The Hidden Intelligence of Forests - How Trees Communicate

There is a forest that has been having a conversation for centuries.

Not in words. Not in sounds you can hear. But in chemical signals drifting through the air, in electrical pulses moving through root systems, in nutrients flowing along fungal threads thinner than a human hair, in warning messages passed from tree to tree faster than most people would believe possible.

The forest around you, if you are lucky enough to be near one, is not a collection of individual organisms competing for light and resources. It is something far more extraordinary than that. It is a community. A network. Something that functions with a kind of distributed intelligence that challenges our most fundamental assumptions about what intelligence is, what communication means, and what it means to be alive.

The science establishing this is not fringe speculation. It is peer-reviewed, replicated, and increasingly mainstream, published in Nature, Science, and the world's most respected ecology journals. It has been built over decades by researchers who spent careers in forests, underground, and in laboratories, patiently uncovering a world of biological complexity that was hiding in plain sight beneath the forest floor.

This article tells that story, what trees actually do, how they communicate, what they share, what they remember, and what it means for how we understand forests, nature, and perhaps intelligence itself.

The Discovery That Changed Everything

The story of forest communication science has a precise and remarkable origin point.

In 1997, a young forest ecologist named Suzanne Simard published a paper in the journal Nature that would eventually change how scientists, and increasingly the general public, understand forests. The paper described a series of elegant field experiments conducted in the forests of British Columbia, Canada, in which Simard and her colleagues demonstrated something that mainstream ecology had never established: that trees of different species were transferring carbon to each other through underground fungal networks (Simard et al., 1997).

The experiment was straightforward in concept, ingenious in execution. Simard labelled carbon dioxide with radioactive carbon isotopes, carbon-13 and carbon-14, and exposed different tree species to these labelled gases. By tracking where the labelled carbon ended up, she could trace the movement of carbon through the system. What she found was that carbon moved from birch trees to Douglas fir trees through the soil, and that the mechanism was the network of mycorrhizal fungi connecting their root systems.

The trees were sharing resources through a fungal internet. The paper was published. It was initially controversial, some ecologists questioned the magnitude of the transfers and their ecological significance. But subsequent research, by Simard and by dozens of other research groups around the world, has confirmed, refined, and extended the core finding many times over.

The forest floor, it turned out, was alive with communication and exchange in ways that had been completely invisible to science.

The Wood Wide Web: Understanding the Fungal Network

Before going further, it is worth understanding what mycorrhizal fungi actually are, because they are the infrastructure through which most tree communication happens, and they are remarkable in their own right.

Mycorrhizal fungi are organisms that form symbiotic relationships with the roots of most land plants, including the vast majority of tree species. They colonise plant roots, extending their own thread-like structures called hyphae into the surrounding soil in vast, branching networks. These networks dramatically extend the effective reach of the plant's root system, accessing soil pores and water sources that roots alone cannot reach.

The relationship is mutualistic, meaning both parties benefit. The plant provides the fungus with sugars produced through photosynthesis, carbon that the fungus needs but cannot produce itself, because fungi cannot photosynthesize. The fungus provides the plant with water, phosphorus, nitrogen, and other minerals that it extracts from the soil with its vastly extended hyphal network, which can be thousands of times more extensive than the root system alone.

What Simard and subsequent researchers discovered is that this fungal network does not just connect individual plants to their own fungal partners. It connects plants to each other. The hyphae of mycorrhizal fungi interweave through the soil and connect the root systems of multiple trees, sometimes trees of the same species, sometimes trees of entirely different species, into a single, continuous network.

This network, which scientists call the common mycorrhizal network (CMN) and which the popular press has dubbed the "Wood Wide Web", allows materials including carbon, nitrogen, phosphorus, water, and signalling chemicals to move between trees through the fungal threads. A single fungal network can connect hundreds of trees across an area of several hectares, integrating what appear to be separate individuals into something more like a superorganism.

The network is not uniform. Research by Simard and colleagues identified that certain trees in a forest act as hubs, highly connected nodes that link more trees to the network than average. These hub trees, which Simard called mother trees, are typically the oldest, largest trees in a forest, the individuals with the most extensive root systems and therefore the most extensive mycorrhizal connections (Simard, 2021).

What Trees Share: Carbon, Nutrients, and More

Once you know that trees are connected by fungal networks, the question becomes: what actually travels through those networks, and what does it mean for the trees involved?

Carbon: The Currency of the Forest

Carbon, in the form of sugars produced by photosynthesis, is the primary currency of the forest network. And what the research has shown is that carbon flows through the network in ways that are not random or indiscriminate. They appear to reflect the needs of the receiving tree.

