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.