Resident functional composition mediates the impacts of nutrient enrichment and neighbour removal on plant immigration rates

Abstract

Summary 1. Theory suggests that herbivory, soil fertility and neighbourhood interactions interactively determine rates of species extinction and immigration in plant communities. Initial community properties and environmental conditions may also exert a profound influence on community responses to external alterations, yet only a few attempts have been made to elucidate their role in assemblage dynamics. 2. I conducted a seed sowing experiment in northern European mountain tundra to investigate the roles of biotic and environmental constraints on plant immigration, and the dependence of these relationships on inherent community properties. I factorially manipulated the presence of grazers and neighbouring established vegetation, soil nutrient availability and pH, and replicated these treatments on five acidic and five non‐acidic sites that showed contrasting patterns of inherent community properties and environmental conditions, i.e. community composition, evenness and soil nutrient availability. A seed mixture of 16 species was added to all experimental plots, and the establishment of seedlings was followed for 4 years. 3. Exclusion of mammalian herbivores had no significant impact on plant colonization rates nor did it interact with any other treatments. However, colonization rates were negatively influenced by fertilization and positively affected by the removal of neighbouring established vegetation, and biomass removal counteracted the negative effects of nutrient enrichment. The effects of fertilization and biomass removal on seedling recruitment were both greater in the non‐acidic habitats; the among‐habitat differences were associated with initial nutrient availability and forb : shrub ratio of vegetation, but were independent of community evenness. 4. Fertilization effects on colonization became increasingly negative with increasing prevalence of forbs which may be explained by the greater biomass increment potential of forbs under conditions of nutrient enrichment. Biomass removal enhanced colonization rates to a smaller extent in high shrub dominance where the overall recruitment was lowest, suggesting that dominant shrubs can create an interference effect that impedes seed germination and persists even after the removal of the dominant vegetation itself. 5. Synthesis. These results highlight the role of initial community properties, i.e. the potential for biomass increment of the resident species and the traits of the dominant plant groups as important mediators of biotic and environmental alterations on plant immigration rates.