The impacts of altered floodplain hydrology of the lower Roanoke River on tree regeneration and floodplain forest composition

Tree regeneration in floodplain forests is broadly determined by the ability of species to tolerate the flood regime at a given location. River regulation affects all aspects of the flood regime, but the effect is not linear across the floodplain or over time. This study assesses regeneration patterns in relation to the inundation regime at small scales across a broad extent in order to anticipate trajectories of forest change. This research is a case study of the lower Roanoke River, a regulated brownwater river on the North Carolina Coastal Plain, but the results presented here have broad applications.;The first two chapters assess the combined effects of flood control and hydropower generation on the flood inundation regime. These changes are then mapped across the floodplain as an index of hydrologic stress. Next, I document variation in tree species regeneration dynamics over time and with respect to hydrologic setting. I then relate seedling dynamics to year-to-year variation in inundation patterns, and develop simple models to predict potential trajectories over time.;There is a spatial and temporal gradient in flooding that is primarily driven by flood control, but also to some extent by hydropower production. For much of the active floodplain, flood frequency has been reduced, but when floods occur they are much longer in duration. Flood timing was found to have the greatest effect on regeneration. Flooding early in the growing season had a positive effect whereas mid-growing-season flooding had a significant negative effect. The analysis presented here projects that if the survival rates of any given year, or a random ordering of years were to continue, few if any seedlings would recruit into the sapling layer. The abundance of small stems declined over 14 years providing support for this prediction. Species that are prolific seeders, widely dispersed and have a broad tolerance range dominate the seedling layer as their regeneration strategy improves the probability of success in this stochastic environment. Without changes in dam operations it is likely that tree density and diversity will decline and that the floodplain will be become dominated by a few opportunistic species.