Understanding how plant-plant interactions change under different conditions improves our ability to predict composition changes in response to management decisions and environmental change. Grazing, competition, and drought all affect the outcome of plant interactions, although their interactions are rarely assessed. In the context of community assembly, a switch among filters from grazing to competition is expected to profoundly affect vegetation composition. Here, we examined the response of two species, Panicum hemitomon, a native grass and, Alternanthera philoxeroides, a non-native forb to neighborhoods and the dominant species (Juncus effusus) along a hydrological gradient in grazed wetlands and in wetlands released from grazing. The study was conducted for two growing seasons: a drought with 57% of the average precipitation and a normal year with 98% of the average precipitation. We hypothesized that grazing and hydrological position would alter the outcome of plant interactions, and the relative magnitude and direction of interactions would differ under variable climate conditions and among species. We calculated relative interaction intensity (RII) using relative growth rates, an index that ranges from +1 to -1, with (+) values indicating facilitation and (–) values indicating competition. We expected facilitation to be prevalent in grazed wetlands and differ among species.
Results/Conclusions
The native grass, P. hemitomon, had facilitative interactions with Juncus in grazed wetlands with greater height and leaf growth, while the interaction was competitive in ungrazed wetlands. Facilitation was stronger in the drought year and at the wetter end of the gradient, where the vicinity of Juncus resulted in higher survival and relative growth rates. Interactions with all neighbors also tended to be facilitative, with greater growth rates and survival with neighbors; however, there was greater variation in the direction of interactions in the drought year.
For A. philoxeroides, interactions with Juncus were not influenced by grazing, hydrologic gradient or year. However interactions with all neighbors were influenced by drought and hydrologic gradient. During drought, competition with all neighbors was greatest in grazed-wet plots while interactions were facilitative in grazed-wet plots in the normal year. This may be explained by different stressors among years: in the drought year, the non-native forb competed with neighbors in wetter plots because there was no inundation but adequate soil moisture, while in the normal year neighbors weakly facilitated the forb under inundated and grazed conditions.
This study highlights the importance of interactions among climate, grazing, and hydrologic gradients on the outcome of plant-plant interactions.