How fundamental plant functional trait relationships scale-up to trade-offs and synergies in ecosystem services

1. Ecosystem services (ES) lie at the core of the interactions among humans and ecosystems. Fundamental understanding of the ecological mechanisms underlying the simultaneous provision of multiple ES has been lagging behind policy and management needs and stands out as a research priority. In this paper, we focus on interactions between ES resulting from fundamental functional mechanisms. 2. Plant diversity contributes significantly to the delivery of ES. Specifically, functional composition strongly determines different ecosystem properties and services. Knowledge on associations and trade-offs among different plant traits is well established, but the consequences for ecosystem functioning and the resulting ability for ecosystems to provide multiple services have only started to be explored. We present a conceptual framework linking environmental change to changes in ecosystem functioning and to changes in ES through plant traits. We explicitly consider the leaf economics and size axes of plant functional variation, and how their responses to key environmental variables are expected to scale-up to ecosystem properties and ES. 3. The framework was tested using a structural equation modelling formalism to understand the trait-based mechanisms driving trade-offs in ES in mountain grasslands. Variations along the leaf economics spectrum (LES) towards more exploitative strategies in response to increasing fertility led to a combined increase in several ES valued by local stakeholders, including agronomic value, cultural value and soil water retention. Surprisingly, and contrary to published hypotheses, soil carbon sequestration in the studied subalpine system did not increase at lower fertility which was associated with more conservative plant strategies. Independent variation in LES and height provided alternative pathways to biomass production. 4. Synthesis: A trait-based framework can support the understanding and aid the management of multiple ES. We recommend testing this framework in a variety of contexts and at larger scales, using additional trait axes such as wood density or seed size.