Riparian vegetation responses to altered disturbance regimes in the Grand Canyon

Abstract: Flooding is a fundamental source of community organization in riparian zones, and as a disturbance force, it generates both stressors and resources that determine the structure and composition of these systems. Historically, the Grand Canyon corridor experienced frequent, high severity flooding disturbances, but after the completion of the Glen Canyon Dam in 1966, the Colorado River between Lee’s Ferry and Lake Mead has experienced less acute, less seasonally variable flows. Due to the alteration of the native disturbance regime, vegetation communities at the old high water line are in decline, while the formerly denuded riverbanks have become occupied by several new vegetation zones, including woody riparian assemblages and marsh patches. These novel assemblages are discussed in the context of their affinity for disturbance, dispersal abilities, and resistance to desiccation. Recent high flow experiments (HFEs) have not approached the historical severity of pre-dam flooding events, and while HFEs may help achieve other management goals, novel riparian vegetation has shown little response to this anthropogenic form of disturbance. The vegetation shifts observed in the Grand Canyon are just one instance of the impacts of altered disturbance regimes. Yet, this case study raises important management questions about the difficulties inherent in conserving ecosystem processes, especially when they conflict with resource use and finer-scale conservation values.