No relationship between trophic and functional complementarity in tree roots reveals new possible biodiversity effects

Background/Question/Methods

Thirty years of studies of the Biodiversity-Ecosystem Functioning relationship (BEF) gave a clear understanding of the functional role of species richness. However, these studies have mostly focused on one trophic level at a time and few empirical studies have so far integrated other interactions. Some recent theoretical work have addressed that issue and uncovered the mechanism of trophic complementarity. Two species sharing a common enemy may interact indirectly via “apparent” competition because an increasing population size of one species will translate into higher enemy pressure for the other. Species sharing less pathogens in common would then be more complementary and mixtures with low functional diversity could still show biodiversity effects such as overyielding. Our goal was to test whether these effects could take place by :1) comparing root microbial communities in tree mixtures versus monocultures; 2) describing the relationship between functional proximity and root microbial beta-diversity. Our experimental design uses two high diversity plantations with multiple combinations of tree mixtures along a functional diversity gradient. It builds on pairs of functionally equivalent tree species, but from either North America or Europe. We used Illumina high-throughput sequencing to characterize the structure of the root fungal and bacterial endophytic communities.

Results/Conclusions

We first showed that root microbial communities from mixtures significantly differed from the communities in monocultures. In other words, the microbial community in mixtures was different from the expected mixture based on the communities found in monocultures. This result suggests the possibility for apparent competition among our trees because root microorganisms that were not abundant enough to be detected by genetic sequencing on one host species can now thrive with a second host species in the vicinity. We then found that tree mixture functional diversity and root microbiome beta-diversity have no clear relationship, suggesting that indirect interactions may create new ground for species coexistence and contribute to species complementarity. This result also suggests that trophic complementarity could explain differences in ecosystem functions between tree species mixtures and monocultures that functional diversity alone could not. Our next project in this regard will be to test the relationship between trophic complementarity and tree mixture overyielding. This study offers new ways to look at the effects of biodiversity on ecosystem functioning and helps to close the gap between BEF and network ecology.