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A guided journey · 5 stops

From forest to fungus

Follow a tree downward — into its roots, across the interface where it trades with a fungus, and out the far side into what happens when it dies.

The route

  1. Trees
  2. Roots
  3. Mycorrhizal networks
  4. Fungi
  5. Decomposition

A tree is the most familiar large organism most people ever stand next to, and almost everything interesting about one happens where it cannot be seen. The visible part is a structure for holding leaves in the light. The part that decides whether the tree lives is underground, and it is not, in the ordinary sense, entirely the tree.

This route goes down. It starts with the tree as a piece of engineering, because the questions people arrive with — how does water get to the top, where does the mass come from — have answers that are genuinely surprising and that everything else depends on. Then it follows the roots, which are shallower and wider than almost anyone pictures. Then it stops at the interface, where a fungus and a root meet and exchange things, because that meeting is where the popular story about forests either becomes interesting or goes badly wrong. Then the fungus itself, as an organism with its own life rather than as a service the tree uses. And finally decay, which is the same relationship running the other way.

The order is doing work. Read the mycorrhizal page first and the trade looks like altruism; read it after the roots page, knowing how little of a root system is fine absorbing tissue and how much soil a hypha can reach that a root cannot, and it looks like what it is — an exchange each partner is under selection to drive a hard bargain in.

  1. Stop 1 of 5. Trees

    Start above ground, where the questions are

    Almost everyone arrives with one of two questions — how does water reach the top of a tall tree, and where does all that wood come from — and both have answers that reset the intuitions the rest of the route depends on. Water is not pumped; it is pulled, by evaporation, through a column under tension that would be physically impossible if water were not unusually good at sticking to itself. And the mass is not taken from the soil. It is very largely carbon that was in the air. Get those two straight and the underground half stops looking like plumbing and starts looking like chemistry.

    Read TreesNo tree has a pump. Water is pulled to the top under tension — and at about 120 metres, that stops working.

  2. Stop 2 of 5. Trees

    Follow it underground, and find less than you expected

    The picture in most heads is a mirror image: as much tree below as above. It is not close. Root systems are typically far shallower and far wider than the canopy, and the fraction of them that is fine, actively absorbing tissue is small. That shortfall is the setup for everything after it. A root cannot explore soil at the scale a tree needs, and it cannot reach into the pore spaces where phosphorus sits. Something has to make up the difference, and the next page is what does.

    Read RootsThe roots of the trees in a wood are not separate systems. They overlap, and they reach two or three times further than the branches above them.

  3. Stop 3 of 5. Fungi

    Stop at the interface, and read the trade carefully

    This is the page the route exists for, and the one where popular coverage is furthest from the literature. What is well evidenced: fungal hyphae are orders of magnitude finer than roots, they reach soil roots cannot, and they exchange mineral nutrients for the tree’s carbon. What is much less well evidenced: that trees use these connections to feed each other, that older trees deliberately support seedlings, or that anything resembling a forest-wide network has been demonstrated. Reading it here, immediately after the shortfall on the previous page, makes the exchange legible as an exchange rather than as generosity.

    Read Mycorrhizal networksThe fungi are real, the connections are real, and carbon does move. Whether anybody is sharing is a different question, and it is still open.

  4. Stop 4 of 5. Fungi

    Turn round and look at the other partner

    Up to here the fungus has been a service the tree buys. It is an organism with its own problem to solve, and it is not a plant that lost its chlorophyll — it is closer to us than to the tree it is wrapped around. Seeing the partnership from the fungal side is what stops the previous page being read as a story about trees with a fungus in it. It also introduces the thing the last step needs: a body made of filaments that grow through solid material and digest it from outside.

    Read FungiThe mushroom is not the fungus. The fungus is the network — and one of them has been mapped across nearly ten square kilometres of Oregon forest.

  5. Stop 5 of 5. Ecology

    Run the relationship backwards

    A tree spends a century taking carbon out of the air and locking it into a polymer almost nothing can eat. Decay is the same interface working in the other direction, and the reason forests are not buried in their own dead wood is a small set of fungi that can break lignin apart. Ending here closes the loop the first page opened: the mass that came out of the air goes back into it, on a timescale set by what can digest what.

    Read DecompositionThe ability to digest wood appears to have been invented once, by fungi, and everything a forest floor does follows from it.

Where this leaves you

You end with a forest that is neither a machine nor a family. The evidence for material moving between trees through fungal connections is real and is mostly from isotope tracing in a small number of systems; the evidence that trees use those connections to help each other is much weaker than the popular telling, and the strongest single result in the whole area — that partners preferentially reward partners who deliver — points at a market rather than a community.

What the route does not settle: how much of what has been shown in a Douglas fir plantation or a laboratory mesh experiment generalises to any given wood, and whether the networks that undoubtedly exist are doing anything at the scale of the forest at all. Both are open, and both are places where confident writing outruns the measurements.

Last reviewed 2026-09-02. All guided journeys