Compound Influences of River Damming and Beavers on Riparian Cottonwoods: A Comparative Study Along the Lardeau and Duncan Rivers, British Columbia, Canada

We investigated interactions between river regulation, riparian black cottonwoods ( Populus trichocarpa ) and beavers ( Castor canadensis ) with a paired comparison between the free-flowing Lardeau River and regulated lower Duncan River in western Canada. Cottonwood saplings occurred more broadly along Lardeau River transects (63 % vs. 38 %) and with increased density. Beavers preferred cottonwoods over other shrubs and cutting was more intense (36 % vs. 7 % of stems cut) along the regulated Duncan River. Beaver cutting occurred in wider bands (25 m vs. 11 m from river) along the Duncan, and there was also evidence for increased cutting of a less-favored alternate, alder ( Alnus incana ), while willows ( Salix spp.) were substantially cut along both rivers. River regulation has apparently reduced cottonwood recruitment along the Duncan River and regulation may also increase beaver accessibility to saplings since higher river levels in late summer and autumn may promote inland access. These ecosystem alterations may thus create an imbalance between bottom-up cottonwood recruitment versus top-down mortality. We overview some of the ecological interactions in riparian woodlands in a schematic model recognizing river flow regime, sediment patterns and bank forms, and other riparian vegetation as key factors influencing cottonwoods and beavers.