Ectomycorrhizas and tree seedling establishment are strongly influenced by forest edge proximity but not soil inoculum
Reforestation is challenging when timber harvested areas have been degraded, invaded by nonnative species, or are of marginal suitability to begin with. Conifers form mutualistic partnerships with ectomycorrhizal fungi (EMF) to obtain greater access to soil resources, and these partnerships may be especially important in degraded areas. However, timber harvest can impact mycorrhizal fungi by removing or compacting topsoil, removing host plants, and warming and drying the soil. We used a field experiment to evaluate the role of EMF in Douglas-fir reforestation in clearcuts invaded by Cytisus scoparius (Scotch broom) where traditional reforestation approaches have repeatedly failed. We tested how planting distance from intact Douglas-fir forest edges influenced reforestation success and whether inoculation with forest soils can be used to restore EMF relationships. We used an Illumina DNA sequencing approach to measure the abundance, richness and composition of ectomycorrhizal fungi on Douglas-fir roots, and assessed differences in Douglas-fir seedling survival and growth near to and far from forest edges with and without forest soil inoculum. Planting Douglas-fir seedlings near forest edges increased seedling survival, growth, and EMF root colonization. Edge proximity had no effect on EMF richness but did change fungal community composition. Inoculations with forest soil did not increase EMF abundance or richness or change community composition, nor did it improve seedling establishment. With Illumina sequencing, we identified two to three times greater species richness than described in previous edge effects studies. Of the 95 EMF species we identified, 40% of the species occurred on less than 5% of the seedlings. The ability to detect fungi at low abundance may explain why we did not detect differences in EMF richness with distance to hosts as previous studies. Our findings suggest that forest edges are suitable for reforestation, even when the interiors of deforested areas are not. We advocate for timber harvest designs that maximize edge habitat where ectomycorrhizal fungi contribute to tree establishment. However, this study does not support the use of inoculation with forest soil as a simple method to enhance EMF and seedling survival.