A Forest Is More Than Trees: What Soil Biodiversity Reveals
We often look at a forest by what protrudes from the ground: trunks, foliage, birds, light. Yet, a large part of its functioning takes place where our eyes almost never go: beneath our feet.
A study published in August 2026 in Nature Ecology & Evolution shows how the diversity of tree species and their type of mycorrhizal symbiosis can shape an entire network of underground life, from microorganisms to soil predators.
An open-air forest laboratory
The researchers relied on the MyDiv experiment, which has been in operation in Germany since 2015. In this young experimental forest stand, they can vary two elements: the number of tree species present and the dominant type of mycorrhizae.
They then observed organisms living in the canopy, on the soil surface, and underground. Nearly 100,000 invertebrates were identified and categorized according to their position in the food chain, in addition to analyzing bacteria and fungi.
More species, more life… especially underground
The result is clear: as tree species richness increases, so too does the abundance and diversity of many subterranean organisms. The effect is apparent in microorganisms and nematodes at lower trophic levels, and then spreads to certain predators in the mesofauna and macrofauna.
Above ground, the effects are much more limited. This means that the benefits of a diverse population can be particularly powerful in the least visible part of the ecosystem.
Mycorrhizae are not a simple detail
The type of mycorrhizal symbiosis also plays a structuring role. Some tree species associate primarily with ectomycorrhizal fungi, others with arbuscular mycorrhizal fungi. These strategies influence nutrients, roots, microorganisms, and, in turn, the soil food web.
Interestingly, the effects of tree diversity and mycorrhizal type were largely additive, not mutually reinforcing. A diverse forest, therefore, does not automatically become "better" simply because of a mix of mycorrhizae; the two factors largely act as separate drivers.
Why this changes the way we think about a forest
These results serve as a reminder that a forest cannot be reduced to the number of trees it contains. A monoculture plantation and a forest composed of several species can have a comparable quantity of timber while offering very different conditions for underground biodiversity.
It is also an invitation to move beyond the sometimes overly simplistic image of the "Wood Wide Web". Mycorrhizal networks are important, but they are part of a much larger whole made up of roots, nutrients, bacteria, fungi, nematodes, invertebrates and predators.
Caring for life also means caring for the invisible.
When we talk about forest restoration or regeneration, the question is therefore not only: how many trees are we going to plant? It also becomes: what species, what interactions, what soil, what ecological continuity, what diversity do we want to allow?
And that changes a lot of things. A living forest is not a collection of individuals lined up. It is a community of organisms, most of which still elude us.
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