Tree Diversity and Fungal Partners Drive Forest Life Below Ground More Than Above

Forests are often celebrated for what we can see — the towering trunks, layered canopies, and the birds and insects that move through them. Yet a new study published in Nature Ecology & Evolution suggests that much of the real action in a forest ecosystem happens underground, where an intricate web of roots, fungi, and soil organisms responds to the composition of the trees above in ways that scientists are only beginning to untangle. The research, based on a large-scale experiment combining tree diversity with mycorrhizal fungal dominance, provides some of the clearest evidence yet that two fundamental features of forest stands — how many tree species grow together and which types of root-associated fungi dominate — independently shape biodiversity across multiple trophic levels.

The study addresses a long-standing puzzle in forest ecology. For decades, researchers have documented that forests with more tree species tend to support more diverse communities of animals, fungi, and microorganisms, a pattern broadly consistent with the idea that greater plant diversity creates more niches, more resources, and more structural complexity. At the same time, ecologists have recognized that nearly all tree species form symbiotic relationships with mycorrhizal fungi — root-dwelling partners that exchange soil nutrients for plant carbon — and that the two dominant types of these symbioses, associated with different nutrient-acquisition strategies, create profoundly different soil environments. What has remained unclear is whether these two factors operate independently, interact with one another, or merely reflect the same underlying gradient of forest conditions.

Disentangling these effects requires a special kind of experiment, one in which tree diversity and mycorrhizal composition are manipulated deliberately rather than merely observed. The researchers behind the new study designed exactly such an experiment, establishing forest plots in which the number of tree species and the identity of the mycorrhizal types associated with those trees were controlled in a replicated design. This approach allowed the team to statistically separate the influence of tree species richness from the influence of mycorrhizal dominance, and to measure how each factor cascades through food webs both above the soil surface and within it.

The central finding is striking: tree diversity and mycorrhizal type each exert their own distinct influence on the diversity of organisms at higher trophic levels, and these influences do not simply collapse into a single combined effect. In other words, a forest’s capacity to support diverse communities of herbivores, predators, decomposers, and microbes depends both on how many tree species are present and on which fungal symbionts dominate the root systems — and knowing one of these factors does not allow scientists to predict the other. This independence has important implications, because it means that conservation and restoration strategies targeting only one of these dimensions may miss critical levers for supporting biodiversity.

Perhaps the most consequential result concerns the direction of these effects relative to the soil surface. While tree diversity and mycorrhizal composition shaped communities both above and below ground, the strength of their influence was consistently greater below ground. Organisms living in the soil — from fungal and bacterial decomposers to root-feeding insects and the predators that hunt them — responded more strongly to variation in tree diversity and mycorrhizal type than did organisms living in the canopy and on the forest floor above. This asymmetry makes intuitive sense once the biology is considered: soil communities are physically close to the roots and fungal networks through which trees channel carbon and draw up nutrients, so any change in the composition of those roots and symbionts propagates rapidly through the belowground food web.

The belowground emphasis of the findings adds to a growing appreciation among ecologists that soils are not merely a substrate supporting plant life but a vast reservoir of biodiversity in their own right. A single handful of forest soil can contain thousands of species of bacteria, fungi, and microscopic animals, many of which remain formally undescribed. These organisms drive the decomposition of organic matter, the cycling of nitrogen and phosphorus, and the formation of soil structure — processes on which forest productivity and, ultimately, the global carbon cycle depend. If the diversity and composition of these communities are governed largely by the trees and their fungal partners above them, then the way forests are planted, managed, and restored will echo through soil ecosystems for decades.

The mycorrhizal dimension of the study deserves particular attention. Trees are commonly categorized by the type of mycorrhizal association they form, with two major groups dominating temperate and boreal forests. One group of fungi is especially adept at accessing nitrogen directly from organic matter in the soil, while the other excels at scavenging inorganic nutrients over larger soil volumes. These different strategies leave different chemical fingerprints on the soil: stands dominated by one mycorrhizal type tend to accumulate organic layers and cycle nutrients more slowly, while stands dominated by the other foster faster decomposition and different microbial assemblages. By manipulating which type dominated their experimental plots, the researchers could show that these fungal legacies shape entire communities of soil organisms independently of how many tree species were planted.

