Understanding how plant communities assemble is at the heart of restoration efforts, understanding response to disturbance (i.e. management), and understanding response to secular changes (eg. climate change). The reason we lack a general model of community dynamics, including community assembly is because of: 1) The overwhelming interactions among populations and 2) The overwhelming variability of disturbance effects. We used small islands of the Mississippi River to examine neutral and niche-based mechanisms of community assembly by examining plant community composition, structure, and diversity along a disturbance gradient (i.e., flooding). Elevations on five lower Mississippi River islands were surveyed and then corrected with the nearest river gage elevation to link island elevation with hydrological patterns. Historical data showed frequency of inundations ranged from 1.32 – 45.26 inundations/yr, with durations lasting from 12.8 – 70.4 days of inundation/yr. Six transects were placed on each island from fore side (main channel) waters edge to zee side (non-main channel) waters edge to encompass all elevations on the island. Two 1 x 2 m herbaceous vegetation plots were placed at every 1 m increase in elevation, with one randomly placed plot in the center of each elevational zone and the other plot positioned to ensure maximum diversity.
Results/Conclusions
Total diversity was regressed with island area and maximum elevation to determine if there were significant relationships. Larger islands tended to have greater elevational change, and the largest island with the greatest elevation change had the highest diversity. However, there were no relationships among total diversity and island area nor between total diversity and maximum elevation, suggesting islands may be acting independently. Nonparametric statistics were then used to test for significant differences in total diversity among elevations (hydrologic regimes), and between fore and zee sides. Zee sides had significantly greater diversity (random=58; high diversity=66) compared to fore sides (random=42; high diversity=53). The elevation with the greatest diversity for random and high plots was 2m elevation with diversity =20, corresponding to a hydrological regime of 37 inundations/year averaging 49 days inundation/year. The results show mid-elevations on islands have higher richness and % cover values. These results are consistent with the intermediate disturbance hypothesis. Experimental plots are being designed in order to examine niche vs. neutral controls on plant community assembly.
