By Annie Chen, Environmental Science and Management ’19

Author’s note: When people think of ecosystems, trees and animals usually come to mind. However, most often we neglect an important part of the ecosystem — Fungi. Without us noticing, the fungi stealthily connects the organisms underground, creating a communication network that helps organisms interact with one another.

Picture yourself walking your dog in a quiet, peaceful natural forest, where you imagine the two of you as the only organisms capable of interacting with one another here. However, you are not alone; the plants can communicate and those trees and grasses are always speaking to each other without you taking notice. The conversation between vascular plants in this forest started before any of us are old enough to remember, and will likely continue if this forest is untouched. These conversations between seemingly disconnected organisms have helped this forest survive and thrive to become what you see today. You might wonder, what do these plants talk about, and most importantly, how do the plants communicate if they cannot move freely and have no vocal cords? The secret lies underground in an extensive network.

The underground network connects different immobile creatures to one another. Much like the above-ground biological interactions, the underground ecosystem is diverse, and not only houses many animals, but also consists of roots of different plants, bacteria, and fungal mycelium. The plant roots interact with their immediate neighbors, but in order for plants to communicate with plants further away from them, they rely on the underground fungal network, or according to Dr. Suzanne Simard who popularized the idea, the “Wood Wide Web” (WWW).

What is the underground “Wood Wide Web”, and how is it built?

This communication network is not made up of invisible radio waves like our Wifi, but rather relies on a minuscule and dense fungi network to deliver various signals and information [6]. These fungi, using their branching and arm-like membranes, build a communication network called the mycelium that connects between individual plants, and even the whole ecosystem. The mycelium deliver nutrients, sugar, and water, and in a more complex dynamic with the plants, deliver chemical signals. The fungi’s ability to expand their mycelium through reproduction and growth of fungi individually helps build these connections within the network. To expand their mycelium and link the network together with different individual plants, it must be evolutionarily advantageous for the fungi species to create such an extensive system. That is where the plant roots and their cooperative interactions come into play.

This communication network builds upon the foundation of mutualistic relationships between plants and fungi called mycorrhizae. Mutualism is the relationship that allows plants to provide sugars for the fungi in exchange for limiting nutrients such as phosphorus, nitrogen, and sometimes water (figure 1). According to an article published by Fleming, around 80-90% of the earth’s vascular plants have this mutualistic relationship, which allows plants and fungi to connect with one another through the plant roots. Without the mutually beneficial relationship, the fungi are not obligated to expand their network to connect to the plant roots and “help” these plants deliver chemical signals. 

Not only is there nutrient and informational exchange, but the plants benefit from fungi priming, where the initial fungi infection that creates the exchange interface between plant roots and fungi cells force the plant immune systems to increase immunity. The increased immunity in the infected plant indirectly increases the chances of the plants in resisting major changes such as a disease in the ecosystem [6]. This continuous plant-fungi network through nutrient exchange and strengthening each species’ survival connects the whole ecosystem together.

Figure 1: A simplified visual of species interactions within the fungal network. 

(Source: BBC)

That being said, the plant and fungal species that make up the WWW can vary by the participants who built the ecosystems. The interaction also means that the plants can selectively provide carbon or release defense chemicals to decide which fungus remains and has a mutualistic relationship with them [1]. When introducing a non-native species, it can alter the new ecosystem by encouraging different types of mycorrhiza. One such example was the introduction of European cheatgrass in Utah, U.S.A. The mycorrhizal makeup in Utah initially does not have significant changes prior to the introduction. However, upon European cheatgrass introduction to the Utahn site, despite the cheatgrass that does not contain European fungi, the site showed a shifted fungi genetic makeup [8]. Each plant individual, or species, using their preferences and abilities to “choose” their mutualistic partners, can diversify the fungal network to become more extensive and powerful, both to benefit and to harm other species of the ecosystem. The interspecies perspective is important in understanding the WWW.

Plants talk and interact through the “Wood Wide Web”

The communication extends to others in the ecosystem — the plants can “speak” to each other interspecifically, too. The individuals in an ecosystem are closely linked to one another, and so are relationships between plant individuals, whether it is directly with each other, indirectly through the fungal network, or both. The indirect communication relies on the fungal network, where various chemical signals pass through. For instance, the increased phosphorus level in the soil signals other plants that there is a plant-fungal interaction, and they may respond to this signal in different ways to ensure the situation is to their advantage — they could try to have their share of nutrients by producing sugars to attract these types of fungi, or they could make their plant competitors less healthy by excreting chemicals to weaken the fungus’ abilities to provide nutrients [13]. The WWW provides an internet that allows the plants to select a variety of methods to interact with one another, near or far.

