Aspen Groves and Fungal Allies Threatened by Climate Change

Beneath the lush canopies of North America’s aspen groves dwells a hidden yet fundamental network of life. In the depths of the forest floor, an ancient symbiosis between aspen trees and fungi allows these verdant forests to thrive.

As climate change quickly progresses, the delicate balance between fungi and tree is under immense threat. Recent findings from Standford University suggest that the resulting shifts in environmental conditions may alter the stability between beneficial and detrimental fungi in Populus trees.

The essential symbiosis between aspen trees and fungi

Aspen trees, belonging to the Populus genus, share a mutually beneficial relationship with many different kinds of fungi, primarily mycorrhizal fungi. This symbiotic partnership is crucial for the overall health and well-being of aspen forests.

There are two main kinds of mycorrhizal fungi: ectomycorrhizal and arbuscular mycorrhizal. Ectomycorrhizal fungi typically associate with aspen trees, wrapping themselves around the roots, forming a protective sheath that acts as a barrier against pathogens and other external stressors. This sheath dramatically extends the range of the tree’s root system, thereby enhancing its ability to absorb water and nutrients such as nitrogen and phosphorus.

In exchange, the fungi receive essential carbohydrates and other organic compounds that are produced by the tree through photosynthesis. This exchange of nutrients is essential to the survival and growth of both organisms.

However, the symbiotic system facilitated by mycorrhizal fungi goes beyond nutrient and water sharing. The connection between the fungi and roots also connects aspens to a more extensive forest network. This network, known as the ‘Wood Wide Web’ allows trees and plants within a forest ecosystem to share nutrients and water and even communicate warning signals to protect against environmental threats. Without the help of mycorrhizal fungi, this complex and vital network would not exist.

Unfortunately, the delicate relationship between aspen trees and fungi is under increasing strain due to the rapid progression of climate change. According to the new Stanford study, rising temperatures and changes in precipitation patterns can alter the sensitive microbial communities in the soil. Some fungi within these communities, such as mycorrhizal fungi, are incredibly beneficial in Populus groves, but pathogenic fungi are still a looming threat in these ecosystems. 

The consequences of climate change on aspen groves

To build upon over a decade of fungal diversity research, the study analyzed fungi associated with five Populus species at 94 groves across 21 different states. Using a simulation, the researchers looked into how the extreme weather and drought predicted from climate change could alter the types of fungi present in an area.

Although the abundance of mycorrhizal fungi initially increased, they also decreased in diversity, and their numbers are expected to drop if temperatures continue to rise (1). This initial increase could be a possible result of a last-ditch effort by capable mycorrhizal fungi to provide extra support to parched trees in times of water scarcity, but since fungi are incredibly vulnerable to changes in their environment, increasingly warmer and drier weather may leave Populus trees with less fungal variants to protect them. Since fungi thrive in humid environments, many of the helpful species may be unable to withstand the changes in moisture.

 “Diversity is really important for stability and overall productivity of these systems, so it’s quite concerning that we might see fungal diversity decline,” said Michael Van Nuland, the co-lead author of the study and lead data scientist at the Society for the Protection of Underground Networks (SPUN). 

A drop in beneficial fungi would open the door for pathogenic fungi to become more dominant in the soil ecosystems of Populus groves, as these harmful fungi thrive in conditions that are less favorable to mycorrhizal fungi. This occurrence would not only impact the health of aspen trees but also affect the entire surrounding forest ecosystem.

The broader ecological implications

Aspens and the mycorrhizal fungi associated with them are not the only organisms under threat due to climate change. This pattern can be observed in various ecosystems around the world, where similar symbiotic relationships are in danger. For example, North America’s Whitebark Pine, European Beech trees, and several Oak species have become more vulnerable to diseases and pests because of increased stress from changing climate conditions (2). 

As the symbiotic systems between trees and fungi weaken, the forest’s overall resilience can lead to increased tree mortality rates and prevent the ecosystem from ever fully recovering from these disturbances. Healthy forests are critical for the survival of other species, and the impact of environmental changes can have a ripple effect that threatens the entire ecological community. The biodiversity of local flora and fauna may suffer as a result due to habitat loss and altered food chains. 

Furthermore, forests are significant carbon sinks that help capture and store carbon. More than 75% of the Earth’s carbon pool is underground, and mycorrhizal fungi play a major role in transferring carbon into soil systems. Because of their connection to plant roots, these fungi move some of the carbon absorbed by trees into their body of mycelium underground. The more nutrients these fungi provide for their host plant, the more CO2 they receive, which is then locked into the ground for up to several centuries.

Trees are viewed as one of the main sequesters of carbon. On average, a single tree can absorb around 48 pounds of CO2 per year. As they grow, much of this stored carbon accumulates in their biomass. However, when trees die, their biomass is released back into the atmosphere, which could further exacerbate the increasing conflict of rising CO2 levels. Increased tree mortality rates would mean that more dead trees release CO2 back into the environment and less can absorb external carbon into the ground. 

Recognizing the escalating need for conservation, scientists are focusing on developing methods to protect fungal species from the impacts of climate change. This attention is required sooner than later, since these fungi are an integral part of a broader ecological process that sustains multiple forms of life and cleans our atmosphere of excess carbon emissions.

Conservation efforts

As growing research reveals the possible decline in fungal diversity, scientists are looking into different methods to assist these fragile ecosystems. One considered approach involves pinpointing the kinds of fungi that are more resilient to increased temperatures and other climate-induced stressors.

If these fungi can be identified, it may become possible to introduce them into the soil or onto the leaves of Populus trees to once again balance the microbial communities essential to their health. This form of targeted intervention can help enhance the resilience of these fungal communities, allowing aspen groves to better withstand the stresses brought about by climate change.

Although researchers may have a long way to go before determining the best means to resolve the conflict looming in U.S. Populus groves, not all hope is lost for the future of beautiful aspen trees. Comprehending the dynamics between Populus trees and their associated fungal communities, along with anticipating how these relationships may evolve, can guide conservation and restoration strategies in forests all over the globe, even in the face of climate change.

“We hope that this work helps shed light on the immense diversity of fungal life that’s in our soils and in our plant communities,” says Kabir Peay, senior study author and associate professor for Stanford’s Earth System Science. “We can’t easily see them with our eyes, but they have a big impact on the world around us.”

References

1. Van Nuland, Michael E., S. Caroline Daws, Joseph K. Bailey, Jennifer A. Schweitzer, Posy E. Busby, and Kabir G. Peay. 2023. “Above- and Belowground Fungal Biodiversity of Populus Trees on a Continental Scale.” Nature Microbiology 8 (12): 2406–19. https://doi.org/10.1038/s41564-023-01514-8.
2. Nguyen, Diem, Bastien Castagneyrol, Helge Bruelheide, Filippo Bussotti, Virginie Guyot, Hervé Jactel, Bogdan Jaroszewicz, Fernando Valladares, Jan Stenlid, and Johanna Boberg. 2016. “Fungal Disease Incidence along Tree Diversity Gradients Depends on Latitude in European Forests.” Ecology and Evolution 6 (8): 2426–38. https://doi.org/10.1002/ece3.2056.