Mycorrhizal Networks

A mycorrhizal network is an underground network of fungal hyphae that connects the roots of two or more individual plants. These networks are a manifestation of a symbiotic relationship known as mycorrhiza, which forms between certain fungi and the vast majority of terrestrial plants. The network acts as a conduit, allowing for the exchange of resources and information, which can significantly impact plant survival, growth, and community structure, as detailed by the US Forest Service.

Formation and Structure

Mycorrhizal networks form when the vegetative part of a fungus, the mycelium, colonizes the roots of a host plant. The fungus receives carbon-based energy (sugars) produced by the plant through photosynthesis. In return, the fungus's extensive network of fine filaments, or hyphae, extends far into the soil, acting as an extension of the plant's root system. According to a study in Nature Ecology & Evolution, this mycelium can then connect with the roots of other nearby plants, linking them together into a common network.

There are two primary types of mycorrhizal associations that form these networks:

–Arbuscular Mycorrhizal (AM) Networks: Formed by fungi from the phylum Glomeromycota, this is the most common type, associating with approximately 70-80% of all land plants, including most agricultural crops and grassland species. The fungal hyphae penetrate the plant's root cells to form highly branched structures called arbuscules, which are the primary sites of nutrient exchange. These networks are crucial in tropical and temperate ecosystems, as noted by Yale Environment 360.
–Ectomycorrhizal (ECM) Networks: These are formed primarily by fungi from the phyla Basidiomycota and Ascomycota. ECM fungi are dominant in temperate and boreal forests, associating with tree families like pine, oak, and birch. Instead of penetrating root cells, the hyphae form a dense sheath, called a mantle, around the root tip and a web-like structure known as a Hartig net between the root cells. This structure facilitates the exchange of resources, as described by The New York Times.

Functions of the Network

Mycorrhizal networks perform several critical functions within an ecosystem.

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Nutrient and Water Transfer: The primary function is the facilitation of resource exchange. The fungal hyphae are much thinner than plant roots, allowing them to explore a larger volume of soil and access nutrient patches that are unavailable to the plant alone. Fungi are particularly efficient at acquiring phosphorus and nitrogen, which they transfer to the connected plants. In exchange, the fungus receives up to 30% of the sugar a plant produces, according to research published in the journal Science.
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Interspecies Resource Sharing: The network allows for the transfer of carbon, water, and other nutrients between connected plants. This is particularly important for the survival of seedlings growing in the understory with limited light. Research by scientist Suzanne Simard and others has shown that established, older trees (sometimes called "mother trees") can transfer carbon to shaded seedlings, increasing their survival rates. This resource redistribution can enhance the resilience of the entire plant community, a concept explored in a paper in Ecology Letters.
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Communication and Defense: Plants can use the network to transmit chemical signals. When one plant is attacked by herbivores or pathogens, it may release signaling compounds that travel through the network to warn neighboring plants. These neighbors can then preemptively activate their own defense mechanisms, such as producing insect-repellent chemicals. Evidence for this signaling was documented in a study in Scientific Reports.
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Carbon Cycling: These networks play a significant role in the global carbon cycle. The carbon transferred from plants to the fungi is not only used for fungal growth but is also incorporated into the soil, contributing to long-term carbon storage. The stability and extent of this fungal biomass make it a critical component of soil organic matter, as explained by researchers at Boston University.

The "Wood Wide Web"

The term "Wood Wide Web" was coined by scientists to describe the complex, communicative, and resource-sharing nature of mycorrhizal networks, particularly in forests. While the concept has gained significant popular attention, some scientists urge caution, pointing out that the extent of cooperative, intentional behavior among plants is still debated. A 2023 review in Nature Ecology & Evolution highlighted that while resource transfer is well-documented, field evidence for complex signaling and kin recognition remains limited and requires more rigorous study.

Formation and Structure

There are two primary types of mycorrhizal associations that form these networks:

–Arbuscular Mycorrhizal (AM) Networks: Formed by fungi from the phylum Glomeromycota, this is the most common type, associating with approximately 70-80% of all land plants, including most agricultural crops and grassland species. The fungal hyphae penetrate the plant's root cells to form highly branched structures called arbuscules, which are the primary sites of nutrient exchange. These networks are crucial in tropical and temperate ecosystems, as noted by Yale Environment 360.
–Ectomycorrhizal (ECM) Networks: These are formed primarily by fungi from the phyla Basidiomycota and Ascomycota. ECM fungi are dominant in temperate and boreal forests, associating with tree families like pine, oak, and birch. Instead of penetrating root cells, the hyphae form a dense sheath, called a mantle, around the root tip and a web-like structure known as a Hartig net between the root cells. This structure facilitates the exchange of resources, as described by The New York Times.

Functions of the Network

Mycorrhizal networks perform several critical functions within an ecosystem.

–
Nutrient and Water Transfer: The primary function is the facilitation of resource exchange. The fungal hyphae are much thinner than plant roots, allowing them to explore a larger volume of soil and access nutrient patches that are unavailable to the plant alone. Fungi are particularly efficient at acquiring phosphorus and nitrogen, which they transfer to the connected plants. In exchange, the fungus receives up to 30% of the sugar a plant produces, according to research published in the journal Science.
–
Interspecies Resource Sharing: The network allows for the transfer of carbon, water, and other nutrients between connected plants. This is particularly important for the survival of seedlings growing in the understory with limited light. Research by scientist Suzanne Simard and others has shown that established, older trees (sometimes called "mother trees") can transfer carbon to shaded seedlings, increasing their survival rates. This resource redistribution can enhance the resilience of the entire plant community, a concept explored in a paper in Ecology Letters.
–
Communication and Defense: Plants can use the network to transmit chemical signals. When one plant is attacked by herbivores or pathogens, it may release signaling compounds that travel through the network to warn neighboring plants. These neighbors can then preemptively activate their own defense mechanisms, such as producing insect-repellent chemicals. Evidence for this signaling was documented in a study in Scientific Reports.
–
Carbon Cycling: These networks play a significant role in the global carbon cycle. The carbon transferred from plants to the fungi is not only used for fungal growth but is also incorporated into the soil, contributing to long-term carbon storage. The stability and extent of this fungal biomass make it a critical component of soil organic matter, as explained by researchers at Boston University.