Mycorrhizae, mycorrhizospheres, and reforestation: current knowledge and research needs

Although not a panacea, management of mycorrhizae and associated organisms is an important reforestation aid. Its threemajor components are protection of the indigenous soil community and evaluation of inoculation needs, integration ofinoculation programs into existing reforestation technology, and research. Clear-cutting frequently results in reducedmycorrhizae formation, particularly when reforestation is delayed and no other host plants are present to maintain fungalpopulations. Implications of such reductions for reforestation vary with environmental factors and tree species. Adequatemycorrhiza formation is especially critical for ectomycorrhizal trees growing on poor soils or in environments where seedlingsmust establish quickly to survive. It may also be important where early successional, noncrop plants do not support the samemycobiont as the crop. In such circumstances, a self-reinforcing trend may develop, with poor mycorrhiza formation reducingseedling survival and poor tree stocking leading to further loss of mycorrhizal inocula. Inoculating nursery seedlings withmycobionts holds promise for improving outplanting performance only if site-adapted fungi are used. A practical alternative is toimprove nursery practices to enhance natural populations of mycorrhizal fungi. Seedlings leaving the nursery with diversemycorrhizae may perform better than those leaving with only one or a few nursery-adapted types. Research is needed in threebroad areas: on adaptations of mycorrhizal fungi to particular environmental factors; on interactions between tree seedlings andprocesses occurring within the sphere of influence of roots (the rhizosphere) or of mycorrhizal roots (the mycorrhizosphere); andon the role of mycorrhizae and associated organisms in ecosystem structure and processes, particularly nutrient cycling,plant—plant interaction, and soil structure.