Forest microbiomes and aboveground forest functioning

Forest soils and roots harbor hyper-diverse microbiomes which strongly shape the growth and development of plant communities. How the biodiversity and functional capacities of the microbiome influence emergent ecosystem functioning is an important next frontier in the biogeosciences with implications for conservation, ecosystem management, and microbiome engineering. Directly testing hypotheses between microbiome diversity and forest functioning has been obstructed by a lack of paired data on microbiomes and in situ observations of forest growth and health. Here, I will synthesize findings from two large-scale European forest soil and root sampling efforts where we identified features of the mycorrhizal fungal, soil fungal, and bacterial communities linked to variation in forest tree growth and nutrition. We sampled roots and/or soils across 285 forest monitoring plots spanning 18 European countries, sequenced full-length fungal ITS and prokaryotic 16S amplicons to relate microbiomes to high resolution observations of tree growth, foliar nitrogen and phosphorus content, and aboveground carbon stocks. We show that the composition, richness, and traits of both root and soil inhabiting symbiotrophic fungal guilds is tightly linked to variation in tree growth, nutrition, and aboveground biomass carbon stocks while the biodiversity of free-living microbiomes was not tightly coupled to these key metrics of aboveground forest functioning. We also produced a roster of fungal biodiversity and compositional traits which are strong positive and negative bioindicators of forest carbon storage and nutrition. These large-scale observations lay important groundwork for experimental studies and demonstrate that soil microbiomes capture unique variation in emergent forest functions that cannot be explained by other physical, chemical, and biological properties.