Beneath your feet right now, an invisible network is quietly determining the fate of our climate. It's not roots, not bacteria, not earthworms. It's fungi—and scientists are only beginning to understand just how much control these underground organisms have over the carbon that forests lock away.

The Hidden Architects of Forests

Walk through any forest and you're seeing only half the story. Below ground, thread-like fungal filaments called mycelium weave through the soil, connecting tree roots in a vast living network. These threads are thinner than human hair but stretch for miles, forming what some scientists call the "wood wide web."

This relationship isn't new. Five hundred million years ago, when plants first attempted to colonize land, they couldn't do it alone. They had no roots yet. Fungi served as their first root systems, helping them extract water and nutrients from rock and soil. Today, over 90% of plant species still depend on these fungal partners, called mycorrhizae—literally "fungus roots" in Greek.

But here's what's changed: we now know these fungi control far more carbon than anyone suspected.

The Carbon Trading Network

Every year, forests pull carbon dioxide from the atmosphere through photosynthesis. Trees convert that carbon into sugars. Then something remarkable happens: they send up to 30% of those sugars underground to feed their fungal partners.

The scale is staggering. Mycorrhizal fungi receive 13.12 gigatons of CO2 equivalent from plants annually. That's roughly 36% of what humanity emits by burning fossil fuels each year. This carbon doesn't just pass through the fungi—it becomes part of them, part of the soil structure they create, part of the vast underground carbon reservoir that exceeds all the carbon in plants and the atmosphere combined.

The fungi earn this payment. They've largely lost the ability to break down wood and leaves like other fungi do. Instead, they've become specialized traders. They mine the soil for nitrogen and phosphorus—nutrients trees desperately need—and exchange them for carbon-rich sugars. It's a marketplace millions of years in the making.

What the Latest Research Reveals

In March 2024, researchers published findings that should change how we think about forest carbon storage. They analyzed 238 forest plots across 15 European countries, measuring everything from tree growth to soil carbon to fungal diversity.

The results were striking. When they controlled for tree species, climate, and environmental factors, one variable emerged as the key predictor of how much carbon a forest stores: fungal composition.

Not bacterial communities. Not soil chemistry alone. Fungi.

The study found a seven-fold variation in tree growth rates and carbon stocks linked to which fungi were present. Forests with diverse communities of ectomycorrhizal and endophytic fungi grew faster and stored more carbon. And here's the kicker: faster tree growth correlated with more carbon locked in the soil below, not less.

This challenges older assumptions. Scientists once thought rapid tree growth might deplete soil carbon. Instead, thriving fungal networks seem to build it up.

Mapping the Invisible

For decades, fungi have been the wild card in climate models. They're notoriously difficult to study. You can't see them without digging. They don't preserve well. Traditional surveys miss most species.

Enter SPUN—the Society for the Protection of Underground Networks. This initiative is creating the first global map of mycorrhizal fungal networks using DNA sequencing. Their database now contains 2.8 billion fungal DNA sequences from 25,000 soil samples across 130 countries.

The maps use one-kilometer resolution to predict where different fungal communities exist worldwide. According to SPUN's lead data scientist Michael Van Nuland, better fungal data could "reduce uncertainty and lead to better predictions" in global climate models.

This matters because current models contain a "missing carbon" problem. Carbon enters and leaves ecosystems in ways scientists couldn't fully account for. Mycorrhizal networks appear to be a major part of that missing piece.

The Soil Respiration Mystery

Here's another wrinkle: fungi are responsible for over 50% of soil respiration in forest ecosystems. That means they're releasing significant amounts of CO2 back into the atmosphere as they break down organic matter and metabolize sugars from plants.

So are fungi helping or hurting carbon storage?

The answer is both, and it depends on context. In nutrient-poor soils, particularly in coniferous forests, trees allocate more carbon to fungi because they need more help accessing scarce nutrients. Some of that carbon gets respired quickly. But much of it becomes incorporated into stable soil structures that can persist for years, decades, or longer.

A 2019 study from the University of New Hampshire and Stanford used a "mass balance approach"—essentially carbon accounting—to track where all the carbon goes. They confirmed that while fungi do respire carbon, they also create the conditions for long-term soil carbon storage.

The mycelium itself becomes part of the soil matrix. When fungal threads die, they leave behind carbon-rich compounds that bind to soil particles. The network creates pathways for water and air, supporting other organisms that further stabilize carbon.

What This Means for Climate Action

Atmospheric CO2 has risen from 280 parts per million in the 1700s to 424 ppm today. Forests represent one of our best tools for pulling that carbon back down. They're among the world's largest carbon sinks and home to roughly 80% of terrestrial biodiversity.

But protecting forests means protecting fungi. And fungal conservation lags far behind efforts to protect plants and animals.

We don't have endangered species lists for fungi. We don't have protected fungal habitats. When forests are logged, replanted, or managed, fungal networks are rarely considered—even though those networks determine how well the forest will grow and how much carbon it will store.

The European study suggests we should be thinking about fungal diversity the way we think about tree diversity. Monoculture tree plantations might not develop the fungal communities needed for maximum carbon storage. Old-growth forests, with their established fungal networks, might be even more valuable than we realized.

The Path Forward

Scientists are calling for fungi to be integrated into climate models and forest management plans. SPUN's mapping project is a start, but it needs to be followed by protection measures.

Some researchers suggest we could even inoculate degraded soils with beneficial fungi to boost reforestation efforts. Others are exploring whether different forest management practices—like reducing soil disturbance or maintaining dead wood—could support healthier fungal communities.

The irony is that fungi have been doing this work for 500 million years. They built the first forests. They've been managing the carbon cycle since before humans existed. We're only now learning to see them.

Without mycorrhizal fungi, we wouldn't have forests, woodlands, jungles, or prairies as we know them. These organisms aren't just part of the ecosystem—they're the foundation. And as we race to stabilize our climate, that foundation might be our greatest ally.

The mushrooms you see after rain are just the tip of the iceberg—fleeting reproductive structures of organisms whose real bodies sprawl invisibly through the soil. Beneath every forest, a hidden kingdom is at work, moving carbon, feeding trees, building soil. The more we understand these networks, the better our chances of keeping forests healthy and carbon locked away.

The wood wide web has been here all along. We're only beginning to learn its language.