Grazing and microhabitat interact to affect plant-plant interactions in subtropical seasonal wetlands, 2007-2008

The stress gradient hypothesis predicts that competition will be important in productive environments while facilitation will be common in environments with high stress or consumer pressure. However, abiotic stress and grazing may vary independently and even occur simultaneously. Here we examine the outcome of plant interactions in grazed wetlands where consumer pressure and abiotic stress occur concurrently. We hypothesized that cattle grazing and microhabitat would alter the outcome of plant interactions. Given that wetland edges are drier and less productive than wetland centers we expected that facilitation would be greatest in drier wetland edges due to greater abiotic stress regardless of cattle presence. We conducted an experiment for two growing seasons in ten wetlands, five exposed to cattle grazing and five fenced. Two wetland obligate plants were included (Panicum hemitomon and Alternanthera philoxeroides), and plots were assigned to three treatments 1) all neighbors removed, 2) all neighbors removed except Juncus effusus, a dominant, unpalatable plant, 3) all neighbors intact (control), in both wetland centers and edges. Differences in survival, change in height and number of leaves were assessed. In ungrazed wetlands, plant survival was higher in wetland edges vs. centers, while it did not differ in grazed wetlands. Survival in wetland edges was further increased by the presence of J. effusus. Positive interactions under grazed conditions were clear when plant height was assessed, but negative interactions affected leaf production in both ungrazed and grazed wetlands. Grazing interacts with wetland microhabitat to alter plant survival. Facilitative interactions on plant height were apparent in grazed wetlands. Understanding how plant interactions change under different biotic and abiotic contexts is important for informing ecosystem restoration and management.