Associate Professor, Biology University of British Columbia, Okanagan campus Kelowna, British Columbia, Canada
Mycorrhizal fungi may influence the direction of plant-soil feedback depending on the type of association formed by the host plant: hosts of ectomycorrhizal fungi (EMF) tend to promote neutral to positive feedbacks, whereas hosts of arbuscular mycorrhizal fungi (AMF) tend to foster neutral to negative feedbacks. Evidence consistent with these expectations has been documented among assemblages of canopy trees, but the understory vegetation – despite harbouring greater plant diversity – has rarely been considered. Using plant surveys from the United States Department of Agriculture’s Forest Inventory Analysis (FIA) database, we test the prediction that (i) the abundance of canopy EMF tree hosts is positively correlated with the abundance of EMF host plants in the ground layer, and (ii) the abundance of canopy AMF tree hosts is negatively correlated with the abundance of AMF host plants in the ground layer. In each FIA plot, we calculated the total proportional cover of EMF and AMF host types in the canopy and ground layers, and used Spearman correlation analysis to test our predictions. We used a randomization procedure to account for potential sampling artifacts and constrained randomizations to within ecoregions to account for potential environmental confounds.
Our results were consistent with our predictions, and the observed correlations remained significant after accounting for sampling artefacts and potential environmental confounds. Understory plants species are often assumed to be almost exclusively AMF hosts, however the plant cover in some plots in our study consisted of up to 50% EMF hosts. Mapping the encounter probability of AMF and EMF hosts between canopy and ground layers shows considerable variation across the western USA (where suitable FIA plots were located), demonstrating the potential for large differences in how canopy and understory species may interact, which should be considered when studying the emergent effects of mycorrhizas on forest communities. Our results highlight the important role understory plants may have on the mycorrhizal dynamics in natural forest ecosystems, which have thus far largely been ignored.