are symbiotic (intimate) associations between fungi and the feeder roots of a vascular plant. The symbiosis is mutually beneficial and is necessary for the fungus to complete its life cycle. Plants supply carbon from photosynthesis to the fungi; in turn, fungi absorb minerals and nutrients from the soil and transfer them to roots. In a mycorrhizal association, the fungus colonizes the host plants' roots, either intracellularly (within cells) as in arbuscular mycorrhizal fungi, or extracellularly (between cells) as in ectomycorrhizal fungi. Ectomycorrhizal fungus (EMF) diversity is an important attribute of many temperate forest ecosystems. For instance, ectomycorrhizae help to stabilize below-ground processes after disturbance. Ectomycorrhiza communities are particularly influential with respect to nutrient availability and tree nutrition, so may be influential in the predisposition of Douglas-fir to SNC. Seedlings associated with a high diversity of ectomycorrhizae may be better adapted to disturbance as compared to seedlings with less diversity, and ectomycorrhiza diversity also seems to increase a tree’s competitive abilities. Douglas-fir has about 2000 EMF symbionts throughout its range, and will not grow in soil without ectomycorrhizal fungi. Although preliminary fertilization trials have not yet found evidence of nutritional amelioration of SNC, it is still plausible that imbalanced nutrition may contribute to the susceptibility of Douglas-fir to SNC.