Fungi are amongst the most mysterious and fascinating organisms, and although they are often overlooked, life on Earth may not have evolved at all without them. The fossil record demonstrates that even the earliest plants needed fungi to connect them to the soil and they continue to be essential to plants and animals today, processing dead organisms and breaking down rocks to make earth. Fungi are the principle decomposers within ecosystems, absorbing dead and decaying plants or animals, and recycling the nutrients so that they are available again for other organisms.
There is a much less well-known function that fungi perform within ecosystems, in providing a vast underground supply and communications network. They interconnect with the root systems of plants and trees, passing nutrients, water, and even chemical communication signals between the different organisms. In fact around 90% of all plant species depend on these mutually beneficial mycorrhizal relationships with fungi to survive. This mycorrhizal network creates a subterranean Wood Wide Web within forests, which powers growth, development, and resilience in woodlands and other habitats around the world. The discovery of this symbiotic relationship has the potential to revolutionise our understanding of how organisms interact within ecosystems and how seemingly disconnected plants are in constant communication. We take a look at what fungi are and how they connect all the organisms within a woodland.
When we think of fungi we picture mushrooms, but these are only the temporary fruit of a much larger organism. Buried deep beneath the comparatively tiny mushrooms that we see are intricate root-like networks of mycelium threads, which form the permanent part of the fungus. These subterranean structures can reach incomprehensible sizes, and the largest organism on Earth is thought to be a honey fungus in the Blue Mountains in Oregon which measures 2.4 miles across and could be up to 9600 years old.
Fungi share characteristics with both plants and animals but are unique enough to form their own kingdom. They have a cell wall and vacuoles like plants, but unlike plants their cell wall contains chitin, which plants do not produce. Fungi reproduce by both sexual and asexual means, and produce spores in common with some of the plant groups such as ferns and mosses. However, they are heterotrophic organisms like animals, meaning that they require energy from external sources and cannot manufacture their own through photosynthesis. Unlike animals, fungi are immobile and cannot move to gather nutrients, so they release digestive enzymes into their environment to obtain molecules that they can use for nutrition, digesting before ingesting.
Fungi are made entirely of fine, elongated, tubular structures called hyphae, which measure 4-6µm in diameter. In animals and plants specialised cells form different parts of the organism, but amazingly almost all fungal structures are made of hyphal cells. Hyphae form aggregations to fulfil a range of ecological functions, acting as a transport network for nutrients and water. Strands of parallel hyphae form the mycelium, a root like structure that seeks out the resources that the fungus needs. The fungus absorbs nutrients from its environment by secreting enzymes from the hyphae into the food source to break it down, and then absorbing them into the mycelium. Fungi can expand into new areas using more densely packed strands of hyphae to form mycelial cords or rhizomes.
Even mushrooms, the fruiting body of fungi, are entirely made from hyphae. Mushrooms start as a nodule on the mycelium which grows into a round egg-like structure made of interwoven hyphae. The mushroom develops inside this cottony layer, known as the universal veil, which tears as the mushroom grows and may form scales or a cup called a volva at the base of the stalk. Not all mushrooms develop within a universal veil and indeed not all of them have stalks – species of puffballs, jellies and earthstars do not have stalks. Once grown the mushrooms release spores from their gills or pores, which then disperse and recombine in the ground to produce more hyphae that form more mycelium.
The fungi within a woodland can be classed in three ways, depending on how they obtain their nutrients. Saprotrophic fungi are the most common and obtain nutrients from non-living organic matter. Parasitic fungi absorb nutrients from a living host, often to the detriment of the host. Mycorrhizal fungi form a symbiotic association with a plant or tree, where they exchange nutrients and carbohydrates.
Saprotrophic fungi are the main decomposers in woodland ecosystems, breaking down dead plants and animals into compounds that can be recycled by other organisms. Their hyphae invade and digest the dead material by releasing enzymes to break down complex polysaccharides such as lignin, cellulose, and chitin into more simple, soluble compounds such as glucose. Indeed they are only organism that can break down lignin, which is found in wood and bark. Fungi digest their food before they ingest it, so once the simple compounds are available, they are absorbed back into the mycelium. These compounds can then be absorbed by the fungus itself or released back into the soil as nutrients for other fungi, plants, and soil invertebrates. They play a vital role in the carbon and nitrogen cycles as the decomposition process releases carbon and nitrogen back into the environment, and in reducing the accumulation of dead organic material in ecosystems. Species of turkey tails (Trametes) and oyster mushrooms (Pleurotus) are examples of saprotrophic fungi.
Parasitic fungi invade the living tissue of their hosts, forming a mycelial network between the cells of the plant, through which they absorb nutrients, causing disease and potentially the death of the host. The spores of the pathogenic fungus fall on the exterior surface of the plant and form a germ tube, which grows until it finds a weakness such as a stoma on a leaf, or a wound in the plant tissue. Some fungi such as Septobasidium species can even involve insects in their parasitism, forming mycelium over an insect colony that is feeding on a tree. The individual insect sinks its proboscis into the bark and then the fungus penetrates the insect, feeding on it without killing it. The parasitised insect remains fixed to the bark for the rest of its life, while uninfected insects reproduce and spread fungal spores via their newly hatched offspring.
