COS 153-3 - Ectomycorrhizal tree islands and soil inoculum enhance ectomycorrhizal colonization of tree seedlings in northeastern secondary forests

Abstract: Land use legacies in the eastern deciduous forest have resulted in greater abundance of arbuscular mycorrhizal (AM) trees like Acer spp. (maple) and decreased abundance of ectomycorrhizal (EM) trees like Tsuga canadensis (hemlock), Pinus strobus (pine), Fagus grandifolia (beech), and Quercus spp. (oak). The region now has many secondary AM maple-dominated forests with few EM trees such as Betula spp. (birch) present, suggesting a heterogeneous distribution of EM fungi in these forests. We hypothesized that EM tree seedlings planted near EM birch or inoculated with EM forest soil would exhibit greater EM percent colonization, greater EM fungal diversity, as well as a different EM fungal community composition compared to seedlings planted near AM maple. We planted EM pine, hemlock, and oak seedlings near scattered EM birch and AM maple trees (representing the AM-dominated forest matrix) at three secondary forests in New York State. At each tree, we also tested if EM forest soil inoculum collected from local EM-dominated forests can transfer EM fungal communities into AM-dominated secondary forests. We harvested seedlings after two years of growth, measured EM colonization of their roots, and then identified EM fungi with PCR-based methods.

Hemlock, pine, and oak seedlings planted near EM birch all had greater EM colonization (p < 0.003) and fungal richness (p< 0.002) compared to those planted near AM maple. All species of seedlings inoculated with EM forest soil also had greater EM colonization (p< 0.001) and fungal richness (p< 0.030), but only when planted near AM maple trees. EM fungal community composition was different between seedlings planted near EM birch versus AM maple (PERMANOVA: p< 0.001; R² = 0.032), and seedlings inoculated with EM forest soil (PERMANOVA: p< 0.001; R²= 0.030). Most EM fungi detected near AM maple were spore-bank forming taxa like Sphaerosporella, Tuber and Suillus, representing a depauperate EM fungal community (bootstrap diversity = 28.4) compared to the communities associated with EM birch (bootstrap diversity = 75.4) or inoculated seedlings planted near AM maple (bootstrap diversity = 51.2). Our results suggest that EM fungal communities are not ubiquitous and are largely restricted to EM patches which challenges assumptions of homogeneous distribution of EM fungal inoculant (spores and hyphae) in undisturbed forests. Additionally, our results suggest that local EM forest soil can be used to reintroduce EM fungi into AM-dominated secondary forests, particularly in absence of established EM trees.