Most land plants live in symbiosis with fungi

Mycorrhiza is an association between a fungus and a plant's roots. Hyphae increase the surface from which the partner can take up water and mineral nutrients, particularly phosphorus and nitrogen. In return the plant passes to the fungus carbon compounds formed in photosynthesis. It is not an unconditional friendship: the balance of exchange depends on the species and the environment, but for a great many plants this arrangement is the basic way roots function.

The popular figure of "about 90 per cent" is a useful shorthand, but it needs explanation. Global reviews separate several types of symbiosis: arbuscular, ecto-, ericoid and orchid mycorrhiza. Their shares do not add up in a simple counter, because data for some families is incomplete and the status of certain plants depends on conditions. One broad synthesis estimated, among other figures, about 72 per cent of species with arbuscular mycorrhiza, 2 per cent with ectomycorrhiza, 1.5 per cent with ericoid and 10 per cent orchids dependent on fungi. About 8 per cent were considered wholly non-mycorrhizal, and some variable or poorly known.

For a mushroom picker, ectomycorrhiza matters most. Boletes, birch boletes, slippery jacks, saffron milkcaps and numerous amanitas form a sheath on the fine roots of trees. The presence of a host therefore narrows where to search: a birch bolete needs the right tree, and a slippery jack is associated above all with pines or larches. The tree alone does not guarantee fruiting bodies, because soil, weather, the age of the stand and competition from other fungi all count.

Mycorrhizal networks are sometimes presented as a "wood wide web". That is a catchy metaphor, but not evidence that a forest makes collective decisions. The flow of compounds between plants through mycelium has been observed, yet its scale and significance in natural forests are still being studied. Something simpler is certain: without fungal partners, most land plants would function entirely differently.

What the evidence actually shows

The familiar “90% of plants” figure combines several mycorrhizal types and rarely states what is being counted. Brundrett and Tedersoo estimated that roughly 72% of land-plant species form arbuscular mycorrhiza, 2% ectomycorrhiza, 1.5% ericoid mycorrhiza, and about 10% orchid mycorrhiza; approximately 8% were classed as non-mycorrhizal, with the remainder uncertain or context-dependent. These are proportions of species, not of individual plants or global biomass. The relationship is not equally beneficial in every setting either. Fungal partners can improve access to phosphorus, nitrogen, and water in exchange for plant carbon, but the balance shifts with soil fertility, light, and partner identity. The defensible conclusion is that mycorrhiza is the dominant condition among land plants, while a single round percentage obscures both biological diversity and uncertainty in the underlying records.

How do researchers test effects on seedlings?

Finding a tree, a seedling and a mushroom beside one another is not enough. Researchers need a comparison that changes access to fungal hyphae. In a Chinese subtropical forest study, seedlings were grown in cores with mesh openings of 35 or 0.5 micrometres. Different permeability allowed the influence of hyphal access on growth to be tested. Positive effects were found in the studied ectomycorrhizal species and some arbuscular species. The authors nevertheless stated that the precise mechanisms producing the benefits remained unresolved. The 2020 paper reports a particular experiment and set of species.

This distinction matters: greater seedling biomass is a measured result, whereas an older tree “feeding its offspring” is an additional interpretation. Demonstrating that account would require establishing the source of the transferred compounds, their route, the recipient and the biological importance of the flow. A beneficial effect of hyphal access does not answer all those questions by itself.

Three levels of claims about a forest network

The first level is an association between a fungus and roots. The second is a fungal connection between several plants. The third is a particular consequence of that connection, such as a change in nutrition, growth or survival. Evidence for the first level is not automatically evidence for the third. Likewise, detecting movement does not establish that the overall balance benefits every partner.

A 2023 analysis by Karst, Jones and Hoeksema identified overinterpretation and selective citation of positive findings in literature on common mycorrhizal networks. The authors discuss the limits of the evidence, rather than disputing the existence of mycorrhiza itself. Readers can make a useful check: does the headline describe the same organism, environment and outcome that the study actually examined?

In the field, this means using tree information with care. A potential host helps describe habitat but does not establish a particular underground network. A photograph of a bolete between two trunks does not reveal which roots its mycelium reaches or which way compounds move. Keep that difference between observation and interpretation in a field note too. It is possible to appreciate a remarkable association while leaving its unmeasured details open.