Isolating the effects of anthropogenic activities on the trajectory of vegetation is an important application of plant ecology. The Roanoke River, as most large rivers in the United States, has been dammed for flood control and hydropower, which has compressed the natural variation in stream flow. Homogenization of in-channel flow dampens the variation in disturbance and resource gradients on the floodplain, which may result in a homogenization of floodplain forest composition over time.  Detecting shifts in floodplain forests attributed to anthropogenic activity is challenging because the natural variation in the vegetation and environment can obscure response signals. However, the first indicators of change in composition are likely to be found in the regeneration layer, because tree seedlings have narrower tolerance ranges than adults. To assess changes in forest structure and composition across the floodplain due to changes in flooding associated with river regulation, we monitored recruitment and survival of tree seedlings from 2007-2009 and re-sampled permanent vegetation plots established in 1994 spanning the current flood gradient on the lower Roanoke River, North Carolina. By monitoring seedling dynamics over time and re-sampling historically established vegetation plots, we are able to document changes in forest structure and composition on the lower Roanoke River and provide a mechanism for the observed differences.

Results/Conclusions

High spatial variation was observed in the regeneration layer across the floodplain as both the abundance and species richness of seedlings declined with increased flooding. The highest variation in both space and time was observed in areas exposed to the highest flood variability, but at the extremes of the wetness gradient seedling patterns were more stable reflecting the stability of the hydroperiod in these areas of the floodplain. Comparing the re-sample plots to the original sample, changes in forest structure and composition reflected the homogenization of the hydroperiod as well. Flood intolerant species such as Fagus grandifolia, and Pinus taeda increased in abundance, particularly in the smallest size classes. In contrast, moderately flood tolerant overstory oak and midstory species decreased in abundance in all size classes, and little recruitment to the lowest size class was observed. The abundance of highly flood tolerant species was unchanged or increased between sampling periods. Therefore, the loss of variability in the hydroperiod appears to be driving compositional shifts from moderately wet forests to dry forest types due to a relaxation of the regulating effect of floods on the regeneration of tree seedlings.