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.