Tree and mycorrhizal fungal diversity drive intraspecific and intraindividual trait variation in temperate forests: Evidence from a tree diversity experiment

Abstract

The study of tree species coexistence is crucial to understand the assembly of forest communities. In this context, trees adjust their traits in response to the interactions with other trees and, specifically, as a result of the competition for resources. Further, mycorrhizal fungal diversity and associations are important drivers of ecosystem functioning in forests, but their role as drivers of intraspecific trait variation has been disregarded. Here, we studied intraspecific trait variation of trees in response to tree and mycorrhizal fungal diversity. We sampled 3200 leaves from 640 trees belonging to 10 native, deciduous species in a tree diversity experiment in Central Germany. This experiment relies on the combination of gradients of tree richness and mycorrhizal associations. To handle large amounts of leaf samples, we acquired leaf-level spectral data and used deep learning to predict values for five leaf traits from the leaf economics spectrum (LES): specific leaf area, leaf dry matter content, carbon to nitrogen ratio, carbon content and phosphorus content. For every tree, we calculated the mean value for every trait and two multi-trait functional indices (functional richness and functional dispersion) based on values for individual leaves. Finally, we used sequencing-based data to assess the richness of mycorrhizal fungi associated with the trees. We found that tree and mycorrhizal fungi richness had an effect on different leaf functional traits. Specifically, tree richness positively affected specific leaf area and, additionally, had a negative effect on the functional indicies, which revealed that the phenotypic diversity within the tree crown decreased with tree species richness. In addition, leaf carbon to nitrogen ratio decreased with increasing arbuscular mycorrhizal fungal richness in both arbuscular and ectomycorrhizal tree species. Finally, we did not find differences between arbuscular and ectomycorrhizal trees regarding their location within the LES. Our results suggest that trees modify their strategy in response to local tree diversity, not only by shifting trait values but also by shifting the variability intraindividually. In addition, higher mycorrhizal fungal diversity does not seem to lead to higher complementarity, but instead, tree and mycorrhizal fungi affect different aspects of leaf traits.