Almost all trees form partnerships with fungi that help them access soil nutrients in exchange for energy. New research by Dartmouth soil scientists shows that some trees are paying more for these nutrients than others.
Trees that associate with ectomycorrhizal (EcM) fungi, like pines and oaks, are inefficient at taking up nutrients when they are plentiful compared to trees that associate with arbuscular mycorrhizal (AM) fungi, like cherries and maples. This difference could explain why AM-associated trees are becoming increasingly dominant in temperate forests, since agricultural runoff and climate change both increase soil nitrogen content.
"Trees that partner with arbuscular fungi are getting their nutrients at a discount, whereas trees that partner with ectomycorrhizal fungi are overpaying, which means they have less energy they can grow with," says Caitlin Hicks Pries, associate professor in the Department of Biological Sciences and senior author of the study published in Global Change Biology. "This might be a reason why we're seeing a pattern of AM trees becoming more successful and replacing EcM trees."
Several undergraduates were involved in the research, three of whom are co-authors on the paper: Zachary Shortt '25, Augustos Vrattos '25, and Ella Laurent '25. The students worked closely with Amelia Fitch, the study's first author, who led the project as a Guarini PhD candidate in Hicks Pries' lab and is now a postdoctoral fellow at Oregon State University.
"Undergrads were a huge part of this study, all the way through—they helped us set up the experiment and plant the seedlings, harvest the plants and separate them into aboveground and belowground components, and analyze the plants' mycorrhizal colonization," says Hicks Pries.
Shortt still works in Hick Pries' lab this fall and is developing a senior thesis that explores how ectomycorrhizal fungi can help restoration projects in California. "Working on this study radically changed how I view ecology and ecosystems, contributing to my current goal of pursuing a PhD in belowground ecology," he says.
Humans are altering the structure of forests globally via climate change, agricultural expansion, and by introducing invasive species. This has important implications for both biodiversity and climate, because forests are an important carbon sink: trees not only store carbon in their wood, but they also secrete large amounts of carbon into the soil. How human-mediated environmental changes are impacting trees' belowground carbon storage is unclear, but Hicks Pries thought that fungi might be involved.
"Trees shape how carbon, nitrogen and other nutrients cycle through soils, so I wanted to investigate how this function is changing, given that the makeup of our forests is changing," says Hicks Pries. "I was curious about whether the type of mycorrhizal fungi that a tree associates might determine how much carbon and nitrogen is in the soil, and how fast it cycles."
There are two major classes of mycorrhizal fungi: EcM fungi, which ensheathe the roots of their host plant, and AM fungi, which have evolved to live inside their host plant's roots. The two types of fungi are associated with different types of plant.
"EcM trees evolved in situations where decomposition is really slow, and nutrients are really hard to come by, so they're very successful in cold ecosystems, like boreal forests," says Hicks Pries. "It's possible that the way we're changing the world—making it warmer and adding a ton of nitrogen onto agricultural lands—is sort of changing the world in a way that makes EcM trees less efficient and less adapted to the Anthropocene."
To investigate how mycorrhizal fungi impact trees' ability to access nitrogen in different contexts, and whether this had any impact on how much carbon the trees stored in the soil, the researchers grew eight different species of temperate forest seedlings in a controlled environment that enabled them to measure how much carbon and nitrogen the plants used. After five months of growth, they quantified the amount of carbon that each plant deposited into the soil and used microscopy and genetic sequencing to examine each plant's fungal associations.
They found that, when exposed to higher levels of nitrogen, EcM-associated seedlings responded to higher levels of nitrogen by releasing more carbon from their roots. In contrast, nitrogen availability had no impact on the amount of underground carbon invested by AM-associated seedlings.
They found that EcM-associated seedlings released more carbon from their roots when they were exposed to higher levels of nitrogen, but nitrogen availability had no impact on the amount of underground carbon invested by AM-associated seedlings. However, despite these differences in carbon input, there was no difference in the overall amount of carbon stored in the soil at the end of the experiment between EcM- and AM-associated seedlings.
"We didn't see any difference in carbon storage overall, because when roots put carbon into the soil, it can actually stimulate decomposition, which causes more carbon to be lost as CO2," says Hicks Pries.
Looking ahead, Hicks Pries is planning to expand and investigate a third type of symbiotic fungi called "ericoid mycorrhizal fungi" that partners with shrubs including blueberries, huckleberries, rhododendrons, and mountain laurel.
"Some forests to the south of us in Connecticut and Massachusetts are beginning to be overrun with these very dense evergreen shrubs that associate with ericoid fungi, so for our next steps we're planning to add those to the mix to see how they are affecting soil carbon and nitrogen," Hicks Pries says. "We're very interested in knowing how much carbon can be stored in different types of ecosystems, because if that carbon is stored in the soil, it's out of the atmosphere, and it's not contributing to climate change."
