Research on biodiversity – ecosystem functioning has demonstrated links between plant diversity and ecosystem functions such as productivity. At other trophic levels, the plant microbiome has been shown to influence host plant fitness and function, and host-associated microbes have been hypothesized to influence ecosystem function through their role in defining the extended phenotype of host organisms. However, the importance of the plant microbiome for ecosystem function has not been quantified in the context of the known importance of plant diversity and traits. We address this lack of knowledge using high-throughput sequencing of the leaf (phyllosphere) bacterial communities of a high-density tree diversity experiment that varied plant community composition along a gradient of functional trait diversity. We ask two questions: 1) what is the relative influence of host species identity, neighbor species identity, host functional trait diversity and host species richness on leaf bacterial community structure and diversity?; and 2) what is the relationship between leaf bacterial diversity and tree productivity? Using a novel tree biodiversity – ecosystem functioning experiment, we provide strong support for the hypothesis that leaf bacterial diversity is positively linked with ecosystem productivity even after accounting for the role of plant diversity.
Results/Conclusions
The drivers of leaf bacterial community structure at the tree-level were host species identity (PERMANOVA; F=12.68, R2=26.6%, P=0.001), plant functional identity (R2=1.7%) and plant functional diversity (R2=1.3%). Likewise, host species identity (F18,535=38.0, P<0.0001), functional identity (F1,474=26.2, P<0.0001) and functional diversity (F1,302=21.9, P<0.0001) influenced leaf bacterial diversity. The effect of leaf bacterial diversity on plant community productivity was significant (0.12; P=0.002) even when accounting for the effects of all other variables (structural equation model; χ2=1.451, P=0.484). Plant species richness (0.61; P<0.001), functional identity (-0.28; P<0.001) and functional diversity (0.26; P<0.001) had a strong impact on productivity, with plant species richness being the strongest determinant of plant community productivity (R2=85%). In addition, plant species richness (0.29; P=0.003), functional identity (0.44; P<0.001) and functional diversity (0.18; P=0.08) also influenced leaf bacterial diversity (R2=41%). Here we show that plant-associated microbial diversity is positively related with plant community productivity, explaining a portion of the variation in productivity that would otherwise have been attributed to plant diversity and functional traits. Using one of the most extensive studies of tree leaf bacterial communities to date, our work suggests that models of biodiversity – ecosystem functioning can be improved by including plant-associated microbial diversity.