Introduction
Biodiversity plays an important role in maintaining ecosystem functions
including primary productivity
(Hooperet al. 2005; Hautier et al. 2015; Delgado-Baquerizoet al. 2020). Understanding the mechanisms underlying positive
diversity-productivity relationships is a major goal in ecological
research. However, species do not exist in isolation, and plant species
interact intimately with microbiota in ways that may mediate
biodiversity-productivity relationships
(Yanget al. 2021). A number of studies have shown that plant
microbiomes are affected by plant diversity
(Rottstock et al. 2014;
Scheibe et al. 2015; Khlifa et al. 2017; Grossman et
al. 2019; Strukelj et al. 2021) and may mediate
diversity-productivity relationships (Maron et al. 2011;
Schnitzer et al. 2011; Laforest-Lapointe et al. 2017), but
the specific mechanisms involved remain unclear.
The positive net diversity effect on productivity (i.e., overyielding)
can occur via ‘selection effects’ or ‘complementarity effects’. Positive
selection effects occur when communities with more productive species
drive the diversity-productivity relationship, while positive
complementarity effects occur when more diverse communities exhibit
enhanced productivity due to niche partitioning, facilitation, and/or
biotic feedbacks
(Loreau
& Hector 2001; Barry et al. 2019). Soil biota can play a role in
complementarity through a variety of mechanisms that involve
decomposers, consumers/pathogens, and mutualists
(Eisenhauer
2011). For example, soil fungi may influence complementarity if plant
species associate with different fungal mutualists that alter resource
acquisition strategies to create greater niche separation among host
plant species (Koide 2000; Hartet al. 2003; Bever et al. 2010; Afkhami et al.2014; Kazenel et al. 2015; Luo et al. 2018). Additionally,
if soil fungal pathogen effects on plant species are density dependent,
improved productivity may result when host species are at lower
densities in more diverse communities
(Maronet al. 2011; van Ruijven et al. 2020). Therefore, soil
fungi could play an important role in complementarity effects on
productivity, but their effects may be context dependent.
Climate change is expected to have large impacts on plant productivity,
for example, by altering soil water availability through shifts in
precipitation regimes (Allenet al. 2010; Felton et al. 2021). Positive
diversity-productivity relationships are expected to be more common
under stressful environmental conditions, i.e., the stress gradient
hypothesis, but support for this hypothesis is mixed
(Maestre et al. 2009; Wanget al. 2013; Belluau et al. 2021). Soil biota may mediate
the influence of soil water availability on diversity-productivity
relationships. Soil mutualists such as mycorrhizal fungi can play a role
in plant water acquisition and drought tolerance, but water availability
can also influence the abundance of pathogens and susceptibility of
plants to pathogen attack
(Desprez-Loustauet al. 2006; Lehto & Zwiazek 2011). Given the potential of
climatic change to alter precipitation regimes, it is important to
understand the role of soil water availability on the
diversity-productivity relationship and whether soil microbes can modify
this relationship
(Verheyenet al. 2008).
The objective of this study was to determine whether soil fungal
communities influence the diversity-productivity relationship in tree
communities exposed to contrasting soil water availability. We
hypothesized that (1) soil fungal richness increases with tree diversity
and is positively linked to tree community productivity, (2) soil fungal
community composition mediates the relationship between tree diversity
and productivity, (3) richness of ectomycorrhizal and plant pathogenic
fungi influence the diversity-productivity relationship, and (4) soil
fungal richness and community composition mediate the effect of water
availability on diversity-productivity relationships.