Mycorrhizal carbon trade

Why does a tree hand a fifth of the sugar it worked all summer to make over to a fungus it can neither see nor control?

A tree spends a season building sugar out of air and light, at the cost of every leaf it made and every litre of water it lost doing it. Then it exports a large share of that sugar underground to a fungus it did not choose, cannot inspect, and cannot dismiss. The obvious reading is that the tree is being farmed.

Before accepting that, ask what the tree is buying, and whether it could get the same thing any other way. If it could not, the payment stops looking like extortion and starts looking like the cheaper of two options.

Start with the commodity, and specifically with phosphate. How does a phosphate ion get from soil into a root? It travels through the water films between soil particles, and it is very bad at travelling, because it binds tightly to the iron and aluminium oxide surfaces of the grains it passes — moving orders of magnitude more sluggishly than nitrate.

Consider what that does around an absorbing root. The root takes up phosphate faster than the soil resupplies it, so within days a shell around it is stripped: a depletion zone a millimetre or two thick, beyond which phosphate exists in quantity and simply cannot arrive. What does that make the limiting factor? Not the root's transporters; double them and nothing changes, because the constraint sits outside the plant entirely, in the diffusion of an ion through wet mineral. The only way to get more phosphate is to put absorbing surface somewhere else — past the depletion zone, into pores the root never entered.

So: what is the cheapest way to buy absorbing surface at a distance? The arithmetic settles it in one line. For a cylinder, surface per unit volume is 2 / radius. A fine root's radius is roughly half a millimetre; a fungal hypha's, roughly five micrometres. A hundredfold difference in radius is a hundredfold difference in absorbing surface per unit of tissue built. The same carbon spent as hyphae rather than root buys about a hundred times more contact with soil — in a form thin enough to thread into pores a root hair cannot enter, reaching centimetres past the exhausted zone.

That is the trade. The tree is not paying for a service it could do more cheaply itself. It is paying for a geometry it cannot build, being made of the wrong-sized parts.

Now the fungus, and resist saying it is helping. Arbuscular mycorrhizal fungi obtain all of their carbon from a plant host, and the dependence is more specific than it looks: they cannot synthesise their own fatty acids, so the plant supplies lipids as well as sugars. The fungus is neither benefactor nor free agent. It stays because it structurally cannot leave, and that constraint, not any disposition to cooperate, keeps it at the root.

Which leaves the control problem. If the tree cannot see the fungus, what stops it taking sugar and withholding phosphate? The evidence — recalling an influential 2011 experiment rather than citing it — is that discrimination runs both ways: plants direct more carbon to partners delivering more phosphorus, and fungi direct more phosphorus toward more rewarding roots. Neither is choosing in any interesting sense. Each allocates more to whichever contact currently returns more, which is enough to make cheating a losing move with nobody assessing anybody.

Three qualifications, in ascending order of how much they matter.

First, the number in the question. I would not state a figure. Estimates of the share of net photosynthate ending up in a plant's mycorrhizal fungi are method-dependent and published values disagree substantially by system, season and technique. A large and non-trivial fraction is what the evidence supports; a fifth sits at the high end of a range whose sources do not agree.

Second, a complication that damages the tidy economic story and should be stated rather than smoothed. In many arbuscular mycorrhizal plants, most phosphate taken up arrives by the fungal pathway even when the association produces no growth benefit at all — the plant having partly shut down its own direct uptake route in favour of the fungal one. The pathway is not switched on and off according to whether it pays this season, which is a real awkwardness for reading the relationship as a market, and it is unresolved.

Third, the claim most overstated in popular accounts: the network. A single mycelium does connect many trees, and isotope tracing does show carbon moving between plants through it. Much weaker is the claim that the transfer matters ecologically to the receiver, or that mature trees preferentially support their own seedlings. Recent reviews argue this framing outran its evidence — carbon entering a shared mycelium may simply be spent by the fungus, and showing a labelled atom moved is not showing anything benefited.

Separate all this from the general question of why symbioses persist, which turns on dependence and sanctions. The point here is narrower and more physical: what is purchased is absorbing surface at a scale the plant body cannot manufacture.

The test follows directly. If the trade exists because phosphate is diffusion-limited, removing the limitation should collapse it: fertilise heavily with phosphorus and colonisation should decline, belowground carbon allocation should fall, and the benefit should shrink toward zero or below. That is broadly what fertilisation experiments show. The refuting observation would be a plant maintaining full colonisation and full carbon export under abundant free phosphate — meaning the fungus was being paid for something other than the ion, and the diffusion account had named the wrong commodity.

• Why ectomycorrhizal fungi, which also mine organic nitrogen from litter, may sit on a different economic footing.
• What becomes of labelled carbon entering a shared mycelium, and how to distinguish transfer to a neighbour from consumption by the fungus.

Every term the collection defines is gathered in the glossary.