What if trees shared food underground?

A forest has lots of trees. Under the ground, their roots spread out.

Tiny fungus threads grow on the roots. Do the trees use them to share food?

Walk through a forest and you see trunks, branches and leaves. Underground there is a hidden world: roots, and wrapped around them, fine threads of fungi.

You may have heard that trees use these fungal threads to pass food to each other, even feeding their own young. It's a lovely idea. In this lab you will find out which parts scientists have shown, which parts are still being tested, and why careful scientists disagree.

A common mycorrhizal network (CMN) forms when the threads of mycorrhizal fungi connect the roots of several plants, of the same or different species. CMNs have been proposed as routes for carbon, nutrients and signals between trees, and the idea has captured the interest of broad audiences in books and the media.

This thought experiment asks what would follow if trees really shared food through fungi, then checks the evidence. Unusually for this lab series, the most important part is not a consequence but a debate: what has been measured, what has not, and how scientists judge the difference.

A fungus grows on a tree's roots. It brings the tree water from the soil.

The tree makes sugar in its leaves. It gives some sugar to the fungus. They help each other!

This part is well established. Nearly 90 percent of all kinds of plants with roots and stems have fungus partners on their roots, called mycorrhizae (my-co-RISE-ee). The fungus spreads a huge web of threads through the soil and pipes water and minerals into the root.

In return, the plant gives the fungus some of the sugar it makes from sunlight. Both partners gain, which scientists call a mutualism. On many forest trees, the fungus wraps the root tips in a sheath; on many other plants, it grows right inside the root cells.

OpenStax Biology 2e describes mycorrhizae as a mutualistic association between vascular plant roots and symbiotic fungi, present in nearly 90 percent of vascular plant species. The fungal mycelium's large surface area absorbs water and minerals more effectively than roots alone, and the plant supplies photosynthetic products to fuel the fungus.

There are two main types. Ectomycorrhizal fungi sheath the root in a mantle and grow between root cells as a Hartig net; arbuscular mycorrhizal fungi (Glomeromycota) form branched arbuscules inside root cells, where nutrients are exchanged. None of this is in dispute.

One fungus might touch the roots of two trees at once.

Some scientists think food might travel from one tree to another. Others think it might not.

A single fungus can touch the roots of more than one plant, linking them into a network. If trees really shared food through these links, a big tree in the sun might help a small tree in the shade. Some scientists, including forest ecologist Suzanne Simard of the University of British Columbia, say trees do exchange resources this way and share nutrients with younger trees.

Other scientists say the evidence isn't strong enough yet. Food could also leak out of one tree's roots, move through the soil, and be soaked up by another tree, with no fungal network needed.

If CMNs moved substantial carbon between trees, shaded seedlings might be subsidized by canopy trees, and forests might behave more like cooperative networks than collections of competitors. Suzanne Simard (University of British Columbia) has argued that trees exchange resources through mycorrhizal networks, sharing nutrients with saplings.

In 2023, Justine Karst, Melanie Jones and Jason Hoeksema reviewed the evidence. They concluded that claims that CMNs are widespread in forests, and that resources moving through them improve seedling performance, are insufficiently supported, because field results vary too widely, have alternative explanations or are too limited to generalize. One alternative is that carbon leaves one tree's roots in liquid, travels through soil and is taken up by another.

Roots and fungi are hidden in the dirt. They are tiny and hard to watch.

So scientists test and check each other's work. That is how science works.

It is very hard to see what happens underground. Fungal threads are tiny, soil is full of other living things, and food can travel by more than one route. So scientists may measure the same thing and explain it differently.

The 2023 review found something else too: the idea that big 'mother trees' send food and warnings to their own seedlings through fungi has no published, peer-checked evidence yet. And over 25 years, research papers had more and more often repeated claims about the networks that the studies did not support. Simard says many pathways exist together, and that reviews and debate are a normal part of science.

Belowground transfer is hard to attribute. Carbon can move through hyphae, through root exudates and soil, or via other organisms, and experiments must rule each out. Karst and colleagues found that the claim that mature trees preferentially send resources and defense signals to offspring through CMNs has no peer-reviewed, published evidence, and that unsupported claims about CMNs in the literature doubled over 25 years, a positive citation bias.

Simard responded to CBC News that the review focuses on a narrow part of the research, that she has never claimed the fungal network is the only pathway, and that this back-and-forth is a normal part of the scientific method. The reviewers conclude that knowledge of CMNs is too sparse and unsettled to inform forest management yet.

We know trees and fungi help each other. We don't yet know how much trees share.

Scientists want to study many more forests to find out.

Here is where things stand. Fungus partners on roots, trading minerals and water for sugar: well established. Fungi linking more than one plant: real, but how common it is in forests is unclear. Trees sharing meaningful amounts of food through the links, or mothers feeding their young: not shown well enough yet.

Part of the problem is that, according to the review, only two kinds of forest have been checked for these networks: Douglas fir forests in British Columbia, Canada, and pine forests in Japan. Melanie Jones, one of the reviewers, points out that no one has mapped these fungal links in the Amazon rainforest.

As CBC News reported, the review notes that only two forest types, Douglas fir forests in British Columbia and pine forests in Japan, have been investigated, and co-author Melanie Jones (UBC Okanagan) calls for mapping fungal connections in other forests, such as the Amazon. Karst stresses that the importance of mycorrhizal fungi themselves is not in dispute.

The honest summary is layered: the mutualism is textbook science; networks exist; how widespread they are, how much they carry and whether it matters for seedlings are open questions. That is a good model for reading any striking science story: ask what was measured, where, and whether there is another explanation.