Tree Fungi Alliances Are Shifting Across Japan as Climate Warms

One of the most consequential partnerships in the world’s forests is quietly being reorganized, and a new study of the Japanese archipelago suggests that the reshuffling is happening on a scale few ecologists anticipated. Across the chain of islands that stretches from the subtropical south to the cool north of Japan, trees that depend on one type of soil fungus are giving way to trees that depend on another. The research, published in Nature Communications, links this broad-scale shift in dominance among arbuscular mycorrhizal trees to two forces that are reshaping forests everywhere: the spread of tree diseases and the steady pressure of climate warming.

Mycorrhizal symbioses are among the oldest alliances in biology. Nearly all land-forming trees and plants rely on fungi that colonize their roots, extending the reach of the root system into the soil and trading minerals and water for sugars produced by photosynthesis. Ecologists divide these partnerships into broad functional groups. Arbuscular mycorrhizal fungi, often abbreviated AM, penetrate the cells of the root cortex and are ancient partners of flowering plants. Ectomycorrhizal fungi, by contrast, wrap themselves around root tips without penetrating cell walls and dominate among conifers and many trees of the oak, birch, and beech families. The two symbioses come with different nutritional economies: AM fungi are generally thought to be less selective foragers that deliver nitrogen-rich nutrients quickly, while ectomycorrhizal fungi excel at mining organic nitrogen directly from litter and soil organic matter. Which symbiosis dominates a forest therefore shapes how fast carbon cycles, how much carbon stays locked in soils, and which seedlings can establish beneath the canopy.

Because of these links to carbon and nutrient cycling, ecologists have long wanted to know whether the balance between mycorrhizal types is stable or whether it shifts with environmental change. Individual plots have shown responses to nitrogen pollution, drought, and warming, but evidence for a coherent, archipelago-wide reorganization has been scarce. The new analysis of forests across Japan provides exactly that: a broad-scale signal that the relative dominance of arbuscular mycorrhizal trees is changing along the length of the country, and that the change is not random.

The study’s central finding is that AM-associated trees are expanding their hold on Japanese forests, and that two explanatory threads run through the pattern. The first is disease. Tree diseases, driven by fungi, oomycetes, and insect vectors that thrive in a warming world, do not strike all tree species equally. Species that harbor ectomycorrhizal partnerships and species that harbor AM partnerships differ in their susceptibility, their rates of recovery, and their competitive ability following damage. When pathogens and pests remove or weaken particular canopy trees, the species that recruit into the gaps may disproportionately belong to the AM group, tipping the local balance and, cumulatively, the regional one. The second thread is climate warming itself. As temperatures rise, the climatic envelopes that once favored cool-adapted, ectomycorrhizal-rich forests in northern and montane Japan are shifting toward conditions that favor warm-adapted, AM-rich communities. Warming thus acts both directly, by altering which species can tolerate the local climate, and indirectly, by amplifying the diseases that thin the canopy.

What makes the result scientifically important is its breadth. Surveys and forest inventories distributed along the Japanese archipelago, which spans a remarkable gradient of climate from subtropical Ryukyu islands to boreal-influenced Hokkaido, revealed a coherent geographic pattern rather than a scatter of local anomalies. The archipelago functions in the study as a natural laboratory: because it is elongated along a latitudinal and thermal gradient, it allows researchers to ask whether the composition of forests is tracking climate in the way theory predicts. The answer is that it is, but with an added twist. The shift in mycorrhizal dominance is not simply a by-product of species moving poleward; it is entangled with the dynamics of disease, which can accelerate, redirect, or amplify the compositional change that warming alone would produce.

The mechanism by which disease and mycorrhizal type interact is an active area of research, and the Japanese findings add an important macroecological perspective to it. Mycorrhizal fungi do more than feed their hosts; they influence host defense, drought tolerance, and seedling survival. Ectomycorrhizal networks can support seedling establishment under parent trees, while AM associations often favor rapid growth and fast nutrient acquisition. A forest in which pathogens selectively remove ectomycorrhizal trees may therefore experience a cascade: fewer ectomycorrhizal adults mean fewer ectomycorrhizal propagules in the soil, less supportive fungal networks for the remaining regeneration, and an increasingly favorable environment for AM seedlings that thrive on disturbed, nutrient-flushed soils. Disease, in other words, can act as a ratchet, converting temporary losses into durable shifts in symbiotic identity.

