A partnership older than forests
Pull up a small tree carefully and examine the finest roots. Most of them are not bare. They are wrapped in or filled with fungal tissue. This partnership is called mycorrhiza, from the Greek words for fungus and root. It is not a rare curiosity: the large majority of land plants, including most trees, form some kind of mycorrhizal association. Evidence suggests that the relationship is very old, dating from near the time when plants first colonized land.
What each partner gets
The basic bargain is an exchange.
The fungus provides: water and mineral nutrients, especially phosphorus and, in many cases, nitrogen. Fungal threads, called hyphae, are far thinner than even the finest tree roots, and they can reach into small spaces in the soil that roots cannot. A network of hyphae greatly increases the soil volume the tree can effectively draw from.
The tree provides: sugars made in its leaves. Fungi cannot photosynthesize, so they depend on their plant partner for carbon. Estimates of how much of a tree’s sugar goes to its fungal partners vary, and the share is thought to be substantial, but the exact amount depends on species and conditions.
Some fungi also help protect roots from certain pathogens or help trees cope with dry or toxic soils, though these benefits vary and are less well documented than nutrient exchange.
Two main types
Ectomycorrhizas
In ectomycorrhizal partnerships, the fungus forms a sheath around the outside of the root tip and grows between root cells without entering them. This type is common in many familiar temperate and boreal trees, including pines, spruces, firs, oaks, beeches and birches, and also in some tropical groups. The fungal partners include many of the mushrooms and truffles seen in forests. When you find a mushroom beneath an oak or pine, it may be the visible fruiting body of a fungus whose main body lies in the soil and joins tree roots.
Arbuscular mycorrhizas
In arbuscular partnerships, the fungus enters root cells and forms tiny branching structures inside them for exchange. This is the most widespread type overall and is found in many tropical trees, maples, ashes, and a great many herbs and crops. The fungi involved do not typically produce large mushrooms, so the partnership is usually invisible without a microscope.
Some trees can host both types, and other forms of mycorrhiza exist in groups such as heaths and orchids. The type of partner varies with the tree species, which influences how forests function, including how nutrients cycle through leaf litter.
Fungi and the soil
The effects of mycorrhizal fungi reach beyond the roots. They contribute to building soil structure, because hyphae and the sticky substances they release help bind soil particles into aggregates. Some fungi can also break down organic matter to release nitrogen. Mycorrhizal fungi are thought to influence how much carbon is stored in forest soils, and the balance differs between ecto- and arbuscular systems. This is one thread in the larger question of soil carbon storage, which researchers are still working to understand.
The “wood wide web” and what is actually known
Fungal hyphae from one tree can connect to the roots of other trees, forming common mycorrhizal networks. Popular accounts describe forests as sharing food and warning signals through these networks, with older “mother trees” nurturing seedlings.
The reality is more cautious. Research has shown that fungal connections between trees exist, and that in some experiments carbon and other compounds can move through them. What is less clear is how much this matters in natural forests, how much trees benefit compared with moving resources through the soil, and whether trees actively favor relatives. Some of the most popular claims have been questioned by other scientists who argue the evidence does not yet support broad statements. A fair summary is that connections are real and likely important in some settings, while the extent and ecological meaning of resource sharing remain under study.
Seedlings and succession
Regardless of the debate over sharing, partners matter for seedlings. A young tree that lands where its fungal partner is already present often establishes more easily, because it can quickly link into an existing network of hyphae. Where the right fungus is absent, seedlings may struggle, which is a consideration in restoration and tree planting on degraded land. Nurseries sometimes add fungal inoculants, though whether this helps depends on the site and the species.
Fungal partners also differ among stages of forest succession, with some arriving early on disturbed ground and others favoring older forests. Some arbuscular fungi tolerate disturbance better than some ectomycorrhizal species, which can be sensitive to soil disturbance, heavy fertilization or removal of host trees.
What can harm the partnership
- Soil disturbance. Heavy compaction, topsoil removal or intense burning can damage fungal networks.
- Excess nutrients. High nitrogen inputs from fertilizer or air pollution can reduce the benefit trees gain from fungi, and in some forests have been linked to changes in fungal communities.
- Loss of host trees. Many ectomycorrhizal fungi depend on living tree roots, so large-scale clearing can reduce them.
- Non-native partners. Introduced trees may bring their own fungi or lack local ones, and invasive species can change the soil fungal community in ways that are not fully understood.
Why it matters to forest care
Mycorrhizal fungi help explain why trees can grow on soils that seem poor. They are particularly important in places where nutrients are scarce or locked in organic matter, as in the nutrient poor soils of many tropical forests. For anyone managing woodland, the practical points are modest but sensible: avoid unnecessary soil disturbance, keep living trees on the site where possible, and treat the soil as part of the forest rather than only as a surface to walk on.