Mycorrhizal legacy mediates seedling success following timber harvesting in Northeastern forests

Overstory trees rely on arbuscular mycorrhizal (AM) or ectomycorrhizal (EcM) fungi for establishment and growth. However, we know very little about how timber harvesting affects the biotic (e.g., mycorrhizal fungi) and abiotic (e.g., nutrient availability) legacies left behind by AM and EcM trees. These belowground mycorrhizal legacies, including potential shifts in abundance of mycorrhizal fungi, may differentially affect seedling success. To address this knowledge gap, we investigated how the legacy of AM and EcM forest stands affected seedling survival and growth, soil fungal communities, and soil chemistry including nutrient availability and pH following timber harvesting in a temperate hardwood forest. We established 16 plots, half of which were AM- and half of which were EcM-dominated. Eight of the 16 plots were harvested and then planted with four AM- and four EcM-associated seedling species in a split-plot design. The eight unharvested control plots allowed us to compare changes in fungal biomass, and soil pH and nutrients in the harvested plots to a baseline. AM fungal biomass decreased in both AM and EcM plots only in the first fall after timber harvesting, while EcM fungal biomass continued to decrease in both AM and EcM plots over three growing seasons. The mycorrhizal legacy also affected seedling success; AM seedlings had higher survival when planted in AM legacy plots, but there was no effect on EcM seedlings. AM and EcM seedling growth was unaffected by legacy. Patterns in foliar nitrogen (N) showed that although AM seedlings closer to plot edges may have acquired more N through mycorrhizal fungi relative to EcM seedlings at plot edges, they also had lower percent N, indicating a tradeoff between increased access to mycorrhizal networks and competition with proximity to live roots. Alternatively, this mycorrhizal legacy effect could have been driven by preexisting soil characteristics. Given that some AM tree species are sensitive to soil calcium (Ca), higher soil extractable Ca in AM legacy plots suggests a complex set of mechanisms driving seedling success beyond potential access to mycorrhizal fungi.