Seedlings establishing themselves on the forest floor face a fundamental challenge: the light they need for photosynthesis is intercepted by the canopy above them, leaving them in deep shade and unable to produce enough carbon through their own photosynthesis to support their growth. Studies have shown that mother trees channel carbon through the mycorrhizal network to seedlings, their own offspring and others, apparently subsidising the growth of the next generation during the critical establishment period (Simard et al., 1997).

The transfer is not trivial in scale. In some studies, up to 40% of the carbon in a seedling has been shown to have come from neighbouring established trees via the fungal network, a substantial subsidy that may make the difference between a seedling's survival and its death.

This finding has provocative implications for how we understand forest succession and regeneration. The capacity of established trees to subsidise seedling establishment through carbon transfer may be one of the mechanisms that maintains forest stability across disturbance events, the large trees supporting the next generation's establishment even under conditions of low light.

Nutrients: Phosphorus, Nitrogen, and the Chemistry of Sharing

Carbon is not the only material flowing through the network. Phosphorus, nitrogen, and other minerals move between trees through mycorrhizal connections in ways that appear to respond to deficiency.

Plants under nutrient stress, lacking sufficient phosphorus or nitrogen, appear to receive preferential supply from the mycorrhizal network. The mechanisms are not fully understood, but the fungal partners appear to be directing resources in response to chemical signals of deficiency rather than simply distributing them uniformly.

This nutrient sharing has particular significance in the context of tree diversity. Mixed-species forests, where trees of different species coexist, appear to show different patterns of nutrient exchange than monoculture forests. Different species have different nutrient requirements, different peak demand periods, and different root depths, which means that resource sharing across species may reduce competition and increase overall forest productivity compared to monoculture plantings (van der Heijden et al., 2015).

Water: The Hydraulic Lift

Some tree species have the ability to move water from deep, moist soil layers to shallow, dry soil layers through their root systems, a process called hydraulic lift, and research suggests this water can then be accessed by other plants through shared mycorrhizal networks.

During dry periods, trees with deep roots that access groundwater can effectively supply water to shallow-rooted plants nearby, through both direct root contact and the mycorrhizal network. This hydraulic redistribution of water across the forest network may help maintain the productivity of the entire community during drought conditions, the deep-rooted anchors of the community providing a buffer against surface moisture deficits.

Warning Signals: How Trees Alert Each Other to Danger

Perhaps the most striking discovery in forest communication science is that trees do not just share resources. They share information, specifically, warnings about threats.

Aerial Chemical Communication

When a tree is attacked by herbivores, insects eating its leaves, it does not simply suffer passively. It responds. Within hours of the initial attack, the damaged tree begins producing chemical compounds called volatile organic compounds (VOCs) that drift through the air and can be detected by neighbouring trees.

The neighbouring trees, on detecting these chemical signals, begin producing their own defensive compounds, tannins, terpenes, and other chemicals that make their leaves less palatable or digestible to herbivores, before the herbivores even reach them. The attack on one tree effectively warns the community, giving its neighbours time to prepare their chemical defences (Baldwin and Schultz, 1983).

This aerial chemical signalling has been documented in multiple tree species across multiple ecosystems. In African savannas, giraffes feeding on acacia trees prompted upwind acacias to increase leaf tannin production within minutes, the acacia detecting ethylene gas released by the feeding tree and responding defensively (Karban, 2015). In Canadian forests, pine trees under insect attack release terpenes that neighbouring pines detect and respond to.

The specificity of the signals is remarkable. Trees can apparently distinguish between damage caused by different types of herbivores, insects versus mammals, for example, and modulate their chemical response accordingly. The signals carry not just "I am being attacked" but something more specific about the nature of the attack.

Underground Warning Signals

Chemical warnings do not only travel through air. They also travel through the mycorrhizal network underground, often faster and to more targeted recipients than aerial signals.

Research has shown that plants under attack by aphids transmit chemical distress signals through mycorrhizal networks to connected neighbours, which respond by upregulating their own aphid defences, even when the neighbours are physically isolated from the attacked plant and could not have detected the airborne signal (Babikova et al., 2013). The signal travelled underground, through the fungal network, carrying information about a specific threat.

This finding is significant because it suggests that the mycorrhizal network is not just a passive conduit for resource transfer. It is an active signalling network through which ecologically relevant information about threats moves between connected organisms.

Electrical Signals in Trees

More recently, researchers have identified what appears to be electrical signalling within trees, analogous, in a very loose sense, to the electrical signals in animal nervous systems.

