Differential facilitative and competitive effects of a dominant macrophyte in grazed subtropical wetlands

1. Plant-plant interactions fluctuate between competition and facilitation depending upon ecological conditions and species traits. Facilitative interactions are expected to increase in frequency via associational defences with increasing consumer pressure. The ability of species to cope with competition and/or ecological stressors may alter the outcome of plant-plant interactions. 2. We conducted a transplant experiment to determine if native and non-native grasses and forbs respond similarly to interactions with Juncus effusus L., an unpalatable benefactor species, along a grazing intensity gradient in two contrasting pasture types: intensively managed and semi-natural. We expected competitive taller, erect species (grasses) and non-natives to obtain stronger facilitative effects than shorter, sprawling species (forbs) and natives. 3. We transplanted two species each of grasses and forbs, including one native and one non-native species of each, into 1-m(2) grazed plots or exclosure plots either with or without Juncus in wetlands experiencing a range of grazing intensities across the two pasture types. 4. Interactions with Juncus varied from facilitation in grazed plots to competition in exclosures for the two grasses and the non-native forb. Juncus did not facilitate the native forb in grazed conditions. Grazing negatively influenced all species. For the grasses, facilitation intensity increased as grazing intensity increased, while facilitation intensity for forbs was unrelated to grazing intensity. 5. Synthesis. Facilitation intensity of a dominant macrophyte increased with consumer pressure for tall grasses in productive subtropical wetlands. These findings suggest that facilitation can allow native and non-native species to pass through human-induced ecological filters, such as cattle grazing, with implications for understanding forces governing community composition in disturbed environments