DURATION OF HERBIVORE REMOVAL AND ENVIRONMENTAL-STRESS AFFECT THE ECTOMYCORRHIZAE OF PINYON PINES

The mutualistic mycorrhizal symbionts of plants have shown mixed responses to herbivory; they either decrease, increase, or show no measurable change. We examined the ectomycorrhizal responses of pinyon pine (Pinus edulis) exposed to I yr of simulated herbivory in two environments, one stressful and one less so. We also compared levels of ectomycorrhizal colonization and conelet production in pinyons from which an important insect herbivore had been removed for either 1 or 10 yr. Pinyons that grew in more stressful cinder soils experienced significant reductions (19%) in ectomycorrhizal colonization following 1 yr of simulated herbivory while pinyons growing in less stressful sandy loam soils did not. These results indicate that the degree of environmental stress experienced by a plant could affect whether mycorrhizal reductions result from herbivory. In addition the reductions in ectomycorrhizal colonization that resulted from chronic herbivory remained for a full year following herbivore removal even though conelet production increased 250-fold in the same time period. Our findings regarding the role of environmental stress and duration of herbivory in affecting mycorrhizal responses may help explain the variable responses found in other systems.