Studies have measured action potential-like electrical signals in trees in response to wounding, light changes, temperature changes, and other stimuli. These electrical signals propagate through the tree and appear to coordinate physiological responses across the whole organism, similar to the way electrical signals in animal nervous systems coordinate responses across the body.

Research published in 2022 found systematic electrical signal propagation in trees in response to heat and cold stress, with signals moving at rates consistent with a coordinated systemic response rather than simple passive diffusion (Chatterjee et al., 2022). The question of whether these signals constitute something like a nervous system analogue in trees is one of the most provocative open questions in plant biology.

Mother Trees: The Intelligence of the Forest Elders

Simard's concept of mother trees, the highly connected hub trees in the mycorrhizal network, has attracted significant attention, some enthusiastic and some sceptical, and it is worth examining the evidence carefully.

The claim, supported by her research, is that the largest, oldest trees in a forest serve as hubs of the mycorrhizal network, connected to more trees, sharing more resources, and playing a disproportionately important role in the health and resilience of the forest community. When a mother tree is damaged or dying, research has shown that it can increase the flow of carbon and defence signals through the network to its connected neighbours, effectively redistributing its resources to the community as it dies (Beiler et al., 2010).

Simard's research also suggested that mother trees preferentially support their own offspring, recognising their own kin and directing greater resource flows toward them than toward unrelated seedlings (Simard, 2021). Kin recognition, the ability to distinguish relatives from non-relatives and behave differently toward them, is a sophisticated biological capability that had not previously been documented in trees, and the finding remains somewhat contested in the scientific literature, with some researchers arguing the evidence for preferential kin support is not yet conclusive.

What is not contested is that the oldest, largest trees in a forest are disproportionately important to the forest network. Their removal, through logging, disease, or climate-related mortality, disrupts the network in ways that affect the entire community, not just the individual tree. This has significant implications for how forests are managed.

Plant Memory: Do Trees Remember?

One of the most recent and most surprising discoveries in plant communication science is the growing evidence that plants can learn and remember, that exposure to a stimulus can alter their future responses to the same stimulus in ways that persist over time.

The most striking demonstration of plant learning comes from work by the ecologist Monica Gagliano, who subjected the sensitive plant Mimosa pudica, famous for folding its leaves when touched, to repeated harmless drops of water. Initially the plant folded its leaves with each drop. Over repeated exposures, it stopped, apparently "learning" that the drop was not a threat and no longer warranting a defensive response (Gagliano et al., 2014).

More significantly, the plants retained this memory for up to a month, outlasting any plausible explanation based on simple mechanical fatigue. The behaviour was stimulus-specific, the plants still responded to other stimuli, suggesting genuine learned discrimination rather than simple desensitisation.

The mechanisms by which plants encode and retrieve such memories are not yet understood. They do not have neurons, synapses, or anything resembling a brain. Whatever the mechanism is, it operates at the level of cellular chemistry, changes in gene expression, ion channel behaviour, or other cellular processes that persist over time. The implications for how we understand cognition, which we have previously treated as the exclusive province of nervous systems, are profound.

The Scent of Stress: Chemical Signals We Can Smell

Some of the chemical signals that trees produce are not invisible. They are the scents of the forest itself.

The distinctive smell of forests, particularly coniferous forests, is largely composed of terpenes: volatile organic compounds released by trees that play roles in communication, defence, and possibly thermoregulation. The aerosol of terpenes that hangs in the air of a dense forest has measurable effects on human biology, reducing cortisol levels, heart rate, and blood pressure in studies of "forest bathing" or shinrin-yoku, the Japanese practice of mindful time in forest environments (Li, 2010).

The research suggests that the chemicals trees use to communicate with each other may have secondary effects on the humans who walk among them, that the pharmacological chemistry of the forest air, evolved for ecological communication, interacts with human physiology in ways that are measurably beneficial.

This finding bridges the gap between the ecology of forest communication and the human experience of forests, suggesting that when we find forests calming, restorative, and somehow alive, we are not simply projecting. We are responding, at a biological level, to the genuine biochemical activity of the community around us.

The Science of Tree Rings: Reading Forest Memory

Trees encode the history of their environment in their wood, the annual rings that accumulate as they grow record not just the passage of time but the conditions of each year: the width of the ring reflecting growing conditions, the density reflecting seasonal temperature patterns, the chemistry reflecting atmospheric conditions, drought events, pest outbreaks, and fire.

Dendrochronology, the science of reading tree rings, has given researchers a window into centuries and even millennia of environmental history, extending the instrumental climate record far into the past. Long-lived trees including bristlecone pines, some of which are more than 4,000 years old, carry within their rings a record of climate variability extending back to the Bronze Age.

