Abstract
Both the direct effects of warming on a species’ vital rates and indirect effects of warming caused by interactions with neighboring species can influence plant populations. Furthermore, herbivory mediates the effects of warming on plant community composition in many systems. Thus, determining the importance of direct and indirect effects of warming, while considering the role of herbivory, can help predict long‐term plant community dynamics. We conducted a field experiment in the coastal wetlands of western Alaska to investigate how warming and herbivory influence the interactions and abundances of two common plant species, a sedge, Carex ramenskii, and a dwarf shrub, Salix ovalifolia. We used results from the experiment to model the equilibrium abundances of the species under different warming and grazing scenarios and to determine the contribution of direct and indirect effects to predict population changes. Consistent with the current composition of the landscape, model predictions suggest that Carex is more abundant than Salix under ambient temperatures with grazing (53% and 27% cover, respectively). However, with warming and grazing, Salix becomes more abundant than Carex (57% and 41% cover, respectively), reflecting both a negative response of Carex and a positive response of Salix to warming. While grazing reduced the cover of both species, herbivory did not prevent a shift in dominance from sedges to the dwarf shrub. Direct effects of climate change explained about 97% of the total predicted change in species cover, whereas indirect effects explained only 3% of the predicted change. Thus, indirect effects, mediated by interactions between Carex and Salix, were negligible, likely due to use of different niches and weak interspecific interactions. Results suggest that a 2°C increase could cause a shift in dominance from sedges to woody plants on the coast of western Alaska over decadal timescales, and this shift was largely a result of the direct effects of warming. Models predict this shift with or without goose herbivory. Our results are consistent with other studies showing an increase in woody plant abundance in the Arctic and suggest that shifts in plant–plant interactions are not driving this change.
Highlights
LITERATURE REVIEWEffects of warming in the subarctic Mean global temperature is predicted to increase 1.8 - 4.0 °C by 2099, following an increase of 0.4 °C over the last century (1906-2005) (IPCC 2014)
Climate change can affect species vital rates and interactions, and the effects of herbivory may be important in mediating climate change effects on plant communities
The results of our study suggest that an increase of less than 2° C could cause a shift in dominance from sedges to dwarf shrubs on the coast of western Alaska
Summary
LITERATURE REVIEWEffects of warming in the subarctic Mean global temperature is predicted to increase 1.8 - 4.0 °C by 2099, following an increase of 0.4 °C over the last century (1906-2005) (IPCC 2014). Direct effects occur when warming alters plant populations through changes in a focal species’ own vital rates (Adler et al 2009). Indirect effects occur when warming alters the vital rates and abundances of neighbouring species, which in turn affects the fitness of the focal species (Adler et al 2009; Gilman et al 2010). In communities with strong indirect effects, climate change projections that do not account for these interactions will not adequately predict future abundances of important species (Suttle et al 2007; Levine, Adler & HilleRisLambers 2008; Tylianakis et al 2008; Mod et al 2015)
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