Updated: Sep 9, 2020
Are there mycelial networks underneath cities? How far does a mycelial network span? Shortish answer: Most likely. Most trees and plants in cities have fungus-roots (mycorrhizae) and are dependent on the underground mycelial networks. The mycelial networks can vary in size depending on age and connectivity -- mycelial networks can become jeopardized when there's habitat destruction, soil compaction and fragmentation, and with use of fertilizers and pesticides/fungicides. There are two main types of fungi forming the networks that are referred to as nature's original internet: Arbuscular mycorrhizal fungi (AMF) and Ectomycorrhizal (ECM) fungi, though Endophytic fungi are almost definitely present (though not so much in the below-ground systems) and are extremely beneficial. This "wood wide web" can be referred to as the Common Mycelial Network (CMN). At least 90% of the world's plants participate in some kind of mycorrhizal relationship in which there have been many measured benefits. The absorptive area of the plant roots can increase 10-1,000 fold, increased growth rate, decreased time taken to mature, enhanced nutrient access, higher nutritional content, higher medicinal quality, and a higher flowering rat are some of them. Long answer: How far do the networks radiate out? Well, the largest organism on earth (if not the aspen root system in Colorado, USA) is dubbed the "humongous fungus" which is a continual network that exists in Malheur National Forest, Oregon, USA. It's size is ~3.4 square miles (5.5sq km) -- roughly 3x the size of NYC's Central Park. This fungus, though, grows huge rhizomorphs and has extended this large likely because it is parasitic and has so much biomaterial to eat and kill. The size of networks generally increase with age. Bigger, older trees (especially old growth forests) are linchpins of the CMN. The larger the tree root, the larger the mycelial network it is engaged in -- they have more root tips, they are participating in more carbon exchange. If you live next to a large, old forest, you could be engaging on the margin of a larger CMN. There are many fungi like Arbuscular Mycorrhizal Fungi (AMF) or short-lived ECM that form relatively limited mycelial growth and they may only connect plant roots within close proximity or for a short duration. Often these kinds of connections are nested within more extensive fungal networks. Generally, it seems to me like mycelial networks have been more recently studied for biomass and 3D complexity in sample clusters than architecture and spatial topology. Observing CMNs is difficult to do without being destructive to the ecosystem and remains largely unknown due to sampling difficulties and adequate resolution. A fair amount of studies on hyphal growth are recreated in labs. Scientists use biochemical markers, fluorescent dye tracers, quantum dots, or radiolabeled nutrients in the soil networks to be able to follow how and where the networks transport water and nutrients. One study found that individual species with the same genetics of ectomycorrhizal fungi may extend to over 50m squared (source). These nets can turn over rapidly, renewing themselves each year. Some studies say that once networks are a few meters in scale, usually the mycelium is forming rhizomorphs or cords which is essentially a bundle of hyphae that can transport flows of water and nutrients at a higher rate. Genera of fungi that can form rhizomorphs include Armillaria, Boletus, Cortinarius, Paxillus, Piloderma, Pisolithus, Rhizopogon, Suillus, and Tricholoma,
Rhizomorphs are bundles of packed hyphae that increase the ability for a mycelial network to transport at a faster rate and at long distances. The red line depicts a cross section shown in the right image. (Image source: Australian National Botanic Gardens.)
Those studies that trace the expanse of a fungal network usually do so by linking a fungal network to a group of trees, using the trees as guideposts. They might take a stand of 50 trees and measure how information flows between these trees. A few studies do look at mycelium architecture and spacial structure. For instance a study featured in this article (great read!) by Beiler et al. (2010) recreated a diagram of a fungal network that links a group of trees, showing the presence of highly connected “mother trees" as the larger and darker circles. I don't know of a study that really traces how large a CMN can get, though there might be one out there hidden. Please email me if you know of more information!
Above image: Beiler et al. (2010) study showing a representation of mycelial connections of trees as green circles connected by networks of mycelium. "Mother trees" are shown as bigger and darker circles.
Another wonderful study, conducted by Simard et al. (2012), provides extensive research into mycelial network spacial topography. This study looked at a 30m x 30m plot of 67 Douglas Fir trees (green shapes; size relative to tree diameter). Lines link tree roots to ectomycorrhizal Rhizopogon fungi. The most highly connected tree (shown by the arrow) was connected to 47 other trees through 8 R. vesiculus colonies and 3 R. vinicolor colonies.
Above image: Simard et al. (2012) study looked at 30m x 30m plot of 67 Douglas Fir Trees (green shapes; size relative to tree diameter). Lines link tree roots to ectomycorrhizal Rhizopogon fungi. The most highly connected tree (shown by the arrow) was connected to 47 other trees through 8 R. vesiculus colonies and 3 R. vinicolor colonies.
