Walking through a forest, one cannot help but feel a deep sense of awe and wonder. The towering trees, rustling leaves, and cool, earthy smell remind us that nature is alive and ancient. The forest thrives through processes that we are only beginning to understand. It seems to have its own wisdom and unseen intelligence that works to nurture and sustain life. Perhaps, this wisdom is expressed via the Mother Tree Hypothesis (MTH), an idea suggesting that the largest, oldest trees — the Mother Trees — are the lifeblood of the forest, quietly ensuring the ecosystem’s survival through a network beneath our feet. Or is this simply a subconscious desire to personify and project our values onto chemical and biological processes?

Suzanne Simard, a professor in the Department of Forest and Conservation Sciences at the University of British Columbia, first proposed that so-called Mother Trees communicate with genetically related seedlings through chemical signals akin to human neurotransmitters. These signals involve ion flow across fungal membranes, allowing Mother Trees to send warning and recognition messages that help them understand their surroundings, distinguish related trees, and preferentially exchange nutrients with genetically related saplings. The hypothesis posits that this is possible due to mycelium, a network of long, filamentous, branching structures that act as the “roots” of fungus found in the soil and around plant roots that absorb nutrients from the surrounding environment.^{1, 2}  In Simard’s hypothesis, the Mycorrhizal Fungal Network (MFN) — comprising the larger, interconnected network of mycelium from multiple plants — is the facsimile of a neural network, with the molecules moving between trees acting as neurotransmitters.^1,2 

The Power of a Mother’s Love: Evidence Supporting the MTH 

Simard conducted a study of stable carbon isotopes and found evidence suggesting that Mother Trees preferentially send nutrients and communications to saplings that are genetically related to them. One experiment studying seed establishment success in Douglas fir forests found that seedling establishment success was significantly greater in areas where seedlings had full access to the mycorrhizal networks of older Douglas-fir trees compared to where they did not.²  Also, kin seedlings in the proximity of healthy Mother Trees had better nutrition than strangers, as  their mycorrhizal fungi received more carbon.¹  This suggests that Mother Trees have definitive preferences, transmitting much of their remaining carbon and energy to their offspring before distributing resources throughout the MFN. But how do these trees identify kin, and how do they send information and nutrients?

The answer lies in the MFN, which helps trees understand their environment, send communication signals, and transfer resources. Before Mother Trees can send nutrients to their seedlings through the MFN, they must first identify which seedlings are their own. Kin recognition between trees may occur via biochemical signals from mycorrhizae or roots, but the process is not largely understood.³ Once trees have identified their kin with help from the MFN, communication between plants and trees results in behavioral shifts expressed as changes in rooting patterns, mycorrhizal network development, nutrient uptake, and defensive enzyme production, which ultimately influences the survival and growth of both sender and receiver plants.^{3, 1} Certain studies have found that higher mycorrhizal colonization rates — where more of the roots are colonized by mycorrhizal fungi — between genetically related trees may result in a survival advantage since more carbon is transferred.³  Recognizing which neighboring plants are kin may also reduce energy costs since doing so can trigger different root responses, like increasing root density without adding biomass.³  

Simard’s theory also proposes that the MFN is similar to the neural network of the human brain.  In her experiments, Simard saw that the amount of carbon relative to nitrogen transferred through the MFN suggested the presence of the amino acid glutamate, one of the most abundant neurotransmitters in the human brain network.^{1, 4} In the MFN, communication occurs at synapses between different plant roots or mycorrhizas, where chemicals are released and transported along electrochemical gradients.¹ Thus, the neural-like physiology stems in part from the presence of neurotransmitters, allowing trees to perceive and respond to their environment through their leaves, stems, and roots.¹ This raises the question: Do Mother Trees exhibit a form of  intelligence, or is this simply complex behavior that allows  for such a vast system?¹  Furthermore, is the preferential treatment towards kin real, and if so, can sentience play a role? 