The plants can choose to actively help each other through this fungal network, and allow both individuals, or species, to thrive in the ecosystem. Evolutionarily speaking, a plant individual could benefit from their own kind to thrive for the benefit of their survival. When an individual plant is thriving and producing excess carbon, they can help other plants by transferring excess nutrients through the fungal network [6]. An older, dying tree can also choose to transfer its resources to the younger neighbors through the fungal network, or donate its stored nutrients to the entire ecosystem through the decaying process that is aided by the extensive fungal network from fungal hyphae growth over the material [5]. Furthermore, through the WWW, the plants are able to communicate with one another about the possible threats including herbivores and parasitic fungi. In the research of Song et al, tomato plants infected with pathogens are able to send various defensive chemical signals, such as enzymes, into the existing fungal network for healthy neighbors in the network, warning them of the dangers nearby before they are infected themselves; using this mechanism, the plants can concentrate defensive chemicals with neighbors to minimize the spreading of this parasitic fungus in the area.

Not only can plants benefit one another, they can also use this network to put others at a disadvantage, such as to wipe out another competing or predating species that threaten their own survival. Allelopathy, or the exuding of chemicals to ward off enemies, usually gives off the impression that the plants use this method to discourage herbivores from consuming them, such as the milky sap that causes skin rashes and inflammation when a cucumber vine is cut, but the allelopathy is also active underground through the WWW. Barto et al, through their research on allelopathy, shows that even within a disturbed habitat, when there is competition between plant species, one species may utilize the regional network of fungi attached to them to deliver allelochemicals from one plant species to its neighboring species, preserving the fitness of their own kind.

Passive Animals, Active Plant and Fungi

We always think of herbivores as active players impacting the ecosystem. In the WWW, they are the last to respond to changes. Plants and fungi signal each other when an herbivore is present in the network, well before it has established its presence in the neighboring plants. Fungi are an important and active part of this ecosystem because they can also choose to exclude herbivores through chemical allelopathy. While it is possible that the fungi can choose to colonize a separate species that provides more benefits for them, they can concentrate their energy on defending its current host. Before the herbivore can expand its population, the plants have already communicated with one another through excretion of allelopathic chemicals, not only to ward off the herbivores that are causing potential damage, but also to warn other plants of the herbivore presence [1]. The fungi colonization of two nightshade species, Solanum ptycanthum and Solanum dulcamara, showed an increase of defense protein levels against the feeding caterpillars. This is just one example of herbivory defense mechanisms that results in decreased predator fitness, specifically in reduced growth rate and feeding rates [11]. When caterpillars feed on the Solanum spp., the active players in this relationship, the fungi and their plant hosts use chemical defense mechanisms indirectly induced by the fungi to discourage herbivores from feeding, and through evolution, eventually they drive out predators who are disadvantageous to the fungi-plant fitness.

Alone without the Wood Wide Web: Human Impacts

The network is built on a web of hyphae connections that is barely visible to the human eye, and even more vulnerable to changes. Older ecosystems not only have a higher percentage of larger trees with broader root systems, but are also denser in number, which both lead to a more extensive mycorrhizal fungal network. The species diversity, on top of age and density, contributes to a complex and healthy WWW that supports all plants in the ecosystem [3]. However, a disturbed ecosystem severs the connections in this network, making the previously extensive system difficult to repair.

Human activities that disturb the soil can affect this fragile yet powerful connection: seasonal tilling in agriculture, intensive logging, and change of soil chemistry and structure by laying concrete inhibits the soil from building an extensive web. Physically turning and chemically altering the soil is a direct human impact that cuts off hyphae connections between plant individuals in the system. According to Dr. Simard’s statement in a Biohabitat interview, the urban plants are less healthy because they lack the WWW to help them thrive through nutrient, water, and chemical signal exchange; they must do all those things or rely on humans to provide these needs. Indirectly, the larger-scale deaths and removal of plant individuals from logging also no longer foster a healthy mutualistic relationship between plants and fungi. If an individual plant is prevented from connecting with its mutualistic partners, whether that is through disturbance in the soil or the death of these partners, prevent the extensiveness of the WWW, and the isolation makes the urban tree population vulnerable to diseases if the humans do not diligently maintain them. 

It is true that the smaller versions of the WWW still develop between periods of disturbance, proven indirectly by the fungal colonization ability in off-site lab experiments such as those included in the studies of Barto et al and Hawkes et al. However, important interspecies collaborations and perhaps lack thereof that is missing compared to a minimally disturbed habitat functions much better in resisting climate change and increased foreign and invasive species that threaten the health of the ecosystem. 

Fortunately, despite the growing demand of land produced by economic growth and population, there is an increased awareness of the importance of plants and the health of the ecosystem. Over the last two decades, the addition of policies and practices indicated that major western conservation agencies have started to take on an interspecies perspective. One notable example is the inclusion of ecosystem management in the Clean Water Act, which adapts to the notion that endangered flora or fauna species is dependent on the health of an ecosystem [14]. The increased understanding of how interconnected the flora species are, in addition to conservation methods that have existed before the western colonization, have changed how governments aim to preserve nature. 

Regardless of the level of human impacts, the WWW holds important communication between plants, fungi, and herbivores through chemical signals and nutrient exchange to sustain or to outcompete each other. The connectivity to relay information within this network is key to the healthy plant community, and further the health of the ecosystem. Next time when you walk your dog in the woods, remember that the plants around you are capable of communicating thanks to this underground network. In order to keep this forest healthy for generations on, it is up to us to rethink development strategies to preserve this network that helps them thrive to continue the species’ communication in the WWW in this forest.

References

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