It's hard to estimate when and where a single species is colonizing an entire area, though it is sometimes easier to tell. As an example, let's say you know that the Imleria badia fungus grows in association with spruce trees and you have seen this mushroom fruiting in your yard (since it's ectomycorrhizal, it can form mushrooms). If there are many spruce trees down the road, it is a good guess that this Common Mycelial Network extends among these other spruce trees. Many fungi like Agrocybe praecox and Coprinus picaceus are known to be short-range foragers whereas species like Phallus impudicus, P. velutina, and Resinicium bicolor are long-range foragers who are interested in more sparsely distributed resources. There are multiple threats that can affect the size and distribution of mycelial networks. Fragmentation of terrain is one of the largest threats to fungi, so for the CMN to be strong and large, there should be no huge strip mall or soil-deep, huge obstruction dividing your garden with the trees and plants down the street. Habitat destruction is the largest threat to fungi, so if the soil between your garden and the end of the street is very disturbed (construction or tilling) or very nutrient poor (if the soil is dependent on fertilizer is one sign or if there are fungicides/pesticides used), that's another way the networks will be cut off from one another. Minimum soil disturbance and permanent plant cover (even if it is just grass) help establish CMN. Extras about mycorrhizal fungi: 90-95% of all plants worldwide form some kind of mycorrhizal relationship, including cacti, flowers, cereal crops, grass, and pine trees. As we discussed in Class 1, there are 7 different types of Mcorrhizae. 1) Arbuscular endomycorrhiza, 2) Ericoid endomycorrhiza, 3) Arbutoid endomycorrhiza, 4) Monotropoid endomycorrhiza, 5) orchidaceous endomycorrhiza, 6) ECTO-mycorrhiza, and 7) Ecto-Endomycorrhiza. Essentially, in temperate city habitats, there are two main types of mycorrhizal fungi that could be involved in forming the CMN: Arbuscular mycorrhizal fungi (AMF) and Ectomycorrhizal (ECM) fungi, though Endophytic fungi are also mycorrhizal and are arguably living in all plant tissue. Ectomycorrhizae are considered the most advanced (/new) of all mycorrhizal relationships. ECM can confer many benefits to their plant partners including antiparasitic, antibiotic, and general fitness aid. Endophytic fungi also provide fundamental immune system support, even though they might not be the main CMN connectors.
Above image: A common depiction of the Common Mycelial Network (CMN) a.k.a the "wood wide web". (1) shows tree roots connected by ectomycorrhizal fungi; (2, 3) shows various plant species and a tree form arbuscular myccorhizal networks (AMF) which are interconnected into the subsoil layers; (4) an orchid forms a 3rd underground mycelial network with orchidaceous endomyccorhizas. More than 6,000 fungal species and at least 90% of the worlds plants confer nutrients through mycorrhizal symbiosis. (Image source: van der Heijden et al., 2014)
Ectomycorrhizal networks are formed when plants associate to ectomycorrhizal fungal partners. These associations can be very specific and some fungi will have very particular tree root hosts. There are more than 6,000 fungal species that form mycorrhizal symbiosis with at least 90% of the world's plants. Some fungi form ectomycorrhizal and ericoid associations. Some fungi can even participate in both myccorhizal and saprophytic (decomposer) lifestyles. For example fungus Boletus elegans only tends to grow with larch trees, though Amanita muscaria will associate with 20 or more tree species. Some trees may form a range of associations, like the Norway Spruce (Picea albies) which can associate with 100 different ECM fungi at a time. One way to speculate is if there are the same tree species in your garden to ones down the road that form ECM. Common tree species include: Douglas Fir (over 2,000 fungal species are known to be associates) Western Hemlock Black Cottonwood Pacific Madrone Red Alder Beaked Hazelnut Do any of these trees have distributions in your local habitat? What other trees unique to your bioregion could be forming mycorrhizal relationships? AMF Fungi (a.k.a. Glomeromycota fungi, who inhabit their own Phylla; there are 7), are responsible for creating soil tilth by exuding the sticky protein glomalin on the surface of their mycelium. Glomalin creates soil structure and allows microorganisms to enliven the soil at deeper layers. In areas in which it is found, glomalin may contain 1/3 of the habitat's sequestered carbon. These AMF fungi can be found in aquatic environments, salt marshes, grasslands, and most ecosystems. They're usually found deeper in the soil (subsoil) whereas the ECM fungi can be found higher up in the soil horizon. Remember, AMF fungi do not produce fruiting bodies (mushrooms). If they do, they are hypogeous (underground) and are the size of peas. This AMF symbiosis is by far the most ancient of all mycorrhizal relationships (evidence dating to 450 million BCE). These fungi may be the most ecologically significant of all fungi. They play a fundamental role in soil fertility and plant nutrition, and can defend against root-infecting pathogens. Lastly, Endophytic fungi are the ones living in arguably all plant tissue (there hasn't been a single scientific study where plants have not contained endophytes). Many of these fungi only inhabit flora in above ground tissue, though quite a number of endophytic fungi also inhabit the roots of flora especially in harsh and arid climates like deserts, coastal plains, and arctic/antarctic zones. Again, these fungi are most likely present and very nourishing for all plants, though they are not central to the CMN. Citations Beiler, Kevin & Durall, Daniel & Simard, Suzanne & Maxwell, Sheri & Kretzer, Annette. (2009). Architecture of the wood-wide web: Rhizopogon spp. genets link multiple Douglas-fir cohorts. The New phytologist. 185. 543-53. 10.1111/j.1469-8137.2009.03069.x. Simard, Suzanne & Beiler, Kevin & Bingham, Marcus & Deslippe, Julie & Philip, Leanne & Teste, Francois. (2012). Mycorrhizal networks: Mechanisms, ecology and modelling. Fungal Biology Reviews. 26. 39-60. 10.1016/j.fbr.2012.01.001. 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 Phytol, 205: 1406-1423. doi:10.1111/nph.13288