The great equalizers? Arbuscular mycorrhizal fungi interrupt ectomycorrhizal forest dominance by promoting plant species coexistence

Background/Question/Methods

The factors which determine species coexistence and maintain diversity in terrestrial systems is a key question in ecology. Forests are complex, multi-species assemblages where empirically, we know that trees coexist. However, stable coexistence is rarely identified in the plant ecology literature. One potential explanation for these results might be that plants are typically removed from their microbial contexts for experimental studies. Yet plants are never found in nature without their microbiomes and these communities can play critical roles in plant establishment and function. For example, root-associated fungal mutualists such as mycorrhizal fungi help to determine plant competitive ability and host niche breath. Using a controlled greenhouse study, we manipulated the presence of two main types of mycorrhizal fungi, ectomycorrhizal fungi (EMF) and arbuscular mycorrhizal fungi (AMF), and used a range of conspecific and heterospecific competitor densities to investigate the role of host-specific mycorrhizal mutualisms on patterns of vegetation structure. We specifically predicted that: (H1) host-specific fungi increase fitness differences, (H2) that EMF would decrease intraspecific competition (due to network effects), and (H3) that when both plants have their host specific fungi, coexistence is likely to due to increased niche partitioning.

Results/Conclusions

In support of our first hypothesis, we found that host-specific fungi increase fitness differences of respective host plants (H1). Preliminary analyses also suggest that EMF decrease intraspecific competition, which leads to competitive exclusion and monodominance of EMF host plants (H2). Growth of EM hosts was positively correlated with colonization by EMF (linear mixed effects model; marginal R2= 0.456, slope=1.34, P< 0.001). Preliminary analyses also suggest that when both plant competitors have their host specific fungi, coexistence is likely due to increased niche partitioning, where, based on natural abundance stable isotope data, preliminary analyses find that plants access different pools of nitrogen (H3). In addition, we find that AMF also increase the carrying capacity of the system, meaning that AMF enable more plants to grow in a given system. In all, we demonstrate how mycorrhizal fungi may help explain a central question in plant community ecology about processes of plant community assembly and coexistence. Specifically, we report that the presence of EMF is one axis by which EMF-hosts form monotypic stands. Further, we indicate that in the many places were coexistence occurs between plants which host these distinct mycorrhizal types, AMF play a critical role in promoting net-stabilizing and equalizing forces.