TOLERANCE OF 4 WETLAND PLANT-SPECIES TO FLOODING AND SEDIMENT DEPOSITION

A greenhouse assessment of the physiological responses of four common Pacific Northwest wetland plant species (Carer rostrata and Carer stip ata (sedges) and the flood-tolerant trees Alnus rubra (red alder) and Fraxinus latifolia (Oregon ash)) to flooding and sediment deposition w as 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 wer e 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 d rying, but sediment deposits resulted in decreased biomass which was d iminished further by high water levels. Static flooding to or above th e soil surface killed saplings of A. rubra and F. latifolia in less th an a week A. rubra saplings exhibited decreased photosynthesis and gro wth when subjected to cycles of flooding and drying, more so if the so il were completely saturated during flooding cycles. F. latifolia sapl ings 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 n egatively impacted the sedges and trees, and A. rubra being more sensi tive than the other three species.