For decades, ecologists have sought to explain why some forests support dozens of plant species packed into a single hectare while others host only a handful. Traditional explanations have emphasized competition for light, water, and nutrients, along with the influence of climate and soil chemistry. But the new research adds a crucial layer to this picture: the identity and composition of mycorrhizal fungal partners. These fungi, which colonize plant roots and extend thread-like hyphae into the surrounding soil, exchange soil-derived nutrients—particularly nitrogen and phosphorus—for the carbon-rich sugars that plants produce through photosynthesis. The study demonstrates that the type of mycorrhizal association a forest’s dominant species depend upon fundamentally alters how plant richness and abundance change along environmental gradients.

The research distinguishes between the two principal mycorrhizal types that dominate forest ecosystems. Arbuscular mycorrhizal (AM) fungi, ancient symbionts that penetrate root cell walls, associate with many tropical and temperate trees and are particularly adept at acquiring nitrogen. Ectomycorrhizal (EcM) fungi, by contrast, envelop roots in a sheath and form extensive hyphal networks in the soil, and are characteristic of many conifers, oaks, and other trees typical of cooler or nutrient-poor environments. Because these fungal groups differ in how efficiently they extract nutrients and how they influence soil organic matter decomposition, forests dominated by one or the other type respond in strikingly different ways to the same environmental pressures.

Drawing on extensive forest inventory data spanning broad environmental gradients, the researchers analyzed how plant species richness and individual abundance co-vary with climatic factors such as temperature and precipitation, as well as with soil properties including pH, nutrient availability, and moisture. Crucially, they found that these relationships cannot be understood without accounting for the mycorrhizal composition of the forest community. In forests where arbuscular mycorrhizal trees dominate, plant richness tends to respond more strongly to soil nutrient gradients, reflecting the AM symbiosis’s dependence on labile nutrient pools and its association with faster nutrient cycling. In ectomycorrhizal-dominated forests, by contrast, richness patterns are often shaped more by climate and by the capacity of EcM fungi to unlock nutrients directly from organic matter, buffering the vegetation against some soil nutrient limitations.

The study also highlights the role of species dominance—a factor that ecologists increasingly recognize as a powerful driver of ecosystem processes. Rather than treating all species as equal contributors, the researchers weighted their analyses by the degree to which particular species dominate local communities. This approach revealed that the dominant trees in a forest, through their mycorrhizal associations, set the stage for the entire understory community. A forest dominated by ectomycorrhizal trees, for example, tends to accumulate thick layers of slowly decomposing organic litter, acidifying the soil and favoring a distinct suite of understory plants whose own fungal partners can tolerate or exploit those conditions. Conversely, forests dominated by AM trees typically feature faster litter decomposition, more neutral soil chemistry, and a different assemblage of herbaceous species competing for readily available nutrients.

These findings have profound implications for how scientists predict the responses of forests to ongoing environmental change. As global temperatures rise and precipitation patterns become more erratic, plant communities are expected to shift, with some species expanding their ranges while others contract. Most existing models of these dynamics treat plant species as independent actors responding directly to climate. The new research suggests that such models may be missing a critical mediator: the fungal networks that supply nutrients and shape competitive hierarchies. If environmental change alters mycorrhizal communities—through shifts in dominant tree species, soil acidification, or disruptions to fungal networks—the cascading effects on plant diversity could be far larger than climate-only models predict.

The mechanisms underlying these patterns are rooted in the fundamentally different nutrient economies of the two mycorrhizal types. EcM fungi produce powerful extracellular enzymes capable of breaking down complex organic compounds, granting their host plants access to nitrogen locked in leaf litter and soil humus. This capability allows ectomycorrhizal trees to thrive on nutrient-poor soils where decomposition is slow, and it creates feedback loops in which the trees’ own nutrient-poor litter further suppresses decomposition, reinforcing EcM dominance. AM fungi, lacking this enzymatic arsenal, rely on rapidly mineralized nutrients and cooperate with decomposer microbes indirectly, flourishing in environments where nutrient cycling is fast. These opposing feedbacks—slow and conservative in EcM systems, fast and open in AM systems—create alternative ecosystem states that respond asymmetrically to gradients of fertility, moisture, and temperature.

The study’s emphasis on dominance adds another layer of nuance. In many forests, a small number of tree species account for the majority of biomass and canopy cover, and it is these dominant species whose root systems and fungal partners most strongly condition the soil environment experienced by everything else. The researchers found that the mycorrhizal identity of dominant species, more than the overall proportion of mycorrhizal types in the community, best predicted how plant richness and abundance changed across gradients. This suggests that the loss of a single dominant tree species—whether through disease, logging, or climate stress—could reorganize the entire soil fungal community and trigger disproportionate changes in plant diversity throughout the forest.

Conservation practitioners and forest managers may find these results particularly consequential. Efforts to restore degraded forests or to plant trees for carbon sequestration often proceed without consideration of mycorrhizal compatibility. The new findings imply that matching planted species to the prevailing mycorrhizal conditions—and to the fungal partners of neighboring vegetation—could dramatically improve establishment success and biodiversity outcomes. Moreover, because mycorrhizal composition influences soil carbon storage, with EcM-dominated forests often storing more carbon in stable soil organic matter, understanding these dynamics is also relevant to climate mitigation strategies that rely on forests as carbon sinks.

The research also speaks to a broader theoretical debate in ecology: the relative importance of environmental filtering versus biotic interactions in structuring communities. Classical theory often treated environmental gradients as filters that permit or exclude species based on their traits, while interactions among organisms were considered secondary. The new work blurs this distinction, showing that a biotic interaction—between plants and fungi—effectively determines how the environmental filter operates. Environmental gradients still matter, but their effects on plant communities are channeled and amplified through the mycorrhizal partnerships that dominate the ecosystem.

Looking ahead, the researchers suggest that future work should extend these analyses to tropical forests, where mycorrhizal diversity is greater and where the AM symbiosis overwhelmingly prevails, and to regions experiencing rapid land-use change. Long-term monitoring of both plant and fungal communities, ideally with molecular tools capable of identifying fungal species from root and soil samples, would allow scientists to test whether the relationships identified here hold as environments continue to change. There is also growing interest in whether mycorrhizal networks facilitate nutrient transfer between trees of different species, potentially softening competition and contributing to the high diversity characteristic of many natural forests.

What emerges from this study is a vision of the forest as an integrated system in which the boundary between above-ground and below-ground life is porous and consequential. The plants that define a forest’s character—the towering canopy trees, the shrubs, the spring wildflowers—do not merely respond to climate and soil on their own terms. They do so in partnership with fungi whose evolutionary histories stretch back hundreds of millions of years, and whose presence or absence can determine whether a forest floor blooms with dozens of species or supports only a hardy few. As environmental pressures intensify across the globe, safeguarding these hidden partnerships may prove as important as protecting the visible forests they sustain.