The role of fungi in diversity-productivity relationships
Our results demonstrate that soil fungal communities play a significant and complex role in the relationship between tree diversity and productivity under contrasting soil water availability. While soil fungal diversity is often hypothesized to be positively correlated with plant diversity, results vary among studies (Mommer et al. 2018; Liuet al. 2020; Shen et al. 2021). Here, we found that tree functional diversity decreased fungal richness, and interestingly, fungal richness was also negatively associated with above-ground productivity. Therefore, fungal richness mediated a positive, indirect effect of tree functional diversity on tree community productivity. This result clearly demonstrates that direct, inter-trophic diversity relationships do not need to be positive to enhance the plant diversity-productivity relationship.
Fungal beta diversity measured as multivariate dispersion along three NMDS axes was related to tree functional diversity and species richness but in different directions among axes, suggesting that soil fungi-plant diversity relationships are complex. We demonstrated that a portion of the apparent direct effects of tree functional diversity and richness on productivity are in fact associated with indirect effects mediated by fungal community composition. Furthermore, both the direct and indirect effects were positive, indicating that fungal beta diversity supports the positive diversity-productivity relationship.
Previous studies have shown that complementarity effects are the primary influence on the diversity-productivity relationship in tree communities (Williams et al. 2017; Luo et al. 2018; Belluau et al. 2021; Yang et al. 2021), although switching from initial selection effects (Tobner et al. 2016) to complementarity can in some cases take more time than observed here (Urgoiti et al.2022). However, CE can have multiple causes including abiotic facilitation, resource partitioning, and biotic feedbacks that may be mediated by soil fungi (Luo et al. 2018; Barry et al. 2019; Yang et al. 2021). We found that fungal communities mediated positive indirect effects of tree species richness on CE, suggesting that fungal community effects on productivity are associated with facilitation, niche partitioning, or biotic feedbacks; however, distinguishing between these mechanisms is difficult and all three mechanisms may overlap (Barry et al.2019). We examined key functional roles of fungal taxa in tree communities that could help to tease apart these complex interactions. Negative biotic feedbacks can promote positive diversity-productivity relationships and CE, for example, where “dilution” of plant pathogens in more diverse communities is associated with negative density dependence (Eisenhauer 2011; Maronet al. 2011, 2016; Schnitzer et al. 2011). In our study, more species-rich tree communities had lower pathogen richness, but pathogen richness alone was not directly associated with productivity. This suggests that pathogen richness is not driving the negative association of total fungal richness with productivity or the positive indirect effect of tree species richness on productivity.
Mycorrhizal fungi could also be a factor driving the diversity-productivity relationships (Jonsson et al. 2001). Mycorrhizal fungi can potentially influence resource partitioning among species, facilitation via sharing of resources between species, and biotic feedbacks (Van Der Heijden & Horton 2009; Bever et al.2010). We found that ECM richness was negatively associated with tree community productivity. It is possible that a greater diversity of ECM fungi could increase competition between tree species by immobilizing resources in their hyphal networks (Franklin et al. 2014). Alternatively, if the dominant ECM taxon is also most beneficial to tree productivity, then increased diversity of ECM fungi could introduce competition and diminish the effect of the dominant taxa (Livne-Luzon et al. 2017). Tree species richness increased ECM richness which mediated a negative, indirect effect on productivity, NE, and CE, in contrast to the positive, indirect effect observed for fungal community composition. More complex interactions such as those between pathogen and ECM richness may also influence the positive, indirect effect of tree richness on productivity for example if ECM protect plants from pathogens (Whipps 2004). Pathogen and ECM richness mediated both positive and negative indirect effects of tree species richness on productivity, and also played a role in NE and CE on productivity. Taken together, these results strongly suggest that plant-soil microbe interactions are critical to understanding the mechanisms underlying plant diversity-productivity relationships.