growth when subjected to cycles of flooding and drying, more so if the soil were completely saturated during flooding cycles. F. latifolia saplings showed no significant response to cycles of flooding and drying. Sediment addition to the soil surface resulted in an immediate drop in photosynthesis for A. rubra. These experiments showed that sediment negatively impacted the sedges and trees, and that flooding differentially affected the trees, with A. rubra being more sensitive than the other three species.