Tree diseases are often caused by parasitic fungi and the effects can be devastating. For example, elms in the US and the UK were nearly eradicated by the fungus that causes Dutch Elm disease (Ophiostoma ulmi), and the fungus that causes ash dieback (Hymenoscyphus fraxineus) is expected to kill 80% of ash trees in the UK. One of the most damaging fungi to trees and plants is the honey fungus genus (Armillaria), which surrounds its host using mycelial cords, parasitises the roots by taking nutrients from the tree, and then saprotrophically digests the dead material.
Mycorrhizal fungi work in a totally different way, forming mutualistic relationships with plants and particularly trees, providing them with water and nutrients such as phosphorus, in return for carbohydrates. There are two main types of mycorrhizal fungi, ectomycorrhizae (found in 2% of plant families) and endomycorrhizae (including arbuscular mycorrhizae, found in 90% of plant families). The difference between the two is that the hyphae of ectomycorrhizal fungi do not penetrate individual plant root cells but endomycorrhizal fungi do. Ectomycorrhiza are associations commonly found between fungi and woody plants including birch, beech, oak, and pine, and individual trees can partner with 15 or more ectomycorrhizal fungi at one time. They form a mantle around the roots of plants and trees, and can be seen with the naked eye. Arbuscular mycorrhiza are associations between microscopic fungi and vascular plants including rowan, aspens, maples, and ash and are essential to many crop plants. Examples of ectomycorrhizal fungi include many woodland species such as boletes (Boletus spp.), Tricholoma spp, and agarics (Amanita spp.). Arbuscular mycorrhiza are microscopic and are all from class glomeromycetes.
Although there are an astonishing 2.2 to 3.8 million species of fungi, only 148,000 species have been formally identified, meaning that we only have the most superficial understanding of how fungi species may be interacting with other organisms. The symbiotic relationship between trees and fungi formed by the mycorrhizal network is what enables a woodland to thrive, bringing benefits for both partners in terms of accessing resources. Mycorrhizal networks carry carbon, phosphorus, nitrogen, water, and sugars between the fungi and their plant hosts, but also link all the plants attached to the network together, creating a vast collaborative framework of shared resources. The size of the mycorrhizal network is extraordinary, the length of the mycorrhizal hyphae in the top 10cm of soil is estimated to be half the width of our galaxy.
Forming a mycorrhizal relationship with fungal partners has many benefits for plants. One of the most studied is the effect that mycorrhizal partnerships have on seedlings, where they increase establishment success, promote higher growth rates, and improve survivorship. Fungi can supply 80% of a plant’s nitrogen requirements, up to 100% of its phosphorus requirements, and also provide water in times of drought or dormancy. In return, plants allocate around 20% of their carbon intake to their fungal partners. The importance of this carbon store in addressing climate change is now being explored, with studies aiming to map mycorrhizal networks to ensure their protection. The exchange of resources between mycorrhizal partners has been shown to increase growth rates and improve survivorship of the connected plants, and also provides essential nutrients to the fungus and ensures the continuation of its supply. Fungi can enable plants to live in hostile environments, which has been shown in an extraordinary series of experiments where a coastal specialist grass was able to survive in hot geothermal conditions and vice versa when it was paired with the appropriate fungal partner.
The relationship between a plant and its mycorrhizal partner is not a simple symbiotic relationship, however, where both partners benefit from the interaction. Because the fungus is also connected to a multitude of other plants, it controls the allocation of resources across the whole network. Trees and plants may form partnerships with a variety of fungal species, making the models of resource allocation across networks even more complex. Resources tend to be taken from plants that have more and distributed to plants that have less, in a source sink model. For example carbon has been shown to be transported from plants with an excess to plants with a scarcity with the use of isotope markers. This means that evergreen trees that photosynthesise over winter can supply carbon to dormant trees, and deciduous trees in leaf provide more carbon to coniferous trees in the spring and summer.
It is tempting to think of plant to plant interactions across the mycorrhizal network as being an altruistic sharing of resources, particularly as a dying tree will dump its resources into the network. Plants also use the network to ‘share’ warnings about predation, with species such as douglas fir sending stress signals into the mycorrhizal network when they are attacked by predators. These signals reach neighbouring trees, which can then mount a defence, and even trees of different species exhibit this response. It is in the interests of the fungus to ensure that all its plant hosts survive and continue to provide resources, so it may well be that it is the fungi controlling the allocation of resources and transmission of infochemical signals. The Wood Wide Web is not just a collaborative framework either, as plants can also emit allelochemicals (chemicals which can affect growth and survival in other plants) more quickly across mycorrhizal networks, reducing growth in competitors.
Experiments have shown that young trees only have one or two connections to the network, but older trees have many more, with a few individuals acting as hubs with hundreds of connections. The loss of these veteran trees through felling operations can have dramatic consequences on the other trees in the forest. There is evidence that trees provide more resources to their relatives, with higher carbon flow between sibling plants, and mother trees nurturing their own offspring more than adjacent plants. This may be due to the fungal partner having greater success with obtaining resources from genetically similar individuals, however.
Understanding these dynamics has the potential to help us create better forests, by retaining legacy trees and improving disease resistance and resiliency. We can also learn how to maximise carbon sequestration within woodland ecosystems, with the knowledge that ecosystems high in mycorrhizal fungi store eight times more carbon. It is clear that we need to consider forests as one integrated organism, with all the connected individuals communicating and collaborating together. Viewing trees as organisms that are able to communicate with each other and recognise offspring revolutionises our understanding of what separates plants and animals and is proving a fascinating avenue for research.