Responses of obligate versus facultative riparian shrubs following river damming

Riparian or streamside woodlands include obligate riparian trees and shrubs (obligates) that are restricted to streamside zones, and facultative riparian species that are abundant in, but not restricted to the riparian areas. Due to their distinctive life history requirements, it may be predicted that the ecological specialist obligates would be more vulnerable than the facultative generalists to impacts from river damming and flow regulation. We tested this along the Snake River through Hells Canyon, USA, where two native riparian shrubs dominate: the obligate sandbar willow (Salix exigua), and the facultative, netleaf hackberry (Celtis reticulata). We assessed riparian conditions over the past century by comparing ground-level and aerial photographs taken after 1907 and in the 1950s in advance of three dams, versus recent conditions. These comparisons revealed three changes downstream from the dams: (1) the depletion of surface sands and sandbars and (2) reductions in sandbar willow versus (3) the proliferation of hackberry in dense bands above the typical high-water line. The willow decline probably resulted from the depletion of sand following sediment trapping by the reservoirs, combined with changes in the seasonal water flow pattern. The increase in hackberry may have resulted from a beneficial 'irrigation effect' of daily water releases for power generation during the summer. The opposing responses reflect the plants' differing life histories and may partially resolve impacts of river regulation on alluvial sediments versus the instream flow pattern. We consider other riparian studies that suggest that obligates such as cottonwoods (Populus angustifolia, P. deltoides and P. fremontii) are highly vulnerable to river regulation, while facultative trees and shrubs such as trembling aspen (Populus tremuloides), wolf-willow (Elaeagnus commutata) and velvet mesquite (Prosopis velutina) are more resilient. These results suggest that conservation of riparian woodlands should emphasize the ecological specialist obligates, while facultative species may be less vulnerable to river regulation. Copyright © 2009 John Wiley & Sons, Ltd.