In 1997, a young ecologist named Suzanne Simard published a paper in Nature that would upend how we think about forests. She had spent years in the woods of British Columbia, injecting radioactive carbon into paper birch trees and tracking where it went. The answer: into neighboring Douglas firs, traveling through an underground network of fungal threads connecting their roots. Trees, it turned out, were sharing resources through what German forester Peter Wohlleben would later dub the "woodwide web."

Nearly three decades later, we're still untangling what this network actually does. The latest evidence suggests these fungal highways don't just move sugar and nutrients between trees—they carry electrical signals too.

The Underground Internet

Mycelium, the thread-like body of fungi, forms networks that wrap around or bore into tree roots. Think of them as biological fiber optics: incredibly thin filaments that create physical connections between separate root systems. In a healthy forest, nearly every tree plugs into this network. The biggest, oldest trees—what Simard calls "mother trees"—function as highly connected hubs, with more fungal links than their younger neighbors.

The arrangement is transactional. Trees produce sugar through photosynthesis and pump about 30% of it into the fungal network as payment. In exchange, the fungi collect phosphorus and other mineral nutrients from the soil and deliver them to tree roots. Saplings growing in deep shade, unable to photosynthesize enough to survive on their own, depend on this system to receive sugar from taller trees overhead.

But the network does more than move molecules. Simard's shading experiments revealed something stranger: the direction of resource flow changes based on need. When she shaded Douglas firs in summer, carbon flowed from birch to fir. In fall, when birch trees lost their leaves, the transfer reversed. The system wasn't just plumbing—it was responsive.

Measuring the Invisible

Proving that fungi transmit electrical signals presents a technical nightmare. The organisms are microscopically small, and the electrical currents they generate are even smaller. Researchers in the 1970s suspected fungal electrical signaling existed but lacked the tools to study it properly. The work went dormant for decades.

Modern techniques have revived the field, though each method comes with caveats. Scientists pierce fungal filaments with glass needles, use vibrating electrodes, or deploy multi-electrode arrays to detect currents. Some researchers use dyes that fluoresce when they encounter electrical changes, though this requires better 3D visualization of mycelium structure than we currently have. Background noise drowns out signals. The detection tools themselves disrupt the very currents they're trying to measure.

Despite these challenges, the evidence is mounting. Studies using chemical inhibitors and genetic mutants have identified membrane channels involved in ion transport—the basis of electrical signaling. These channels work differently than animal neurons, but they move charged particles all the same. The currents appear in specific areas of the mycelium, particularly in response to food sources.

A 2025 review in FEMS Microbiology Reviews laid out both the progress and the persistent methodological problems. We know electrical signals exist in fungi. What they communicate, and whether they actually transmit information between trees, remains murkier.

What Trees Say to Each Other

If the fungal network carries electrical signals, what messages are trees sending? The most compelling evidence involves distress calls.

When researchers injure Douglas fir trees, the wounded trees dump carbon into the mycorrhizal network. Neighboring trees—including different species like ponderosa pine—absorb this carbon. More tellingly, both the injured tree and its neighbors ramp up production of defense enzymes. Something in the network is signaling danger.

Mother trees appear to detect ill health in their neighbors and respond by sending nutrients. They can even recognize their own offspring. A University of Reading study on Douglas fir showed that trees identify the root tips of relatives and preferentially direct carbon and nutrients to their kin through the fungal network.

The most poignant finding might be what happens when old trees die. Simard's research suggests dying mother trees transfer their "legacy" to younger trees that will replace them, especially as climate shifts favor different species. They send both carbon and what appear to be warning signals to neighboring seedlings.

The Fungus Among Us

This cooperative picture of forest life contradicts the competitive narrative that dominated ecology for decades. But cooperation might be the wrong word. The fungus isn't altruistic—it's securing its own future.

By directing carbon transfer between different plants, the fungal network maintains diverse food sources. If one tree species fails, others remain. The fungus acts as a broker, taking its 30% cut while keeping the whole system stable. Trees benefit from the arrangement, but they're also somewhat trapped in it. A sapling in the shade has no choice but to plug in and pay the toll.

The electrical signaling adds another layer of complexity. If fungi can transmit information about threats or resource availability, they're not just passive conduits but active participants in forest decision-making. The network becomes less like plumbing and more like a nervous system—one that operates on timescales of days and weeks rather than milliseconds.

When the Network Breaks

Understanding these fungal electrical signals matters because we're disrupting the networks at scale. Clear-cutting removes mother trees. Fungicides kill mycorrhizal fungi. Soil compaction from heavy machinery crushes the delicate hyphal threads.

We're only beginning to grasp what we're breaking. If electrical signals through fungal networks help forests respond to drought, disease, or changing climate, then fragmenting those networks doesn't just remove individual trees—it severs the forest's ability to communicate and adapt. The woodwide web might be less like the internet and more like a brain: damage enough connections, and the whole system stops thinking.

The challenge now is moving from proof of concept to practical understanding. We know signals travel through the network. The next question is whether we can learn to listen.