Herbivore and neighbour effects on tundra plants depend on species identity, nutrient availability and local environmental conditions

Abstract

1 I performed a factorial transplant experiment to test the roles of plant–plant interactions, herbivory by mammal grazers and resource availability for plant performance in two contrasting habitat types in a mountain tundra environment. 2 Three perennial dicot herbs, Solidago virgaurea, Erigeron uniflorus and Saussurea alpina, were used as target plants to study the effects of neighbour removal and grazer exclusion, and nutrient enrichment and liming on plant growth, survival and reproductive success. These treatments were replicated in two contrasting habitat types, infertile acidic and fertile non-acidic tundra heaths. 3 The effects of plant–plant interactions on Saussurea varied from facilitation in infertile acidic habitats to competition in fertile non-acidic habitats and in nutrient-enriched conditions, while the overall performance of Saussurea was strongly negatively influenced by the presence of grazers, the effects being greater when plants were fertilized and in fertile non-acidic heaths. Erigeron performed better under nutrient-enriched conditions than in unfertilized plots, when neighbours had been removed. Solidago was negatively affected by grazing and this impact was greater in nutrient-enriched plots and in non-acidic heaths than in acidic heaths and for unfertilized controls. There were no interactions between neighbour removal and herbivory in any of the three species, indicating that these processes operated independently. 4 Grazer-preferred tall plants are strongly limited by consumption by mammal herbivores in nutrient-enriched conditions and in inherently fertile habitats. By contrast, arctic–alpine specialists and species of low stature experience increased competition with neighbouring vegetation in fertile habitats and in enriched nutrient conditions. 5 Synthesis. Overall, the results suggest that the strength and directions of plant–plant and plant–herbivore interactions depend on plant species identity and are modified by soil edaphic factors to govern vegetation processes in tundra plant communities. These findings have important implications for understanding the forces structuring vegetation in barren tundra ecosystems under a changing environment.