Impacts of seven years of experimental warming and nutrient addition on neighbourhood species interactions and community structure in two contrasting alpine plant communities

Abstract

Global change is predicted to have major impacts on alpine and arctic ecosystems. Plant fitness and growth will be determined by how plants interact with each other at smaller scales. Local-scale neighbourhood interactions may be altered by environmental pertubations, which could fundamentally affect community structure. This study examined the effects of seven years of experimental warming and nutrient addition on overall changes in the community structure and patterns of interspecific interaction between neighbouring plant species in two contrasting alpine plant communities, mesic meadow and poor heath, in subarctic Sweden. We used a network approach to quantify the dissimilarity of plant interaction networks and the average number of interspecific neighbourhood interactions over time in response to different environmental perturbations. The results revealed that combined warming and nutrient addition had significant negative effects on how dissimilar plant interaction networks were over time compared with the control. Moreover, plant–plant neighbourhood interaction networks were more dissimilar over time in nutrient-poor heath than in nutrient-rich mesic meadow. In addition, nutrient addition alone and combined nutrient addition and warming significantly affected neighbourhood species interactions in both plant communities. Surprisingly, changes in interspecific neighbourhood interactions over time in both communities were very similar, suggesting that the nutrient-poor heath is as robust to experimental environmental perturbation as the mesic meadow. Comparisons of changes in neighbouring species interactions with changes in evenness and richness at the same scale, in order to determine whether diversity drove such changes in local-scale interaction patterns, provided moderate evidence that diversity was behind the changes in local-scale interspecific neighbourhood interactions. This implied that species might interact at smaller scales than those at which community measures were made. Overall, these results demonstrated that global change involving increased nutrient deposition and warming is likely to affect species interactions and alter community structure in plant communities, whether rich or poor in nutrients and species.