Plant functional group interactions intensify with warming in alpine grasslands

Plant–plant interactions regulate plant community structure and function. Shifts in these interactions due to global climate change, mediated through disproportional increases of certain species or functional groups, may strongly affect plant community properties. Still, we lack knowledge of community-level effects of climate-driven changes in biotic interactions. We examined plant community interactions by experimentally removing a dominant functional group, graminoids, in semi-natural grasslands in Southern Norway. To test whether the effect of graminoid removal varied with climate, the experiment was replicated across broad-scale temperature and precipitation gradients. To quantify community-level interactions across sites, we tested for changes in the remaining vascular community (i.e. forbs) cover, richness, evenness, and functional traits reflecting leaf-economic investment and plant size over five years. The effect of graminoid removal on forb community structure and functioning varied over time, and along the climate gradients. Forb cover increased in response to graminoid removal, especially at warmer sites. Species richness increased following removal irrespective of climate, whilst evenness increased under warmer and wetter conditions irrespective of removal. No climate or removal effect was found for species turnover. Functional trait responses varied along the precipitation gradient – compared to controls, forb mean SLA decreased in drier conditions after graminoid removal. Leaf thickness increased under cooler and drier conditions irrespective of removal. These community structure alterations demonstrate stronger competitive interactions between forbs and graminoids under warmer conditions, whilst functional trait responses indicate a facilitative effect of graminoids under drier conditions. This indicates that both competition and facilitation regulate plant communities, suggesting complexity when scaling from populations to communities. Finally, both temperature and precipitation determine the direction and intensity of biotic interactions, with ecosystem-wide implications for forb persistence and ecosystem functioning under future climates. Further work is needed to generalise the role of changing interactions in mediating community responses to climate change.