THE mutualistic ectomycorrhizal fungi associated with the roots of woody perennials can enhance nutrient uptake and provide protection from pathogens in exchange for up to 40% of the photosynthate produced by host plants1-9. By removing photo-synthetic tissue, herbivores could reduce the amount of photosynthate available for maintaining this mutualism10-15, 29. Here we examine how ectomycorrhizal levels vary between trees resistant and susceptible to an insect herbivore, and demonstrate how mycorrhizal levels respond to the experimental removal of a native herbivore under natural conditions. We find that pinyon pine trees susceptible to chronic insect attack have 33% fewer ectomycor-rhizae than resistant trees, demonstrating that the herbivore-mycorrhizae-host plant interaction differs between resistant and susceptible trees. We removed insects from susceptible trees and find that the mycorrhizal levels of these trees increased to a level comparable to that of resistant trees. This demonstrates that herbivores negatively affect the mutualism between ectomycorrhizal fungi and susceptible trees, and that mycorrhizal levels can rebound after herbivore removal. The dynamics of these interactions on resistant and susceptible plants could be important for understanding plant-pest interactions in natural and managed systems.
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