But tree ring records also reveal something about forest community dynamics, the synchronised responses of trees across a landscape to drought years, volcanic eruptions that reduce sunlight, or pest outbreaks. The rings of trees across a forest often show correlated patterns that reflect shared environmental history, evidence that the community has been shaped by the same forces, recorded in the biology of every surviving individual.

What This Means for Conservation and Forestry

The science of forest communication has practical implications for how forests are managed, implications that are beginning to influence conservation policy and forestry practice.

Old trees matter more than we thought. If mother trees are hubs of the mycorrhizal network that support seedling establishment, transmit resources to stressed neighbours, and play disproportionate roles in forest resilience, then the widespread practice of removing the largest, oldest trees in commercial logging operations, the practice of taking the best timber, has effects that go far beyond the loss of individual trees. It disrupts the network infrastructure of the forest community.

Research supports this concern. Clearcut forests, where all trees are removed simultaneously, show significantly slower regeneration and lower seedling survival than forests managed with retention of veteran trees, which maintain the mycorrhizal network and provide the hub connections that support new tree establishment (Teste et al., 2010).

Monoculture forestry reduces resilience. The mycorrhizal networks in monoculture plantations, forests composed of a single tree species, are less diverse, less connected, and less capable of facilitating resource sharing than the networks in mixed-species natural forests. The reduced diversity of both tree species and fungal species in monocultures translates into reduced functional redundancy, fewer alternative pathways for resources and signals to flow, and reduced resilience to disturbance.

Forest restoration needs to consider the network. Replanting deforested areas with seedlings grown in nurseries may not be sufficient for forest restoration if those seedlings are not inoculated with appropriate mycorrhizal fungi and planted in proximity to established trees that can provide network connectivity. Forest restoration science is increasingly incorporating mycorrhizal ecology into restoration protocols.

Indigenous forest management encodes this knowledge. Many indigenous communities with long relationships with specific forests have developed management practices that, in retrospect, reflect an intuitive understanding of forest communication and network dynamics, practices of selective harvesting, retention of veteran trees, and minimal soil disturbance that maintain the integrity of the forest network. The science is, in some respects, catching up with knowledge that traditional ecological wisdom has encoded for generations.

The Philosophical Dimension: What Is Intelligence, Really?

The discoveries of forest communication science raise questions that extend beyond ecology into philosophy, specifically, the philosophy of mind and the question of what intelligence actually is.

Intelligence, in its standard definition, involves the capacity to acquire and apply knowledge, to adapt to new situations, and to solve problems. It has been assumed, almost universally in Western intellectual tradition, to require a nervous system, neurons, synapses, the centralised information processing architecture that animal brains exemplify.

The evidence from plant biology suggests that this assumption may be too narrow. Plants acquire information about their environment through multiple sensory systems. They process that information, integrating signals from roots, leaves, and other organs, and generate adaptive responses. They communicate information to other members of their community. And they remember past experiences in ways that alter future behaviour.

None of this happens through a nervous system. It happens through biochemistry, through the extraordinary chemical computing that occurs within and between plant cells, regulated by molecules and processes that we are only beginning to understand. Whether this constitutes intelligence in a meaningful philosophical sense is a question that different thinkers answer differently.

What it does require is a reconsideration of the sharp line we have drawn between organisms that have minds and organisms that do not. The forest, it turns out, is doing something, processing information, communicating, adapting, remembering, that we do not have adequate language for, because our existing concepts of intelligence and communication were built around animals and, ultimately, around ourselves.

The philosopher Michael Marder has argued for what he calls "plant thinking", not thinking in the human sense, but a distributed, non-centralised form of information processing and response that is genuinely cognitive even if it lacks the architecture of animal cognition (Marder, 2013). This view is not universally accepted, and the debate it has sparked is one of the most interesting in contemporary philosophy of biology.

What is not debatable is that forests are doing far more than most people imagined. The wood is not silent. The trees are not isolated. The forest is, in some meaningful sense, alive in ways that go beyond the biological aliveness of individual organisms, alive as a community, as a network, as a system that communicates, shares, warns, supports, and remembers.

The Bottom Line

Walk into a forest with this knowledge and it transforms.

What looks like stillness is not stillness. The trees around you are communicating through the air with chemical signals you cannot smell and warning each other of threats you cannot see. Beneath your feet, a network of fungal threads thinner than spider silk is carrying carbon and nutrients between trees, redistributing resources across the community, maintaining the connections that make a forest a forest rather than a collection of competing individuals.

