Effects of a short-term climate change experiment on a sub-Antarctic keystone plant species

The cushion plant Azorella selago is widespread across the sub‐Antarctic, and is considered a keystone species in the dominant fellfield vegetation. However, the impact of current changes in climate in the region (increasing temperature and declining rainfall) on this species is unknown. Here, the response of A. selago to reduced rainfall (a direct effect of climate change) and increased shading (a predicted indirect effect of increasing temperatures, via enhanced growth and wider distribution of more responsive competitors and epiphytes) was experimentally determined. Reduced rainfall increased stem mortality and accelerated autumnal senescence. Furthermore, under this treatment senescence was unequally distributed across individual plants, hypothesized to be a consequence of an interactive effect between rainfall and wind patterns. Shaded stems grew more, and carried larger leaves with lower trichome densities, than their exposed equivalents. As a result, shaded plants were less compact and their surface integrity reduced. The species' response to combined drying and shading was generally similar to its response to shading alone, suggesting that, at least over the short term, the indirect effects of climate change could be more severe than the direct effects. Thus, despite the species' slow growth rate and the short duration of the experiment, persistent direct and indirect effects were observed, both with potential longer‐term consequences for A. selago populations. Climate change is, therefore, likely to impact negatively on this long‐lived keystone species, with significant implications for the structure and functioning of fellfield systems.