Long‐Term Warming and Dominant Species Removal Alter Plant Community Composition in Invaded Alpine Tussock Grasslands in New Zealand

ABSTRACT Climate change threatens alpine communities as faster warming leads to plant invasions and local extinctions, but effects vary along environmental gradients. Experiments accounting for interactions of warming and species turnover along environmental gradients can clarify mechanisms of community change. Methods In a field experiment in grasslands, we tested warming and removal of dominant ericaceous shrubs: Calluna vulgaris (invader, low elevation) and Gaultheria colensoi (native, high elevation). Aim Species presence and cover were recorded annually to assess community composition, richness and functional diversity, interpreted with respect to the stress gradient hypothesis (SGH). Further, we explored drivers of cover over elevation. Results Independent of treatments, cover of the dominant species increased significantly over time. At high elevation, an 8.5% increase in Gaultheria cover was accompanied by a 0.2% increase in rare species, while at low elevation, a 10% increase in invasive Calluna cover coincided with a 0.1% reduction in rare species. Treatment effects on subordinate cover differed between elevations. At high elevation, warming reduced the probability of occurrence of subordinate species by 20% and species richness by 2.3, with forbs most negatively affected. However, Gaultheria removal had a positive effect on functional diversity, increasing species evenness over time. At low elevation, invader removal increased evenness by 10% through a positive effect on forb cover. Diversity marginally increased (5%) in the combined warming and removal treatment, suggesting that invasion may suppress the response of plant functional diversity to warming at this site. Conclusion We found support for the SGH only under ambient conditions and mainly for rare, low‐cover species. At high elevation, warming weakened positive Gaultheria –subordinate interactions. At low elevation, Calluna cover suppressed rare species under ambient conditions, but warming did not promote invasion, likely due to warming‐induced drought. Removal increased subordinate diversity but had little effect on cover, potentially revealing longer‐term reductions in productivity. Our study highlights the utility of experimental warming and dominant removals for revealing plant community responses to global change and suggests that the lee side of Mount Ruapehu will be a strategically important location for the conservation of drought‐adapted native species as climate warms.