The experimental design also allowed the team to examine how the two factors play out across trophic levels — the successive tiers of a food web, from primary producers through herbivores and decomposers to predators. The results indicate that the influence of tree diversity and mycorrhizal type propagates upward and outward through these levels, affecting not only the organisms that directly consume plant material or live on roots, but also the predators and higher-order consumers that depend on them. This multitrophic perspective is critical for understanding ecosystem functioning, because the diversity of consumers influences processes such as herbivore control, pollination, and the rate at which organic matter is broken down and its nutrients returned to the soil.

For forest managers and policymakers, the findings arrive at a moment when tree planting and forest restoration have become central pillars of climate and biodiversity policy around the world. Ambitious pledges to plant billions of trees frequently emphasize quantity — how many trees, how many hectares — while giving far less attention to which species are planted and in what combinations. The new study suggests that such considerations are not ecological fine print but fundamental determinants of how much biodiversity a restored forest can support. Mixtures of tree species spanning different mycorrhizal types, the results imply, are likely to support richer and more functionally robust communities both above and below ground than monocultures or narrowly composed plantations.

The research also carries implications for how scientists model and predict the consequences of global environmental change. As climate shifts, air pollution alters nutrient deposition, and land-use change simplifies forests, both tree diversity and the relative abundance of different mycorrhizal types are expected to change — often in ways that reinforce one another. Understanding that these two drivers operate independently gives modelers a clearer framework for predicting how forest biodiversity will respond, and it highlights the belowground realm as the arena where those responses will be felt most strongly and most rapidly. It is a reminder that the invisible architecture of roots and fungi beneath a forest floor may be as important to the future of biodiversity as the visible trees rising above it.

Beyond their immediate findings, studies of this kind contribute to a broader methodological shift in ecology. Observational surveys, while valuable, often struggle to separate correlated variables, because in natural forests tree species richness and mycorrhizal composition frequently co-vary with soil age, moisture, and land-use history. Manipulative experiments such as the one underlying this research allow ecologists to assign plots to combinations of tree species and mycorrhizal types at random, so that differences in biodiversity among plots can be attributed with confidence to the manipulated factors rather than to confounding environmental gradients. This is the same logic that underpins long-running grassland biodiversity experiments, which helped establish the relationship between plant diversity and ecosystem productivity decades ago, now extended to forest systems where trees interact with symbiotic fungi over much longer timescales.

The distinction between the two major mycorrhizal types also connects to fundamental biogeochemistry. Because one type mobilizes nitrogen from organic residues while the other relies more heavily on inorganic uptake, the two associations are associated with different rates of carbon storage in soils and different patterns of nutrient loss through leaching. Communities of decomposer animals, bacteria, and protists assemble differently under these contrasting conditions, which helps explain why belowground food webs responded so strongly in the experiment. For restoration practice, the practical takeaway is that species selection lists for planting programs could usefully include mycorrhizal type alongside growth rate and climate suitability, ensuring that new forests recreate not only the visible structure of natural stands but also the subterranean partnerships that sustain their biodiversity.

Subject of Research: The independent effects of tree diversity and mycorrhizal fungal type on above- and belowground multitrophic biodiversity in forests.

Article Title: Tree diversity and mycorrhizal type independently shape multitrophic biodiversity, with stronger effects belowground than aboveground

Article References: Yi, H., Ferlian, O., Becker, P. J., Christel, H., Huang, Y., Köhler, M., Meier, I. C., Ul Haq, H., Wubet, T., & Eisenhauer, N. (2026). Tree diversity and mycorrhizal type independently shape multitrophic biodiversity, with stronger effects belowground than aboveground. Nature Ecology & Evolution, 10(9), 1628-1643. https://doi.org/10.1038/s41559-026-03147-6

Image Credits: AI Generated

DOI: 10.1038/s41559-026-03147-6

Keywords: tree diversity, mycorrhiza, forest ecology, biodiversity, soil food webs, belowground ecology, ecosystem functioning, symbiosis, trophic levels, forest restoration, Nature Ecology & Evolution, multitrophic interactions