Over a third of the world’s tree species, ranging from tropical magnolias to mountainous pines, are at risk of extinction. This alarming update was provided by the International Union for the Conservation of Nature in October, as part of its Red List of threatened species. Trees now constitute more than a quarter of all species on the Red List and face extinction risks in almost every country. The absence of fungi, or the inability of fungi to adapt, could be partly responsible for trees' struggle to cope with climate change. Most tree species rely on underground symbiotic fungi, known as ectomycorrhizal fungi, for essential nutrients and water (SN: 7/13/09). Like other organisms, these fungi may be having difficulty adapting to climate change, particularly heat and drought. However, scientists still have much to learn about how these crucial fungi are responding to climate change and how this impacts trees' survival in various regions.
“These interactions are absolutely vital for both the aboveground and belowground ecosystems,” says Michael van Nuland, a soil microbe scientist specializing in tree-fungi relationships. “Yet, we are still grappling with understanding how these relationships will evolve with climate change.” Van Nuland, working at the Society for the Protection of Underground Networks, and his colleagues published a study in PNAS earlier this year that explored how climate change is reducing the overlap between trees and underground fungi, thereby limiting the areas where trees can relocate. The researchers utilized North American distribution data for 50 tree species and 402 species of soil fungi, along with their DNA, to map “suitable habitat” where tree species and soil fungi overlapped. They also incorporated climate data to analyze the current conditions in these habitats and modeled future climate scenarios to predict how both trees and fungi would respond.
The resulting maps indicated that, as anticipated, suitable habitat for both trees and fungi tends to move northward into cooler and wetter conditions. However, 35 percent of all tree-fungi pairings face diminishing areas where both can survive. Without the appropriate fungi, trees cannot migrate north in tandem with their changing climate. “If we truly aim to conserve trees and their diversity, we must comprehend mycorrhizal plant interactions,” says Aimée Classen, a soil ecologist at the University of Michigan, who was not involved in the study. “I believe we are heading in the right direction.” Van Nuland was somewhat surprised that only about a third of tree-fungi pairs faced habitat shrinkage. “It feels a bit low, which likely means it’s a conservative estimate,” he notes.
This type of habitat loss differs from traditional extinction risk assessments, such as those conducted by the IUCN. It’s not just about habitat size; it’s also about ecological function. “It’s about the loss of species interactions,” van Nuland explains. “You’re missing a critical element necessary for survival, much like lacking the right climate.” Deforestation also poses a threat to fungi. “When you clear-cut a forest, you’re also destroying the belowground fungal network,” van Nuland points out. “It’s just not as visible.” The data revealed that suitable habitat shrinkage was largely due to a lack of soil fungi biodiversity at the edges of the overlap. Trees that could migrate in response to climate change had more options for soil fungi at the edges of their suitable habitat, increasing their chances of finding a suitable partner. Conversely, tree species that lagged in migration were in areas with lower soil fungi diversity.
“It truly demonstrated that fungi play a role in aiding trees to move across the landscape in response to climate change,” van Nuland concludes. “Fungi unlock the potential for trees to escape.”
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