[1] For three decades, the prevailing account of temperate forest ecology held that trees are joined belowground into cooperative collectives by common mycorrhizal networks—the symbiotic fungal filaments that colonize plant roots and, on this account, knit neighboring individuals into a single conduit for water, nutrients, and even photosynthetic carbon. [2] The image of a “shared” forest, in which mature trees subsidize shaded seedlings and warn their neighbors of stress, drew on a celebrated series of field experiments in which isotopically labeled carbon, introduced to one tree, was subsequently detected in the tissues of a second. [3] That such transfers occur is not seriously disputed; what a growing body of skeptical scholarship contests is the inference that the fungal network functions as a channel of interplant altruism, or indeed that the transfers are ecologically consequential for the recipient.
[4] The reinterpretation rests on two observations. [5] First, the quantities of carbon documented moving between trees are, in most studies, a negligible fraction of a recipient’s photosynthetic budget—sufficient to be traced by sensitive instruments but far too small to underwrite the survival of a struggling seedling. [6] Second, and more fundamentally, the fungus is not a passive pipe but an autonomous organism with interests of its own; carbon that enters the network may simply be retained by the fungus or routed wherever its own growth is favored, with any arrival in a second plant an incidental byproduct rather than a directed gift.
[7] Reframed this way, the network is better understood as a marketplace than as a commune. [8] The mycorrhizal symbiosis is fundamentally a trade: the plant supplies the fungus with carbon, the fungus supplies the plant with mineral nutrients it is better equipped to scavenge, and each partner appears to reward the more generous and penalize the less. [9] On this model, apparent “sharing” among trees is an epiphenomenon of the fungus’s own resource management, not evidence that trees have evolved to aid competitors with whom they contend for light and soil.
[10] Critics of the cooperative account are careful to specify what they do and do not claim. [11] They do not deny that networks exist, that they are extensive, or that they materially affect how forests function; they deny only that the existing evidence licenses the further, and more romantic, conclusion that trees use them to nurture one another. [12] The disagreement is thus less about data than about the burden of proof: whether a striking phenomenon, once demonstrated, should be presumed adaptive until shown otherwise, or held to be incidental until its benefit to the purported beneficiary is established.
