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Functional implications of inter- vs. intraspecific trait diversity in European forests

Final Report Summary - DIVERFOR (Functional implications of inter- vs. intraspecific trait diversity in European forests)

The main goal of DIVERFOR is to understand the extent and role of the inter- and intraspecific trait variability of tree species on ecosystem functioning in European forests. In other words, it aims at disentangling the contribution to the overall functional diversity of the variability given by individuals within a species. In the last two decades, the remarkable and increasing biodiversity loss derived from global change has speeded up efforts to quantify its consequences on ecosystems. Results from different studies have drawn an overall conclusion that correlates diverse communities with a positive response in productivity and stability against environmental fluctuations or disturbances, pointing at that more diverse communities are more likely to include species with different requirements (complementarity) and high performing species (sampling effect). To this regard, the inclusion of trait-based approaches to diversity has greatly contributed to its understanding, because the biodiversity effect arises from functional differences (trait variation) among plants. However, most of the previous studies only considered the interspecific diversity (different species), ignoring the differences among individuals within a species, i.e. the so-called intraspecific trait variability, precisely what DIVERFOR tackles.
To achieve the main goal, four specific objectives were posed to be addressed in collaboration with another European project (FunDivEUROPE: ‘Functional significance of forest biodiversity in Europe The specific objectives are:
1) To address a selection of traits to quantify the inter- and intraspecific variability in tree species;
2) To analyse the extent of within- and between-species trait variability in the different types of forests across Europe and their contribution to the total functional diversity in a community;
3) To analyse the scale in which trait variability is distributed (at individual, species, plot or regional scale);
4) To analyse the diversity-functioning relationships along diversity gradients of tree species in different European forests.
Firstly, a thorough review of literature on intra- and interspecific trait variability was done, on the basis of which traits to assess the functional diversity were selected. Field work was carried out in three of the “Exploratory study sites” established for the EU project FunDivEUROPE: a boreal forest in Finland, a temperate forest in the Carpathian Mountains in Romania and a Mediterranean forest in Spain. In each site, plots were established along a diversity gradient in terms of species richness of the tree dominant species. In these plots, architectural traits from more than 4000 trees were compiled, and nearly 1800 trees were sampled to assess leaf traits, that were recorded in more than 15000 leaves/needles. These tasks took place during the first year of the project.
The first result drawn from DIVERFOR was the outstanding relevance of the within-species component of the functional diversity of tree species in European forests, showing in some cases over 50% of the total variability at community level. European forests are relatively species-poor (low diverse) compared to similar regions in other continents, in terms of dominant tree species. This is partly due to natural factors such as glaciations and the geographical situation of mountain ranges (impairing species migration during ancestral climatic shifts), but also to the long human occupation of the land that has caused uncountable disturbances altering the original ecosystems. Hence, it is confirmed that in these relatively species-poor stands, the extent of the differences among individuals within a species may represent an important part of the total diversity, and improve the power to detect relationships between biodiversity and ecosystems services.
Another important result is that trees adjust their characteristics in response, not only to the abiotic environment (light, soil nutrients and water, climatic conditions, etc.), but also to the biotic conditions, including the diversity of the neighbours (the species richness). In other words, trees showed a high phenotypic variability in the measured traits. Our results showed that individuals of the same species presented differences in some leaf and architectural characteristics when they grew among individuals of the same species compared to those sharing the place and resources with other species, and the direction of these trait shifts was site-specific. These shifts involve an increment of the functional diversity within a single species along the different patches of a forest (at meta-population level), that may confer on the species more chances to cope with environmental fluctuations and disturbances.
This project can be considered a starting point for studies focusing on more types of forests and species. It provides new insights regarding different ecological questions like the study of biodiversity-ecosystem functioning relationships, the community assembly, and species coexistence mechanisms, but also may provide important knowledge for forest managers and policy makers. We need to acknowledge that the response of communities to global change drivers (climatic change, loss of habitat, invasive plants, etc.) depends on their ability to cope with changes in the environment and adapt to new conditions. Communities with higher functional diversity, i.e. with characteristics more variables (either this variability comes from differences between or within species) are expected to have a higher resistance, stability and resilience under global change scenarios. This conclusion advocates the preservation of diverse communities, not only in number of species (the so-called alpha-diversity) but including patches with different structures and compositions (the so-called beta-diversity). This conclusion is also supported by the linked project FunDivEUROPE, which proved the relevance of these heterogeneous patches in a forest to maintain a high numbers of services provided by ecosystems (van der Plas et al 2016 PNAS).