Warmer seasonal temperatures can shift the direction and intensity of plant-plant interactions and, therefore, species abundances. Community shifts may be driven by the ‘direct’ effects of climate, through changes in a species own vital rates, or by ‘indirect’ effects, through alterations in the abundance of competitors/facilitators (due to a direct effect on the other species vital rates). To better predict climate change effects, it is important to understand how these direct and indirect effects contribute to the mechanism of plant community change. Warming is known to influence community composition and contribute to shrub expansion especially in northern latitudes; however, herbivory has been found to counteract the effects of warming in this region. We conducted a two-year field experiment in the Yukon-Kuskokwim Delta, Alaska to investigate how warming and goose herbivory influence the interactions and abundances of two species, Carex ramenskii (sedge) and Salix ovalifolia (dwarf shrub). We used results from the experiment to parameterize competition models, predict the equilibrium abundances of the two species, and determine the contribution of direct and indirect effects to community change. We were also interested in determining whether the presence of goose herbivory can mediate community change caused by warming.
Warming and herbivory altered the equilibrium abundances of both species and modified plant interactions. In ambient environments, C. ramenskii was dominant. In warmed environments, S. ovalifolia overtook C. ramenskii as the dominant species due to reduced interspecific competition. However, herbivory mediated some of the effects of warming; it kept the cover of both species similar to that of the natural ambient, grazed landscape. Direct effects controlled most of the reduction in C. ramenskii caused by warming while indirect effects were almost entirely responsible for increasing S. ovalifolia in warmed environments. Indirect effects were strong enough to reverse the outcome of competition; when only direct effects were considered C. ramenskii remained the dominant species (even in warmed environments) and S. ovalifolia increased minimally or not at all. Our findings suggest that, with a temperature increase of ~2°C, the expansion of this deciduous dwarf shrub is possible in the currently sedge-dominated coastal, low elevation portions of western Alaska. However, if grazing pressure by goose herbivores continues at the present rate, it may help retain current community composition. Our study also supports previous work which highlights the importance of considering competitive interactions and indirect effects when modeling climate change effects on plant communities.