Evidence of mycorrhizal host generality for hemlock woolly adelgid-infested Tsuga canadensis trees growing in a Quercus-dominated landscape

Background/Question/Methods

The ectomycorrhizal eastern hemlock (Tsuga canadensis; hereafter “hemlock”) is subject to herbivory by the invasive hemlock woolly adelgid (Adelges tsugae) which is causing widespread mortality. In addition, a warming climate is predicted to reduce the importance value of hemlock in much of the eastern US, while creating favorable conditions for the establishment of oak species. Over the long term, the ectomycorrhizal fungal (EMF) communities presently associated with large conspecific hemlock stands could become uncommon, as stands become fragmented and hemlock is found less often as a foundation species than as a tree with a minor presence in oak forests. Oaks and hemlock are both ectomycorrhizal trees that rely on their fungal mutualists to provide soil-associated nutrients. The degree to which hemlock is a mycorrhizal specialist may have a bearing on its survival and success at reestablishment within oak-dominated forests. This study sought to discover EM fungi shared by hemlock and oak trees and assess hemlock-associated EMF diversity measures and adelgid-related defoliation across a range of hemlock densities within oak-dominated forests. The study was conducted in the Black Rock Forest in Orange County, NY, where oak species and hemlock dominate tree composition, and where adelgid-related hemlock defoliation is widespread. Twelve 0.04 ha plots were chosen to represent a variety of hemlock and oak densities; a soil core was taken on three different dates at 1 m from the bole of a hemlock centered in each plot (36 cores total). Mycorrhizal root tips from all cores were visually identified as to morphotype. DNA from 121 root tips representing the morphotypes was analyzed to discover both fungal species and tree genus. EMF diversity measures and hemlock defoliation were compared with the 12 hemlock/oak plot tree densities using correlation.    

Results/Conclusions

Of a total of 36 EMF taxa, 12 were common to both hemlock and oak root tips while nine were found only on hemlock tips and 15 were found only on oak tips. EMF diversity was higher (Shannon r = 0.553, P = 0.062; evenness r = 0.618, P = 0.032) and there was lower plot-wide hemlock defoliation (r = −0.653, P = 0.019) as oak density increased in relation to combined hemlock/oak density. The results suggest that oak forests may present an ideal habitat for surviving and reinvading hemlock trees, by supporting EMF inocula common to both tree genera, by supporting EMF diversity, and by diminishing the defoliation of adelgid-infested hemlock trees.