The Underground Networks That Connect Trees

Beneath a forest, most of the action is invisible. Roots spread through the soil, fungi thread between them, and nutrients and water move through spaces too small to see. For decades, scientists have been trying to understand how much of this hidden world connects individual trees. In one well-known 1997 forest experiment, researchers exposed trees to carbon dioxide containing a traceable form of carbon. Later, they detected that carbon in neighboring trees. Somehow, material had moved below the forest floor from one tree to another.

The discovery helped inspire the "wood wide web," likening how forests are connected to the internet. Yet trees don't send messages through buried cables, and scientists disagree about what these transfers mean. The real system involves roots, microscopic fungi, and exchanges that researchers are working to understand.

What Actually Connects Trees Underground?

Tree roots form partnerships with fungi known as mycorrhizae. In this relationship, a tree supplies fungi with carbon-rich sugars made during photosynthesis. In return, the fungi help the tree obtain water and nutrients, including nitrogen and phosphorus.

Fungi gather these resources using narrow filaments called hyphae. These threads can enter soil spaces that are too small for roots and extend beyond a tree's immediate root zone. Together, the threads form a larger fungal body called a mycelium. A single fungus may colonize the roots of several plants. When its hyphae link those plants, they form what scientists call a common mycorrhizal network.

Different trees partner with different groups of fungi. Ectomycorrhizal fungi commonly associate with various trees, including pines, oaks, beeches, and birches. Maples, ashes, and many other plants usually associate with arbuscular mycorrhizal fungi. The relationship is often described as cooperation, but it's also a trade. A fungus is a living organism pursuing its own survival. It may distribute resources in ways that benefit itself rather than treating every connected tree equally.

How Scientists Found the Hidden Connections

Scientists have known about fungal partnerships with roots since the 1800s. Evidence that these fungi could connect separate plants emerged from laboratory experiments during the 20th century.

One widely recognized field experiment involved Douglas-fir and paper birch seedlings. Researchers supplied the plants with different traceable forms of carbon and later detected carbon moving between the two species. The results suggested that their shared ectomycorrhizal fungi provided a possible route.

Modern experiments often place mesh barriers between plants. Fine fungal hyphae can pass through some of these barriers, while the thicker roots cannot. Researchers can then compare connected plants with plants whose fungal connections have been cut off.

Natural forest soil makes the work difficult. Carbon might pass between trees through fungal hyphae, but it can also move through leaking roots, microbes, or decomposing material. Roots may even touch or fuse directly.

Finding labeled carbon in another tree shows that movement occurred. Proving its exact path and whether that movement meaningfully helped the receiving tree is much harder.

What Moves Below the Forest Floor?

Researchers have studied the underground movement of various materials including carbon, nitrogen, and defense-related chemicals. Fungal partners also make these materials easier for trees to obtain without necessarily transferring them from one tree to another.

The exchange between a tree and its fungal partners is well established. A tree gives up some of the carbon it captures from the air, while fungal hyphae expand its reach through the soil. This can be especially valuable when water or nutrients are unevenly distributed.

Experiments have also suggested that connected plants may respond differently to insects, drought, or shade. A chemical change associated with an attacked plant, for example, might be followed by a defensive response in another plant.

That doesn't necessarily mean the first plant sent a deliberate warning. Chemicals may move because of differences in their concentrations, while the fungus may control or influence the route. Scientists continue to investigate which signals travel through fungal networks and whether they produce important effects under natural forest conditions.

Do Older Trees Feed Their Seedlings?

The "mother tree" idea proposes that large, mature trees act as important network hubs. These trees are said to support nearby seedlings, particularly their own offspring, by sending them carbon or other resources through fungi.

Several parts of this picture are possible. Large trees can associate with extensive fungal systems, and seedlings can connect to fungi used by older trees. More specific claims remain contested.

A major 2023 review in Nature Ecology & Evolution concluded that evidence was insufficient to say common mycorrhizal networks are widespread throughout forests or regularly improve seedling performance. Its authors found no published peer-reviewed evidence that mature trees preferentially send resources or defense signals to their offspring.

Other scientists responded that carbon transfer through fungal networks has been demonstrated and deserves continued study. However, even they acknowledged unanswered questions about how important the transfer is to receiving trees.

A seedling beside a mature tree is also affected by shade, root competition, moisture, soil chemistry, and many kinds of microbes. Being connected doesn't automatically mean it's being cared for.

How Fire and Logging Change the Network

Many of America's forests are working landscapes. Unlike national parks, national forests are managed by the USDA Forest Service for multiple uses, including recreation, wildlife habitat, and resource extraction.

These activities can affect underground fungi. Roads and heavy equipment may compact soil, reducing the spaces that hold air and water. Timber removal changes shade, moisture, living roots, and the supply of dead wood and leaves. However, results vary with the forest, fungal species, harvesting method, and severity of the disturbance.

Fire creates another major change belowground, but it's a natural part of many American forests. Low- and moderate-severity fires may leave deeper fungi and roots alive. Some fungi persist as spores, while others survive on the roots of living plants.

Severe fire can heat surface soil enough to kill fungi and bacteria. Forest Service research has found that higher-severity fires generally produce larger declines in fungal abundance and variety, especially near the surface. Large burn patches can also slow recolonization because surviving trees and fungi are farther away.

Recovery still occurs, but how fast depends heavily on what kind of fire it was. Fire-adapted plants return, wind and animals carry spores, and "fire-loving" fungi become more common. In ponderosa pine stands treated with prescribed fire and mechanical thinning, mycorrhizal fungi returned to levels resembling untreated controls after more than a decade. Recovery from high-severity wildfire is slower. Thirteen years after one Arizona fire, fungal communities inside large high-severity patches had still not caught up with the moderately burned and unburned ground nearby.

Why Fungal Partnerships Matter

Possible tree-to-tree transfers are only one part of the story. Mycorrhizal fungi help drive nutrient cycling, carbon movement, and seedling establishment. These functions influence which trees grow successfully and how forests respond to disturbance.

A Forest Service study used inventory records from more than three million trees to map the dominant mycorrhizal associations across the contiguous United States. Researchers found that climate was strongly associated with the distribution of the two main mycorrhizal tree groups.

The balance has also shifted in parts of the eastern US. The study connected increasing dominance by arbuscular mycorrhizal trees with several factors, including nitrogen deposition, fire suppression, and changing climate conditions. Such shifts could alter how forests store carbon and cycle nutrients.

A Real Network With Uncertain Rules

Trees aren't isolated organisms. Their roots interact with fungi, bacteria, neighboring plants, and constantly changing soil conditions. Fungal threads can connect multiple plants, and experiments show that materials can move between them.

What remains uncertain is the scale, direction, purpose, and ecological value of those transfers. The evidence doesn't prove that forests operate as harmonious societies or that trees consciously look after one another.

The reality is strange enough without turning trees into people. Every forest rises from relationships among organisms that hikers rarely notice, forming a living system beneath each step.