In the summer of 1997, Suzanne Simard published a paper in Nature that forest ecologists still cite by its first line of data. Working in stands of paper birch and Douglas fir in the interior of British Columbia, she had injected seedlings with radioactive and stable carbon isotopes and then tracked where that carbon went. It did not stay put. Carbon moved between species, through the soil, along threads of fungus connecting one root system to another. Birch sent carbon to fir in summer. Fir sent it back in other seasons. The forest, it turned out, was trading resources through a buried network that nobody had mapped before.
Simard, now at the University of British Columbia, spent the following two decades turning that single finding into a research program. Her Mother Tree Project, launched in 2015, tracks how older, larger trees in a stand, the ones with the densest mycorrhizal connections, preferentially route carbon and nitrogen toward kin and toward stressed or shaded seedlings. The chemistry of that exchange is well established. What is new, and what is pulling plant physiologists and bioelectricity researchers into the same field sites, is the discovery that the network carrying those nutrients also carries voltage, and that the voltage patterns are not random.
The Mycelial Network Was Never Just Chemical Plumbing
Mycorrhizal fungi connect roughly 90 percent of land plant species to shared underground hyphal networks. For most of the twentieth century, foresters treated these networks as passive plumbing, conduits for water and carbohydrate exchange driven entirely by concentration gradients. That framing never quite fit the data. Nutrient transfer along fungal hyphae happens faster, and in more directionally consistent patterns, than diffusion alone explains.
Plant electrophysiologists had a parallel history that forest ecology mostly ignored. Harold Saxton Burr, a Yale anatomist working from the 1920s through the 1940s, spent years running electrodes into the trunks of trees on the Yale campus and in Connecticut woodlots, logging voltage differences that shifted with the seasons, with sap flow, and with lunar cycles he never fully explained. Burr called the phenomenon the tree’s electrodynamic field. His instruments were primitive by current standards, string galvanometers and hand-wired electrodes, but the voltage signals he recorded were real and reproducible, and they outlasted his own explanations for them.
That line of inquiry resurfaced decades later under a different name. Researchers including Stefano Mancuso at the University of Florence’s International Laboratory of Plant Neurobiology began describing something they call the plant electrome: the full set of electrical signals, action potentials, slower variation potentials, and ionic currents that move through a plant’s tissue in response to touch, injury, light, and drought. Mancuso’s group and others working in the same tradition, including Frantisek Baluska at the University of Bonn, argue that these signals function as a distributed communication system, not unlike a nervous system, even though plants have no neurons.
Reading Voltage Across a Root System Like a Circuit Diagram
The mechanism is grounded in basic cell biology, not metaphor. A wound, a drought stress event, or an insect bite triggers calcium influx at the site of damage. That influx depolarizes the cell membrane, which in turn opens voltage-gated channels in neighboring cells, propagating an electrical wave through plant tissue at speeds that can reach several centimeters per minute, far faster than chemical diffusion through xylem or phloem alone. Fungal hyphae, it turns out, support a comparable kind of ionic signaling, using proton pumps and potassium channels to move charge along their own filaments.
Put electrodes into a root system and a fungal mat at multiple points, and the recorded voltage traces start to look less like noise and more like a wiring diagram. That is the basic method researchers are now applying across small experimental forest plots: dozens of paired electrodes and wireless data loggers, some built on the TreeTalker platform developed by Italian forest researchers, attached to trunks and fine roots and left running for months. The resulting time series data can be aligned spatially to the same stands where isotope tracing already mapped which trees send carbon to which seedlings.
The fungus was never just plumbing. It carries current the same way it carries carbon.
Where the two datasets overlap, a pattern holds up across multiple stands: the trees acting as carbon sources in Simard’s isotope work also show distinct voltage signatures, typically more stable, lower-amplitude baseline potentials, compared with the sharper, more reactive voltage spikes recorded in young seedlings under stress. The correlation does not prove the electrical signal causes the nutrient transfer. It does suggest that resource allocation in a forest has an electrical component running alongside the chemical one, and that the two are not independent systems bolted together by coincidence.
