Plant traits mediate consumer and nutrient control on plant community productivity and diversity

Background/Question/Methods

Consumers and nutrients are two interacting factors that are predicted to exhibit major impact on plant community productivity and diversity, yet their joint effects on terrestrial ecosystems are poorly known. One understudied element is the functional traits of the species present in the resident vegetation, which may drive community responsiveness to alterations in herbivory and nutrient availability. Plant functional traits associated with growth rate and rates of nutrient acquisition (e.g. C:N ratio, specific leaf area [SLA], C-based secondary metabolites) are also connected to avoidance and tolerance of herbivory, and may predict community responsiveness to both of them. Here, we report results of a six-year full-factorial field experiment using mammalian herbivore exclusion and fertilization in alpine tundra of Northern Finland. The experiment was carried out in two habitat types, fertile and infertile tundra heaths, which differ in plant functional composition. We asked (1) what are the joint effects of mammalian herbivory and fertilization on plant community productivity (i.e. live and dead biomass) and diversity? (2) What is the role of resident community traits (C:N ratio, SLA, condensed tannins) in predicting productivity and diversity responses to herbivory and fertilization?

Results/Conclusions

We found that (1) Herbivory and fertilization exerted a joint impact on plant communities, such that herbivory counteracted the effect of fertilization on biomass. However, live biomass dominated the response to this interaction in infertile habitats, where tannin-rich evergreen shrubs were prevalent; litter accumulation dominated the response in fertile habitats, where tannin-poor perennial herbs were abundant. (2) Reflecting the negative effects of litter accumulation, species richness declined under herbivore exclusion and fertilization at fertile habitats. (3) Plant functional traits in resident communities played significant roles in mediating these effects: Fertilization decreased and herbivory increased dominance in communities originally dominated by N-poor plants (i.e. plants having high C:N ratio) whereas fertilization increased and herbivory diminished dominance in communities where relatively N-rich plants prevailed (i.e. plants having low C:N ratio). Our results highlight the close interdependence between consumer effects, soil nutrients and plant functional traits. Consumer-plant theory will benefit from better consideration of plant traits, especially those related to the capacity for rapid resource acquisition and growth as well as palatability to herbivores.