The oldest, largest trees are hubs of a network that has been growing and deepening for centuries, accumulating connections, maintaining the infrastructure of the community, supporting the next generation of seedlings with resources flowing through threads of living fungi that have been in this ground since before the oldest tree in the forest was a seedling.

This is not metaphor. It is not romantic projection. It is ecology, documented in peer-reviewed journals, demonstrated in field experiments, observed through isotopic tracing and chemical analysis and molecular ecology methods that could not have existed a generation ago.

The forest has always been this way. We simply could not see it.

Now we can, partially, imperfectly, with many questions still unanswered. And what we can see is extraordinary enough to warrant a fundamental revision in how we think about forests, about plants, about communication, about intelligence, and about the community of life we are part of on this planet.

The trees have been talking all along. We are only now learning to listen.

Cover image by Freepik [www.freepik.come]

References

Babikova, Z., Gilbert, L., Bruce, T.J.A., Birkett, M., Caulfield, J.C., Woodcock, C., Pickett, J.A. and Johnson, D. (2013) 'Underground signals carried through common mycelial networks warn neighbouring plants of aphid attack', Ecology Letters, 16(7), pp. 835–843. doi:10.1111/ele.12115.

Baldwin, I.T. and Schultz, J.C. (1983) 'Rapid changes in tree leaf chemistry induced by damage: evidence for communication between plants', Science, 221(4607), pp. 277–279. doi:10.1126/science.221.4607.277.

Beiler, K.J., Durall, D.M., Simard, S.W., Maxwell, S.A. and Kretzer, A.M. (2010) 'Architecture of the wood-wide web: Rhizopogon spp. genets link multiple Douglas-fir cohorts', New Phytologist, 185(2), pp. 543–553. doi:10.1111/j.1469-8137.2009.03069.x.

Chatterjee, S.K., Das, S., Maharatna, K., Masi, E., Santopolo, L., Mancuso, S. and Bhattacharyya, P. (2022) 'Exploring strategies for classification of external stimuli using statistical features of the plant electrical response', Journal of the Royal Society Interface, 12(104), article 20150225. doi:10.1098/rsif.2015.0225.

Gagliano, M., Renton, M., Depczynski, M. and Mancuso, S. (2014) 'Experience teaches plants to learn faster and forget slower in environments where it matters', Oecologia, 175(1), pp. 63–72. doi:10.1007/s00442-013-2873-7.

Karban, R. (2015) Plant sensing and communication. Chicago, IL: University of Chicago Press.

Li, Q. (2010) 'Effect of forest bathing trips on human immune function', Environmental Health and Preventive Medicine, 15(1), pp. 9–17. doi:10.1007/s12199-008-0068-3.

Marder, M. (2013) Plant-thinking: a philosophy of vegetal life. New York: Columbia University Press.

Merckx, V.S.F.T. (ed.) (2013) Mycoheterotrophy: the biology of plants living on fungi. New York: Springer.

Murchie, E.H. and Ruban, A.V. (2020) 'Dynamic non-photochemical quenching in plants: from molecular mechanism to ecophysiology', Plant Journal, 101(4), pp. 885–896. doi:10.1111/tpj.14601.

Simard, S.W. (2021) Finding the mother tree: uncovering the wisdom and intelligence of the forest. London: Allen Lane.

Simard, S.W., Perry, D.A., Jones, M.D., Myrold, D.D., Durall, D.M. and Molina, R. (1997) 'Net transfer of carbon between ectomycorrhizal tree species in the field', Nature, 388(6642), pp. 579–582. doi:10.1038/41557.

Song, Y.Y., Simard, S.W., Carroll, A., Mohn, W.W. and Zeng, R.S. (2015) 'Defoliation of interior Douglas-fir elicits carbon transfer and stress signalling to ponderosa pine neighbors through ectomycorrhizal networks', Scientific Reports, 5(1), article 8495. doi:10.1038/srep08495.

Teste, F.P., Simard, S.W., Durall, D.M., Guy, R.D., Jones, M.D. and Berch, S.M. (2010) 'Access to mycorrhizal networks and roots of trees: importance for seedling survival and resource transfer', Ecology, 91(10), pp. 2940–2951. doi:10.1890/09-1934.1.

Trewavas, A. (2014) Plant behaviour and intelligence. Oxford: Oxford University Press.

van der Heijden, M.G.A., Martin, F.M., Selosse, M.A. and Sanders, I.R. (2015) 'Mycorrhizal ecology and evolution: the past, the present, and the future', New Phytologist, 205(4), pp. 1406–1423. doi:10.1111/nph.13288.

Wohlleben, P. (2016) The hidden life of trees: what they feel, how they communicate. Translated by J. Billinghurst. Vancouver: Greystone Books.