Is it Truly a Mother’s Love: Discourse Over the MTH

While the MTH is an exciting hypothesis, it has faced significant scrutiny, appearing to conflict with established observations regarding forest tree growth. Numerous studies indicate that belowground competition between mature trees inhibits the establishment of seedlings.^{5, 6, 7, 8, 9, 10}  For example, experiments in temperate beech forests have demonstrated that techniques like trenching, aimed at preventing root competition, promote seedling growth.¹¹ Additionally, there is lack of strong evidence that carbon transfer via the MFN significantly benefits the receiving tree, countering MTH claims. In fact, studies have detected only very small amounts of resources like carbon and nitrogen in the receiving plants.^{5, 12, 13, 14, 15} Even in her own studies, Simard found that Mother Trees did not consistently send significant amounts of carbon to their kin; at times, the Mother Trees transferred as much carbon to the mycorrhizas of a stranger as her kin.^{13, 1} It is also difficult to determine whether any carbon that is transferred is actually absorbed into the roots of the receiving plant or simply retained at the root-fungal interface between the tree roots and the mycorrhizal fungi.^{16, 5}

Additionally, Simard’s work fails to consider whether or not there are alternative pathways for nutrient transfer. Researchers found that there was significant carbon transfer between saplings that did not share the same mycorrhiza, suggesting that carbon transfer could occur through pathways other than the MFN.¹⁷ The experiments also noted significant carbon transfer between trees that do not make mycorrhizal connections, which may be because some plants, like poplar trees, have dual mycorrhizal colonization, allowing them to receive nutrients from plants attached to a different MFN.^{17, 18} The ability for dual mycorrhizal colonization may be more widespread than previously thought.¹⁹ Therefore, one cannot confidently conclude that the MFN is the sole mode of information and nutrient transfer, as posited by the MTH.

Controversy also exists over the MTH’s position that Mother Trees exhibit sentience when prioritizing their seedlings in resource distribution. It is important to note that the presence of molecules that are similar to neurotransmitters does not immediately imply intelligence and sentience.²⁰ It is longstanding consensus that plants such as trees are not sentient: they do not move and thus do not need a nervous system.  As a result, there is no evolutionary reason for plants to have emotions as they are stationary and do not need internal motivation to move. Also, plants lack a mechanism similar to an animal nervous system for transmitting information and do not possess brains.²⁰

While the idea of sentience is compelling, we must be cautious to not let such metaphors influence our interpretations of data. Exaggerating the significance of data interferes with our understanding of chemical and biological processes; furthermore, it assigns value to trees solely based on their resemblance to human intelligence, which risks overlooking the intrinsic worth of nature. 

Implications of the MTH on Environmental Protection and Scientific Understanding

One argument highlighting the dangers of ascribing sentience to the trees within MFNs suggests that recognition and signal-sending between trees are merely complex reactions not associated with thought. However, does the human brain not work the same way, sending communications through neurotransmitters?

These questions of sentience and intelligence may impact how we view and interact with nature. If we attribute kin recognition, communication abilities, and resource sharing to consciousness, then humanity may have further reason to reconcile with the harms we have committed and continue to commit against forests via deforestation, climate change, and environmental degradation. 

Even if these trees and the MFN are not sentient, as significant evidence suggests, should this impact how we treat nature? Is protection and the freedom to thrive only granted to those who are conscious and thus, more similar to humans? Humanity’s relationship with nature is inconceivably complex. However, regardless of sentience, the presence of older organisms and complex networks are undeniably essential to the health and well being of ecosystems and humans. Ultimately, the MTH reminds us to  recognize and respect the role that nature plays in  sustaining life, regardless of any human resemblance.

Acknowledgements 

I would like to thank Nils Henriksson, forest ecophysiologist in the Department of Forest Ecology and Management at  the Swedish University of Agricultural Sciences, for so generously reviewing my article for accuracy and providing plenty of detailed and thoughtful feedback. Additionally, I would like to thank BSJ Features Editors Luyang Zhang and Aashi Parikh for their assistance throughout the writing process. 

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