Context-dependent effects of Juncus effusus on survival and growth of native and exotic plants in Florida grazed wetlands

Background/Question/Methods

The net outcome of plant interactions depends on ecological context, with facilitative interactions expected to increase under ecological stress or consumer pressure. For example, unpalatable plants may benefit neighboring palatable plants via protection from herbivores, but may inhibit neighbors through competition under reduced herbivory. We conducted a 16-month experiment investigating the effects of a native, dominant unpalatable macrophyte, Juncus effusus, on survival and growth of wetland plant communities embedded in a central Florida cattle ranch. Our objective was to determine the role of Juncus in persistence of native species in grazed wetlands and also to determine if Juncus provided refuge for exotic species. We hypothesized that palatable plants neighboring Juncus would be protected from grazing. We used an experimental approach including grazing exclosures, removal of Juncus, and transplants of four different species (Panicum hemitomon (native grass), Diodia virginiana (native forb), Panicum repens (exotic grass) and Alternanthera philoxeroides (exotic forb)) across two pasture types.

Results/Conclusions

We assessed survival using loglinear analysis and Akaike Information Criterion (AIC). Across all species, higher mortality occurred in grazed plots compared to ungrazed plots. There were strong facilitative effects of Juncus for the native grass in grazed plots, while facilitative effects were weaker for the exotic grass and exotic forb. Juncus strongly inhibited the native forb in both grazed and ungrazed plots.  In ungrazed plots, all positive effects of Juncus on survival disappeared and less survival occurred in Juncus plots compared to non-Juncus plots suggesting that competition occurred. Analysis of variation of relative neighbor effect (RNE) based on relative growth rate (RGR) indicated that facilitation was strongest in grazed plots for both native (p≤0.001) and exotic grasses (p≤0.001) and the exotic forb (p≤0.001) compared to ungrazed plots where RNE indicated competition. In these grazed wetlands, facilitative effects of Juncus play a large role in the persistence of the native grass and a lesser role in the success of exotic species.