Community shifts along a disturbance gradient

Background/Question/Methods

The complexity of vegetation dynamics resides in species-specific responses, historical effects, propagule pressure, and variation in the frequency and magnitude of disturbances. Such knowledge is imperative for understanding vegetation shifts in response to climate and nested vegetation dynamics in response to altered disturbance regimes. We use Mississippi River islands as a model system with a known historical disturbance regime and similar propagule pressure on five replicate islands. We examined community similarity in relation to flooding disturbance along a disturbance gradient (elevation gradient of island) as well as year to year variation (2007, 2008). In addition, we examined the difference between channel (fore, greater magnitude of flood disturbance) and bank (zee) sides of islands to separate the effects of disturbance frequency and magnitude. We hypothesized that vegetation communities would be most similar (Bray-Curtis similarity index) at nearer elevations along the gradient and that increased magnitude of disturbance (flood of 2008) would lead to a shift of communities along the gradient. A Monte Carlo permutation test was used to test for randomization across gradients. It compares dissimilarity values among islands by randomly permuting elevations, calculating a new dissimilarity value each permutation, and comparing the new value to actual dissimilarly from the data.

Results/Conclusions

Results indicate that both sides of the island, fore and zee, had elevation zones that were different from random, less or more similar than expected (significant at the 0.05 level). In an analysis across years 2007 and 2008, before and after a major flood event, we found that individual elevations among years were not random. On both the zee and fore sides, elevations were found to be significantly different; again, either less or more similar than expected. These results suggest that communities are structured along a disturbance gradient on Mississippi River Islands, suggesting niche controls, and that such assemblages shift in response to increased disturbance (a shiift in the disturbance gradeint). While species-specific responses were evident, community shifts were also evident suggesting predictions of community shifting in response to changes in disturbance regimes are possible. We explore the possibility of extrapolating this result to major latitudinal and elevational shifts due to climate change.