Why Age, Not Just Size, Changes the Wiring
Simard’s original framing emphasized size and connectivity. Bigger trees have more root surface area, more fungal partners, more physical bandwidth. Newer analysis layering electrical recordings onto the same plots points to age as a separate variable, one that does not simply track with trunk diameter. Two Douglas firs of similar size but different ages, planted a decade apart after a disturbance, show measurably different baseline voltage profiles. Older individuals, even when matched for size, behave more consistently as electrical and nutrient sources. Younger individuals, even large ones, behave more often as sinks.
Specificity comes from decades of root development, not just how much wood a tree has put on.
That distinction matters for how the hub-and-spoke model of mycorrhizal networks gets interpreted. The original picture, popularized as the wood wide web after Simard’s 1997 paper, treated big trees as hubs mostly because of their physical connectivity. The electrical data suggests something closer to seniority. An older tree, regardless of current size, has had more time to build stable fungal partnerships and a more mature internal signaling architecture, and that maturity shows up as a measurable electrical property, not just a structural one. Age appears to set a kind of electrical priority level inside the network, favoring established trees as sources even when a younger neighbor has grown physically larger.
What Older Instrumentation Could Never Show
Isotope tracing, the method Simard relied on, requires injecting a labeled compound and then destructively sampling tissue, or using expensive repeated scanning, to see where it ended up. It produces a snapshot, taken after the transfer has already happened. Electrical recording runs continuously and in real time, which is the main reason researchers are excited about combining the two approaches rather than treating the newer method as a replacement.
| Method | Era / Key Figures | What It Measures | Main Limitation |
|---|---|---|---|
| Radioisotope/stable isotope tracing | Suzanne Simard, 1990s to present, University of British Columbia | Carbon and nitrogen transfer between individual trees | Snapshot in time, often destructive sampling |
| Trunk electrode voltage logging | Harold Saxton Burr, 1920s to 1940s, Yale University | Standing voltage differences across a single tree | Single-tree scale, no network context |
| Plant electrome recording | Stefano Mancuso and others, 2010s to present | Action potentials and variation potentials across tissue | Requires continuous power and data logging infrastructure |
| Wireless multi-point root and fungal electrodes | Current forest plot deployments, built on platforms like TreeTalker | Spatial voltage patterns across a stand, correlated to isotope maps | Still limited to small experimental plots, not full forests |
What the Electrical View Still Cannot Explain
None of this amounts to proof that trees make decisions. The electrical signals recorded so far are consistent with allocation patterns already documented through isotope work, but consistency is not mechanism. Researchers have not captured a case where a voltage change at a hub tree demonstrably preceded, rather than simply accompanied, a shift in carbon flow to a seedling. Species identity, fungal partner diversity, and soil chemistry all still shape the network’s topology in ways the electrical data has not disentangled from age effects. Current instrumentation covers isolated experimental plots, mostly in temperate conifer and birch stands in North America and a smaller number of mixed hardwood sites in Europe. Nobody has wired an entire natural forest.
Funding for that kind of scale-up remains the open question. Isotope tracing work has decades of forest service and university backing behind it. Electrical monitoring equipment is comparatively cheap per unit but requires density, hundreds of paired electrodes per hectare, to produce a usable map rather than a scatter of disconnected readings. That is an infrastructure problem more than a scientific one, and it is the reason this remains an active, unsettled corner of bioelectricity research rather than a closed case.
What the data does establish, cautiously, is that the wood wide web was never only a chemical system. It carries current the way a nervous system carries current, using the same basic electrochemistry that runs through a human neuron: calcium influx, membrane depolarization, propagated voltage. The open question is whether that electrical layer is driving the forest’s economy of sugar and nitrogen, or simply riding along behind it, a readout of decisions made somewhere else in the tree’s physiology. Simard’s isotope maps showed where the resources went. The voltage maps may eventually show the moment before they moved.
Credit: Kadir Celep on Unsplash