What Is Mycorrhiza? The Basics of Fungus-Tree Symbiosis
Beneath every forest trail, just a few centimetres under the leaf litter, life is as intense as it is in the treetops above. That is where fungi and roots form one of the oldest and most productive partnerships in evolutionary history: mycorrhiza. It is a symbiosis in which a fungus wraps around or grows into a tree’s root cells, forming an extensive network of threads called mycelium. The tree hands over some of the sugars it produces through photosynthesis, and in exchange receives water and minerals it could never reach on its own. This exchange has continued uninterrupted for over 400 million years, and it is largely thanks to it that plants were able to colonise dry land at all.
Not every mycorrhizal relationship works the same way. Ectomycorrhiza wraps a sheath of fungal tissue around the outside of the root and is typical of oaks, beeches, and pines, while endomycorrhiza grows directly inside root cells and is found in most herbaceous plants and many broadleaf trees. Both forms serve a similar purpose: they dramatically increase the surface area through which a root can absorb water and nutrients. For anyone interested in the foundation’s approach to ecological education, this is a striking starting point for a conversation about how much of a forest’s life happens where nobody can see it.
Types of Mycorrhiza Found in Temperate Forests
Temperate forests are typically shaped by three main forms of fungus-root symbiosis, and their presence determines how quickly a damaged ecosystem can recover:
- ectomycorrhiza – typical of pine, oak, beech, and birch, producing the visible mushrooms we recognise as edible fungi;
- endomycorrhiza (arbuscular) – dominant in maples, ash trees, and most forest floor plants;
- ericoid mycorrhiza – a specialised form found in blueberries and heathers, essential on poor, acidic soils.
The Wood Wide Web – How Trees Talk Underground
The term Wood Wide Web was popularised by the research of Canadian ecologist Suzanne Simard, who was among the first to demonstrate that trees in a forest are linked by a shared mycelial network stretching across many metres, sometimes even kilometres. Using labelled carbon isotopes, she showed that an older birch could transfer sugars to a neighbouring fir, even though the two belong to entirely different species. This discovery upended the popular image of a forest as a collection of competing, independent organisms. Instead, scientists began describing forests as superorganisms, in which resources are shared with whoever needs them most at a given moment.
The mycelial network works somewhat like a biological internet, carrying not only carbon and nitrogen but also chemical warning signals. When one tree is attacked by pests, it can send a signal through the network to its neighbours, which then begin producing defensive compounds before the threat even reaches them. This mechanism shows that forest resilience is a collective phenomenon, not simply the sum of individual trees’ strategies. It is one of the strongest arguments for protecting whole, intact forest complexes rather than isolated, scattered trees.
Fungi as Architects of Fertile Forest Soil
Mycelium does more than move substances between trees – it actively builds the structure of the soil it grows in. The fine fungal threads, called hyphae, wrap around soil particles and secrete sticky compounds that bind sand, clay, and organic matter into stable aggregates. As a result, forest soil becomes airier, holds water more effectively, and resists erosion even during heavy downpours. Without this invisible architecture, many forest soils would long ago have been washed away by rain or scattered by wind.
Mycorrhizal fungi also act as key nutrient recyclers, though in a different way than the saprotrophic fungi that feed on dead wood. Their role is to release phosphorus, nitrogen, and trace elements locked up in minerals and organic debris, and then hand them over to roots in an easily absorbable form. Scientists estimate that the mycelial network of a single hectare of healthy forest can contain several hundred kilometres of threads. That figure alone shows how much invisible work lies behind what, on the surface, simply looks like a quiet, green forest.
Mother Trees and the Forest’s Intergenerational Memory
Among the trees linked by mycelium, the oldest and most developed individuals play a particularly important role; ecologists call them mother trees, or hub trees. They typically have the densest network of mycorrhizal connections and the largest carbon reserves to share, making them central nodes of the entire underground network. Research has shown that mother trees can recognise seedlings growing in their immediate vicinity and preferentially direct more nutrients to them than to random, unrelated saplings. This behaviour resembles parental care seen in the animal world, though it unfolds on a completely different timescale.
