Ectomycorrhizae and tree seedling nitrogen nutrition in forest restoration

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 ECM fungal partners. Changes in ECM communities through primary and secondary succession are reviewed and related to the influence of N availability. The effect of N-related functional traits of ECM fungi on their distribution is discussed. Part II focuses on the ECM plant partners. The influence of ECM fungi on plant N uptake, and effects of N deposition and fertilization are presented. The benefit of ECM inoculation under different disturbance regimes and the benefit of greater ECM diversity are reviewed. Variations among and within tree and ECM fungal species in the forms of N taken up and utilized are highlighted. Conclusions include recommendations for including ECM fungi in forest restoration projects.