title: Taking a closer look: disentangling effects of functional diversity on ecosystem functions with a trait-based model across hierarchy and time.
abstract: Biodiversity and ecosystem functioning (BEF) research has progressed from the detection of relationships to elucidating their drivers and underlying mechanisms. In this context, replacing taxonomic predictors by trait-based measures of functional composition (FC)-bridging functions of species and of ecosystems-is a widely used approach. The inherent challenge of trait-based approaches is the multi-faceted, dynamic and hierarchical nature of trait influence: (i) traits may act via different facets of their distribution in a community, (ii) their influence may change over time and (iii) traits may influence processes at different levels of the natural hierarchy of organization. Here, we made use of the forest ecosystem model 'LPJ-GUESS' parametrized with empirical trait data, which creates output of individual performance, community assembly, stand-level states and processes. To address the three challenges, we resolved the dynamics of the top-level ecosystem function 'annual biomass change' hierarchically into its various component processes (growth, leaf and root turnover, recruitment and mortality) and states (stand structures, water stress) and traced the influence of different facets of FC along this hierarchy in a path analysis. We found an independent influence of functional richness, dissimilarity and identity on ecosystem states and processes and hence biomass change. Biodiversity effects were only positive during early succession and later turned negative. Unexpectedly, resource acquisition (growth, recruitment) and conservation (mortality, turnover) played an equally important role throughout the succession. These results add to a mechanistic understanding of biodiversity effects and place a caveat on simplistic approaches omitting hierarchical levels when analysing BEF relationships. They support the view that BEF relationships experience dramatic shifts over successional time that should be acknowledged in mechanistic theories.
authors.affiliation: AG Spezielle Botanik und Funktionelle Biodiversität , Institut für Biologie, Universität Leipzig , Johannisallee 21, 04103 Leipzig, Germany.
authors.forename: Frédéric
authors.initials: F
authors.lastname: Holzwarth
authors.affiliation: German Centre for Integrative Biodiversity Research (iDiv) Halle-Jena-Leipzig , Deutscher Platz 5e , 04103 Leipzig, Germany ; Smithsonian Tropical Research Institute , Apartado 0843-03092 , Balboa Ancón, Panama.
authors.forename: Nadja
authors.initials: N
authors.lastname: Rüger
authors.affiliation: AG Spezielle Botanik und Funktionelle Biodiversität , Institut für Biologie, Universität Leipzig , Johannisallee 21, 04103 Leipzig, Germany ; German Centre for Integrative Biodiversity Research (iDiv) Halle-Jena-Leipzig , Deutscher Platz 5e , 04103 Leipzig, Germany.
authors.forename: Christian
authors.initials: C
authors.lastname: Wirth
keywords: LPJ-GUESS; biodiversity and ecosystem functioning; forest succession; functional composition; hierarchical trait influence; trait-based modelling