The Mother Tree Effect
Removing a single large, old tree through clear-cutting therefore means far more than losing one trunk – it often severs a hub through which resources flowed to dozens of younger plants. This is why foresters in many countries are increasingly promoting a model of mycorrhiza-friendly forest management, in which some of the oldest, best-connected trees are deliberately left standing during harvesting. This approach fits naturally with the philosophy behind protecting old-growth stands, which environmental foundations work to preserve precisely because of their unique, decades-long role in the ecosystem.
Mycorrhizal Networks and Forest Resilience to Drought and Climate Change
As climate change brings increasingly frequent and intense droughts, the role of mycorrhizal networks in maintaining a forest’s water balance is becoming ever clearer. Fungal threads can reach far deeper and wider than tree roots alone, tapping into pockets of moisture the root system could never access on its own. During drought periods, trees strongly connected through mycorrhiza show less water stress and recover their stomatal function faster after rainfall than trees growing in isolation or on soils degraded by earlier disturbance.
Drought and Water Stress
Some species of mycorrhizal fungi additionally produce compounds that increase a plant’s tolerance to osmotic stress, meaning roots linked to mycelium can keep functioning even at lower soil moisture levels. This has real practical significance for organisations involved in tree planting, since the survival of young saplings in their first years largely depends on how quickly they establish a relationship with the local mycelium. For this reason, more and more forest nurseries now inoculate saplings with mycorrhizal fungal spores before they are ever planted in the ground.
Underground Biodiversity – The Invisible Layer of the Ecosystem
When people talk about biodiversity, they usually picture birds, mammals, or flowering plants visible above ground. Yet forest soil hides an equally rich, far less appreciated world. Scientists estimate that a single gram of healthy forest soil can contain several hundred metres of fungal thread along with millions of bacteria, nematodes, and microscopic arthropods, all forming a complex web of feeding relationships. This underground biodiversity is the foundation on which the rest of the ecosystem rests.
A healthy mycelial network also supports other organisms that feed on its fruiting bodies or benefit from the soil structure it creates, including:
- wild mammals, such as deer and wild boar, for which mushrooms are an important part of the autumn diet;
- soil invertebrates, including mites and springtails, which feed on mycelium and help regulate its growth;
- forest birds, which benefit indirectly from the abundance of insects living in fertile, well-structured leaf litter.
Threats to Mycorrhiza: Monocultures, Soil Chemicals, and Deforestation
Despite their remarkable resilience, mycelial networks are surprisingly vulnerable to human activity. Forest monocultures, where a single tree species is planted across a large area, significantly reduce the diversity of mycorrhizal fungi, since different trees prefer different symbiotic partners. Heavy use of phosphorus and nitrogen fertilisers paradoxically weakens mycorrhiza, because when a plant can easily access nutrients directly from the soil, it stops investing energy in its relationship with a fungus that has suddenly become less worthwhile.
Forest Fragmentation
Just as serious a threat is forest fragmentation caused by logging, road construction, or urban development. When a continuous forest is cut into isolated patches, the mycelial network loses its continuity, along with its ability to distribute resources over long distances. This effect is amplified wherever heavy machinery is used for logging, since soil compaction physically tears apart the delicate hyphal structures. Rebuilding such a network after serious damage can take decades, even if new saplings quickly appear on the surface.
How to Protect Underground Networks – Practical Steps and the Role of Tree Planting
Protecting mycorrhiza starts with a thoughtful approach to the tree planting process itself. Saplings with an intact root system and mycelium native to a given site have a far better chance of survival than those placed in degraded or artificially sterilised soil. That is why organisations running planting campaigns, including our own foundation, place strong emphasis on choosing native species and minimising disturbance to soil structure during fieldwork.
Anyone who cares about forest protection can support mycorrhizal networks through everyday choices:
- reducing the use of chemical fertilisers and pesticides in private gardens and plots;
- leaving dead wood and leaf litter in forests, since they serve as nourishment for fungi;
- supporting reforestation projects that rely on native species and local mycelium;
- avoiding excessive soil compaction by staying on marked trails in ecologically valuable areas.
The underground fungal network is a reminder that a forest is far more than the sum of its individual trees – it is a living, interconnected system in which every part supports the rest. By protecting this invisible infrastructure, we protect forests’ capacity to regenerate, adapt to climate change, and sustain biodiversity for generations to come. It is worth remembering this even when planting a single tree, because the real success of that act plays out underground.