Climate warming feeds this ratchet in several ways. Warmer winters fail to kill off insects and pathogens that cold once suppressed, extending their active seasons and geographic ranges. Warmer, sometimes drier summers stress trees, making them more vulnerable to attack. Extreme events such as typhoons, which regularly strike Japan, create large areas of disturbed forest in which fast-growing, disturbance-adapted species, many of them AM-associated, gain a foothold. Each of these processes has been documented in local studies; what the new research contributes is evidence that their combined effect is visible at the scale of the entire archipelago, registered in the shifting balance between mycorrhizal types.

The consequences of such a shift extend well beyond the identity of the trees themselves. Because AM and ectomycorrhizal forests differ in how they process nitrogen and store carbon, a wholesale conversion of forest symbiosis has implications for ecosystem function. Ectomycorrhizal-dominated forests are often associated with slower decomposition and greater storage of carbon in soil organic matter, partly because their fungi produce compounds that slow the breakdown of litter and because their nitrogen-mining strategy can suppress decomposer microbes. AM-dominated forests, in contrast, tend toward faster nutrient cycling, faster decomposition, and soils in which carbon is more exposed to microbial attack. A broad-scale transition from ectomycorrhizal toward AM dominance could therefore reduce the capacity of forest soils to lock away carbon, creating a feedback that adds to, rather than offsets, the warming that triggered the shift in the first place. The researchers emphasize that this is a hypothesis grounded in the established functional differences between the two symbioses, and that verifying the magnitude of any carbon feedback will require long-term monitoring of soils alongside vegetation.

There are also implications for biodiversity and forest management. The species that make up the AM and ectomycorrhizal pools differ in their economic and cultural value, in the wildlife they support, and in their responses to silvicultural treatment. Foresters in Japan have long managed stands of sugi, hinoki cypress, and other conifers, many of which rely on ectomycorrhizal partnerships, while broadleaved evergreens of the warm-temperate forests are predominantly AM-associated. A shift toward AM dominance would alter regeneration dynamics, the incidence of certain pests, and the suitability of land for different management objectives. Understanding the disease component of the shift gives managers an actionable lever: reducing pathogen spread, diversifying plantations, and protecting resistant genotypes could slow the conversion and buy time for adaptation.

The study also speaks to a growing recognition that global change operates through interactions rather than single causes. Warming alone would move species ranges; disease alone would reshape forests locally; but together, as the Japanese data show, they can produce a coordinated, archipelago-scale reorganization of one of the fundamental functional axes of forest ecosystems. For scientists modeling the future of the biosphere, the lesson is that predicting vegetation change requires tracking not only temperature and rainfall but also the health of the trees and the hidden fungal partnerships beneath their roots. For the forests of Japan, the finding is a warning and an opportunity in equal measure: the symbiotic identity of the woods is changing, and the window for understanding, anticipating, and perhaps guiding that change is open now, while the process is still measurable and, possibly, still manageable.

As monitoring continues, the Japanese archipelago will remain a bellwether. Its steep environmental gradients, rich forest flora, and dense long-term observational infrastructure make it one of the best places on Earth to watch the interplay of climate, disease, and symbiosis unfold in real time. The evidence assembled in this study indicates that the shift already underway is broad, structured, and driven by identifiable forces, and it establishes a baseline against which the forests of the coming decades will be judged.

Subject of Research: Climate warming and tree diseases driving a broad-scale shift in mycorrhizal tree dominance across Japanese forests

Article Title: Broad-scale shift in dominance of arbuscular mycorrhizal trees along the Japanese archipelago associated with tree diseases and climate warming

Article References: Schaefer, H., Yamashita, N., Hashimoto, S., Inagaki, Y., Kawanishi, A., Chatani, S., Shimadera, H., Furusawa, H., & Imaya, A. (2026). Broad-scale shift in dominance of arbuscular mycorrhizal trees along the Japanese archipelago associated with tree diseases and climate warming. Nature Communications. https://doi.org/10.1038/s41467-026-77711-w

Image Credits: AI Generated

DOI: 10.1038/s41467-026-77711-w

Keywords: arbuscular mycorrhiza, ectomycorrhizal fungi, Japanese forests, climate warming, tree diseases, forest ecology, mycorrhizal symbiosis, carbon cycling, soil fungi, forest composition, biogeography, Nature Communications