Refuge effects of Juncus effusus in grazed, subtropical wetland plant communities

Unpalatable plant species often act as biotic refuges by protecting neighboring plants from herbivores. This positive interaction can increase functional diversity in grazed ecosystems by protecting species sensitive to grazing. While many studies investigate pair-wise interactions between benefactors and beneficiaries, few show that these interactions result in community composition effects. We studied the effect of an unpalatable plant, Juncus effusus, on wetland plant communities in grazed and ungrazed plots. We tested the following predictions: (1) Juncus would increase plant functional diversity in grazed wetlands; (2) Juncus would have significant effects on community composition; and (3) the effects of Juncus on other species would change across the grazing gradient. We found that Juncus preserved functional diversity in grazed wetland communities by protecting species that decrease with grazing pressure. In multivariate analyses, grazing was the strongest driver of species composition but we found significant effects of Juncus on both vegetation change and species composition in grazed plots. Interactions with Juncus were facilitative in grazed plots and competitive in ungrazed plots, but did not vary along the grazing gradient (0.15-1.7 cows/ha). These results indicate that Juncus has significant community composition effects in grazed wetland plant communities. Understanding the effects of plant interactions such as these at the community level is essential to applying plant interactions to restoration or management.