Understanding the relative role of disturbance and species interactions in shaping Mississippi River island plant communities

Background/Question/Methods

To understand the factors governing community assembly on riverine islands, we conducted a greenhouse experiment that examined the effects of inter and intra-specific competition and three disturbance treatments on growth performances of three island-ubiquitous herbaceous plant species (Amaranthus palmeri, Cyperus strigosus, and Xanthium strumarium). We predicted A. palmeri to be most affected by the flooding treatment based on its facultative upland status, suggesting that it should be a poor tolerator of flooded conditions. X. strumarium and C. strigosus, both facultative wetland species, were expected to tolerate flooding, but we predicted that C. strigosus should have greater stress tolerance due to its perennial life history. Because we find these species coexisting at the same elevation zones on riverine islands, we also predicted stronger intraspecific competitive effects. We used a completely randomized design with 36 pots species^-1 with a pot density of 6 for inter (2 plants species^-1 pot^-1) and intra (6 plants pot^-1) in three disturbance treatments (drained, drought, and flood). We used general linear multiple analysis of variance (MANOVA) and individual ANOVA’s on the dependent variables: root biomass, shoot biomass, root/shoot ratio, total biomass, and lnRR for each species individually, using Tukey’s adjustment for multiple comparisons for each dependent variable.

 Results/Conclusions

MANOVA results showed four significant main effects, specifically for A. palmeri and X. strumarium for competition effects. Disturbance effects on root biomass for X. strumarium were significant; contrarily, A. palmeri and C. strigosus showed no significant differences in response to disturbance or competition. X. strumarium showed significantly greater shoot biomass compared to A. palmeri and C. strigosus. Only X. strumarium and A. palmeri showed a significant competition effect with X. strumarium exhibiting greater shoot biomass with conspecifics compared to heterospecifics. Total biomass results mirrored shoot biomass results; the only difference was a significant effect of disturbance on A. palmeri. Competition intensity (lnRR) showed significant effects of disturbance and competition on A. palmeri and X. strumarium. A. palmeri exhibited significantly greater interspecific competition effect than intraspecific and the effect was more for drained treatments. X. strumarium showed significantly less growth with conspecifics suggesting that X. strumarium competes poorly with itself. In conclusion, it is apparent that flooding alters competitive abilities of some plants. We did see an increase in competition with decreased severity of environmental stress for A. palmeri and X. strumarium. Our results indicate that factors in addition to competition and flood tolerance may be controlling island plant community dynamics.