COS 238-3 - Mycorrhizal-mediated management in Northeastern forests

Abstract: Mycorrhizal fungi are essential for tree growth, providing nutrients and water in exchange for photosynthate, and many other ecosystem services including soil carbon storage and pathogen resistance. New England forests are dominated by two types of mycorrhizal fungi—arbuscular mycorrhiza (AM) and ectomycorrhiza (EcM) depending on the tree species present. Forests dominated by EcM or AM fungi differ in seedling survival, nutrient availability, and how soil carbon is stored. As climate change and invasive insects shift the species composition of New England forests, the composition of mycorrhizal fungi will also change. With the impending loss of ash due to the emerald ash borer, northern forests lose one of the few overstory trees that associate with AM fungi. Along with the long-term decline in sugar maple, another AM-associated canopy tree, northern forests may become more dominated by EcM trees, which will have cascading effects on the ecosystem services New England forests provide. Consequently, we sought to understand how the legacy of dominant mycorrhizal associations affects how forests regenerate and cycle nutrients following timber harvesting. In this field experiment, we investigated how changes in soil fungal communities and nutrient availability affect seedling survival and growth in AM- and EcM-dominated forest stands after timber harvesting. We planted eight species of seedings, half of which were associated with each mycorrhizal type, across eight plots, half previously dominated by AM and half by EcM fungi. An additional eight plots served as unharvested controls to compare soil processes including nutrient content, moisture, and temperature. Following two years of growth, our results show that the legacy of mycorrhizal type in harvested forest plots significantly affects soil nitrogen availability, and seedling survival and growth. AM-associated seedlings had a higher rate of survival in previously AM than EcM-dominated plots, and overall had higher survival than EcM-associated seedlings. AM plots also had higher levels of soil available nitrogen, which was positively related to seedling growth rates. This difference in mycorrhizal-associated survival and growth rates indicates that planting AM-associated tree species in soil formally dominated by AM-associated trees could help declining species populations such as sugar maple, whereas EcM-associated seedlings such as red oak could offer a more general approach as it can thrive when planted in either mycorrhizal legacy.