Flood flow attenuation diminishes cottonwood colonization sites: an experimental test along the Boise River, USA

The Boise River in southwest Idaho, USA, flows from forested mountain headwaters to the sagebrush steppe where the river was first regulated in 1909 for irrigated agriculture. Large upstream dams constructed in 1915, 1950, and 1955 are now operated to manage water for both irrigation and the attenuation of flood flows. This, in combination with bank armoring and channelization, has produced a narrowed river channel with simplified morphology. After damming, the native black cottonwood forest, Populus trichocarpa, expanded onto positions previously within the active channel. Since then, sexual reproduction of these cottonwoods appears to have become limited, apparently because of the lack of colonization sites devoid of competing vegetation. We investigated this hypothesis by constructing a channel that simulated recruitment sites previously formed by flood flow events. We observed extensive black cottonwood seedling recruitment along this channel in contrast to nearby reference sites where there were no seedlings, only saplings that originated as clonal root suckers. We conclude that the attenuation of flood flows has removed the geomorphic disturbance that previously created sites for colonization. As a consequence, the forest currently propagates primarily by clonal suckering that fails to introduce genetic diversity essential for adapting to change in the environment. To compensate for this loss, we recommend selective clearing and regrading of the floodplain according to an engineered design as one method to create cottonwood colonization sites. Copyright © 2015 John Wiley & Sons, Ltd.