In natural environments, tree roots are almost always in intimate, symbiotic association with particular species of fungi through the formation of mycorrhizae. Most mycorrhizal fungi provide soil resources, particularly nitrogen (N), phosphorus and/or water to the tree, and can increase the abiotic and biotic stress resistance of their hosts. The fungi benefit by receiving fixed carbon from the tree. The association is of particular benefit on harsh or degraded sites. This review surveys recent literature on ectomycorrhizal (ECM) associations of temperate and boreal forest trees as it relates to N-nutrition and restoration of forests on sites where native mycorrhizal communities have been altered or depleted. Part I emphasizes the [...]