Direct Effects of Warming Increase Woody Plant Abundance in a Subarctic Wetland

Climate change is expected to continue to cause large increases in temperature in Arctic and sub-Arctic ecosystems which has already resulted in changes to plant communities; for example, increased shrub biomass and range. It is important to understand how warmer temperatures could affect the plant community in a wetland system because this region provides crucial high-quality forage for migratory herbivores during the breeding season. One mechanism by which warming could cause change is directly, where warming influences the vital rates of a species; these effects may be either positive or negative. Warmer temperatures may also affect a species indirectly, by impacting neighboring plants which compete with, or facilitate that species. Altering interspecific interactions may affect the abundances of the surrounding species. Recent research shows these ‘indirect’ effects which are mediated by biotic interactions may be important enough to reverse ‘direct’ effects of climate change in some plant communities. Furthermore, herbivores have been shown to mediate the effects of warming, in some systems, even preventing shrub expansion. However, the abundance of herbivores may change because of climate change so it is important to understand the role of herbivores in mitigating climate change effects to inform management strategy. Therefore, we aimed to determine the importance of direct and indirect effects of warming on this plant community while considering changing herbivore pressures. We conducted a two-year field experiment in the coastal wetlands of western Alaska to investigate how warming and herbivory will impact the abundances of two common species, a sedge and a dwarf shrub. We used the results from the experiment to predict the equilibrium abundances of the two species under different climate and herbivory scenarios and determine the contribution of direct and indirect effects to predicted community change. The sedge, Carex ramenskii, remained dominant in under ambient conditions, but the dwarf shrub, Salix ovalifolia, became dominant in warmed treatments. Herbivory mediated some of the effects of warming; where grazing was present community composition did not change as much as where it was not grazed. Results suggest that in the absence of goose herbivory, a 2°C increase could cause a shift from sedge to woody plant dominance on the coast of western Alaska. However, if grazing pressure by geese continues at the present rate, it may help retain the current community composition, though herbivory pressure was not sufficient to entirely reverse the effect of warming. Finally, we found that direct effects were more important than indirect effects in causing changes to this plant community