Environmental determinants of vegetation in the drawdown zones of a Columbia River Treaty reservoir: a template for ecosystem enhancement.

Water storage reservoirs alternately inundate and expose the drawdown zones, limiting riparian vegetation that provides wildlife habitats and contributes to the aquatic food-web. To characterize plant distributions and hydrogeomorphic associations, we inventoried quadrats in transects extending from the full-pool (FP) margin, downwards 12 m through the drawdown zones at sites around the Duncan Reservoir in British Columbia, Canada. Among the 69 plant species, black cottonwoods (Populus trichocarpa), willows (primarily Salix sitchensis) and other trees and shrubs occurred sparsely, rarely extending below 2 m below FP. Perennial herbaceous plants, especially horsetail (Equisetum arvense) and sedges (primarily Carex utriculata), were most common, extending down ~5 m below FP, and ruderal annual plants occurred sparsely at greater depths. Vegetation Cover and Species Richness were correlated with environmental factors, with (1) Elevation being highly influential, reflecting inundation duration and depth. (2) Position, longitudinal location, reflected greater vegetation diversity downstream of the reservoir. (3) Finer Substrate texture was favorable to retain moisture, but coarse sediments would resist erosion. (4) Shallow Slope was favorable to reduce drainage and included finer sediments. (5) Distance from the FP shoreline could reflect seed source proximity. Stepwise linear modeling with combined environmental factors accounted for ~30% of the variation in Vegetation Cover and Richness, and Canonical Correspondence Analysis revealed plant groupings relative to the environmental influences. At this and other storage reservoirs, regimes that reduce the frequency and duration of inundation could promote vegetation in locations with suitable environmental conditions in the upper drawdown zones, thus providing ecosystem enhancement.