LB 19-201 - Islands in the shade: scattered ectomycorrhizal trees influence soil inoculum and heterospecific seedling response in a secondary forest

Background/Question/Methods:

The eastern deciduous forest exists as a mosaic of arbuscular (AM) and ectomycorrhizal (EM) trees but anthropogenic effects, such as agricultural land use have altered overstory tree composition. One such result is an increase in the abundance of AM trees like Acer spp. (maple) with a concomitant decrease of EM trees like Tsuga canadensis (hemlock) and Pinus strobus (pine) in many forests. While the effects of agricultural land use have been well documented for understory plant and overstory tree communities, there is little known about the availability of EM fungi in post-agricultural secondary forests. We used a factorial growth chamber bioassay to investigate whether soils near scattered EM Betula lenta (black birch) support a greater abundance and diversity of EM fungi compatible with hemlock and pine seedlings than the surrounding AM-dominated secondary maple forest, and whether their growth was responsive to fungal propagules associated with scattered overstory birch trees. We also examined how inoculation with soil from an adjacent EM-dominated old-growth forest influenced mycorrhizal colonization and growth of pine and hemlock seedlings compared to secondary forest soil alone.

Results/Conclusions:

We found that scattered birch trees support communities of EM fungi that were mostly lacking from the surrounding maple forest but supported a lower overall EM fungal diversity than old-growth forest soil. Seedlings grown in soil collected near birch trees were dominated by Cenococcum geophilum, Hyaloscypha bicolor, Tomentella sublilacina, and Tuber spp. Tuber arnoldianum was the only EM fungus identified from seedlings grown in surrounding maple forest soil. Hemlock seedlings grown in soil collected near birch trees grew about twice as large as those grown in maple forest soil (p< 0.001), while hemlock and pine seedlings both exhibited increased growth from inoculation with old-growth forest soil (p< 0.001). Seedling EM fungal richness was positively related to total biomass for both hemlock (p=0.002) and pine (p< 0.001), but a stronger effect was detected with hemlock. Our results suggest there is a lack of EM fungal propagules in secondary AM-dominated forests and highlight the importance of scattered EM trees for supporting EM fungi critical to the growth of establishing pine and hemlock seedlings. By factoring in mycorrhizal associations of desirable tree species, restoration ecologists and foresters may improve EM tree seedling regeneration to fulfill management objectives.