Functional diversity and functional identity determine carbon storage and tree productivity in Spanish forests

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Biodiversity loss could reduce primary productivity and carbon storage provided by forests. Thus, biodiversity-ecosystem functioning relationships (BEF) are an important topic in ecology and have been broadly studied during the last decades. However, the underpinning mechanisms of biodiversity effects on multiple forest functions are not completely understood. Two non-exclusive mechanisms of the effects of diversity have been proposed: the complementarity effect (i.e. functionally diverse forests could enhance resource use efficiency and nutrient retention through niche partitioning and facilitation) and the selection effect (i.e. dominant species determines the level of ecosystem functioning, where there is a high probability that the most productive species will become dominant in the community). We used the Spanish Forest Inventory and maximum likelihood techniques to quantify how climate, stand structure and diversity shape carbon storage and tree productivity. Diversity effects included both complementarity and selection mechanisms, measured respectively through functional diversity and functional identity measures. Diversity had a significant effect on both carbon storage and tree productivity, even when controlling for climatic and stand structural confounding factors. Our results indicate that BEF relationships are maintained through both complementarity and selection effects. A consistent positive effect of functional diversity on carbon storage and tree productivity was observed from Mediterranean to Atlantic Forests, but the largest impacts occurred at low functional diversity levels (i.e. from monospecific to functionally diverse forests we observed an average increase of 32% in carbon storage and 21% in tree productivity). Selection effects were particularly important in deciduous and Mediterranean pine forests, but had very little effect on mountain pines. We conclude that management efforts should aim at promoting both functionally diverse forests and functionally important species, which could ensure the maintenance of key ecosystem functions such as carbon storage and tree productivity.

Supplementary information
The National Forest Inventory Platform is coordinated by Prof. Christian Wirth (Leipzig University, Germany) and supported by the group of Prof. Miguel A. Zavala (University of Alcalá, Spain). A major contribution of this platform has been the integration of a wide set of forest data included in the National Forest Inventories of different countries across Europe (i.e. Spain, Wallonia, Germany, Finland and Sweden). This is undoubtedly valuable information with data at two different levels of organization: tree and community. Next to forest data integration it has been harmonized with climatic and topographic information from Worldclim and GTOPO30 datasets. As an initial study we used c. 54,000 plots of the Spanish Forest Inventory and based on species abundance we classified each plot in the main forest types present in the Iberian Peninsula, from Mediterranean pine forests to Atlantic deciduous forests. Using maximum likelihood techniques we quantified the relative importance of stand structure, climate and diversity on both carbon storage and tree productivity. We also measured how functional diversity and functional identity (i.e. complementarity and selection mechanisms) shapes carbon storage and tree productivity, using key traits of tree performance obtained from TRY Initiative (see
Biological diversity, or the shorter "biodiversity," means the diversity, or variety, of plants and animals and other living things in a particular area or region. It describes the variability among living organisms from all sources, including terrestrial, marine, and other aquatic ecosystems and the ecological complexes of which they are part. Biodiversity includes diversity within species, between species, and between ecosystems. (MA, 2003)
Description of tag diversity.
Description of tag diversity.
The value, range, and relative abundance of traits present in the organisms in an ecological community ( MA 2003). The range and value of those species and organismal traits that influence ecosystem functioning (Tilman. 2001).
Description of tag diversity.

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