The Hidden Web: Underground Fungi Networks
Introduction
Trees can talk to each other, albeit not literally. Scientific research has shown that trees of the same species are communal, forming interdependent relationships, communicating with one another, and interacting with the forest environment. Trees exchange information through an underground network of fungi, which are called mycorrhizal networks.
Mycorrhizal Networks
Derived from the Greek words “myco” meaning fungus and “rhiza” meaning root, the term mycorrhiza was first introduced in 1885 by Albert Bernhard Frank in his paper. It describes the symbiotic association between the tree roots and below-ground fungi, where the mycelia of fungi connects the plants in the soil.
Currently, there are four principal types of mycorrhizal fungi. However, they can be primarily differentiated into two dominant types: ectomycorrhizal (EM) and endomycorrhizal (AM) fungi (which is sometimes known as vesicular arbuscular mycorrhizae).
Figure 1: Diagram showcases the differences between ectomycorrhizal fungi and endomycorrhizal fungi in roots. (Source: 2013 Nature Education Bonfante, P. & Genre, A.)
Ectomycorrhizal fungi form networks by their hyphae wrapping around the plant root, but not penetrating the root cells, instead forming a mantle. This phenomenon is commonly found in the roots of woody plants, such as pine trees and oak trees. In this type of root symbiosis, the fungal mantle surrounds the roots with a Hartig net around the root cells. For additional context, a Hartig net is essentially a lattice-structured network of inward-growing hyphae, taking up the space between plant cells and acting as a site for mutualistic exchange of resources.
Conversely, endomycorrhizal fungi penetrate the root cells using the hyphae. It forms relationships with over two-thirds of terrestrial plants and angiosperms, making it the most common type. Thus, they can be found in the roots of crops, including but not limited to wheat, maize, and soy. As the hyphae penetrates the root cells, fungal structures are formed within and between the root cells, such as arbuscules, vesicles, and coils. Additionally, it consists of an extraradical mycelium in the soil.
In both scenarios, the fungi provides nutrients for the plant – including phosphorus, nitrogen, sulfur, and so on – in exchange for photosynthetically-produced sugars. The difference lies in their nutrient acquisition methods. EM fungi mineralises nutrients from organic matter and directly accesses some organic nutrients (Phillips et al., 2013). On the other hand, AM fungi take nutrients released by saprotrophic microbes.
As a result, this relationship is mutually beneficial for the plants and microscopic fungi. Scientific evidence demonstrates that around 30 to 40% of minerals absorbed by the fungal network return to the plant roots. In exchange, the fungi are provided with around 40% of the photosynthesised sugars.
These relationships are so essential that so-called pioneer plants, which colonise virgin areas and therefore lack mycorrhizae, compensate for the absence of the fungal helper by developing root structures that mimic mycelial filaments.
WWW: Wood Wide Web
The mycorrhizal networks form the foundation of the network of resource sharing and communication among trees, especially defense signalling. This social network composed of plants, fungi, and bacteria has since been dubbed as the ‘Wood Wide Web’ due to its similarity with the Internet we know today.
Figure 2: Diagram depicts the ‘Wood Wide Web’, including the mycelial network. (Source: Angrish, 2022)
Through this network, trees are able to share resources between one another. ‘Mother trees’ – otherwise known as hub trees – are older and larger trees that supply younger trees with various nutrients like carbon to increase survival rates. Ecologist Suzanne Simard and her colleagues used radioactive carbon isotopes to track the movement of carbon between trees. Through the study, they discovered that the biggest and oldest trees were the most highly linked to other trees. Moreover, it was found that up to 40% of carbon in a tree’s roots could be transferred from other trees. This form of assistance is also seen during situations where the trees are ill or dying. Neighbouring trees share nutrients to aid it until it is capable enough to help itself, and may even connect with trees of different species. Even in death, some trees would transfer their stored nutrients to neighbouring trees.
Furthermore, the ‘Wood Wide Web’ acts as a site of communication between trees for defense warnings. In instances where trees are attacked by their predators, they warn the surrounding trees of the danger by sending chemical, hormonal, and electrical signals (Bücking et al., 2016). However, the information is passed more slowly compared to other organisms; the electrical signal is transmitted at one centimetre per minute. Once other trees receive the signal, a defence strategy is set up in the form of antibody production, making the leaves inedible. For example, trees transmit underground signals to one another to warn of an aphid infestation (Babikova et al., 2013).
The Web in Climate Change
In addition to the benefits of the ‘Wood Wide Web’, it plays an essential role as a major carbon pool. According to a recently-published study, over 13 billion metric tonnes of carbon dioxide is passed from plants to the fungal network each year, which is equivalent to 36% of annual fossil fuel emissions (Hawkins et al., 2023). Hence, damage to the underground network could lead to an increase in the feedback loop of warmer temperatures and carbon emissions. EM fungi was previously the dominant one in the environment at about 60% of all plants. EM fungi fixes carbon in the soil and prevents it from escaping into the atmosphere, which slows down the feedback loop with carbon emissions. Due to a rise in temperature, EM fungi is decreasing rapidly in numbers and being replaced by AM fungi, which may increase the rate of climate change. Fungal growth is likely to see a decline as climate change worsens, resulting in less biodiversity and disrupting mycorrhizal networks.
As aforementioned, mycorrhizal networks carry ecological significance by promoting biodiversity, carbon sequestration, and ensuring forest regrowth in extreme climate disasters or human activity. Because of that, the importance of mycorrhizal fungi as well as the need to protect this underground system has become increasingly recognised. For example, initiatives, such as the Society for the Protection of Underground Networks (SPUN), are mapping these networks globally. At the same time, more research is being done about this hidden ecosystem.
Figure 3: Diagram demonstrates the carbon sink. (Source: Groundwork BioAg)
Conclusion
Mycorrhizal networks, though unseen to the naked human eye, are crucial to the ecosystem by maintaining connections between plants for communication and nutrient-sharing. More importantly, they help in maintaining the health of the planet through its role in climate resilience and carbon storage. Although controversy remains over the ‘Wood Wide Web’ theory, it is clear that these networks are necessary for their ecological purposes, and thus require targeted conservation efforts.
Article prepared by: Estelle Sia Yu Qi, MBIOS R&D Director 24/25
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References
Babikova, Z., Gilbert, L., Bruce, T. J., Birkett, M., Caulfield, J. C., Woodcock, C., Pickett, J. A., & Johnson, D. (2013). Underground signals carried through common mycelial networks warn neighbouring plants of aphid attack. Ecology letters, 16(7), 835–843. https://doi.org/10.1111/ele.12115
Dong Y, Wang Z, Sun H, Yang W, Xu H. The Response Patterns of Arbuscular Mycorrhizal and Ectomycorrhizal Symbionts Under Elevated CO2: A Meta-Analysis. Front Microbiol. 2018 Jun 11;9:1248. doi: 10.3389/fmicb.2018.01248. PMID: 29942293; PMCID: PMC6004511.
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Hawkins, H. J., Cargill, R. I. M., Van Nuland, M. E., Hagen, S. C., Field, K. J., Sheldrake, M., Soudzilovskaia, N. A., & Kiers, E. T. (2023). Mycorrhizal mycelium as a global carbon pool. Current biology : CB, 33(11), R560–R573. https://doi.org/10.1016/j.cub.2023.02.027
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Zhang, L., Zhou, J., George, T. S., Limpens, E., & Feng, G. (2022). Arbuscular mycorrhizal fungi conducting the hyphosphere bacterial orchestra. Trends in plant science, 27(4), 402–411. https://doi.org/10.1016/j.tplants.2021.10.008
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