Fungi: The Invisible Architects of Forests

Have you ever wondered why Lithuanian forests look so different from tropical rainforests?

Probably not. But now that you’re here, it is actually a pretty fascinating question.

And since you’re reading a mushroom blog, you probably won’t be shocked to hear that fungi may have a lot to do with the way forests are structured.

Most plants live in close partnership with fungi through a relationship called mycorrhiza. In this partnership, fungal hyphae connect with plant roots and exchange nutrients with their plant partners.

There are two major types of mycorrhiza that are especially important when we talk about forests.

Ectomycorrhiza – the partnership common in our forests

In ectomycorrhizal associations (ECM), fungal hyphae form a sheath around tree roots and grow between root cells.

These fungi are especially important in many temperate and boreal forests, including the forests we know in Lithuania. Some very familiar mushrooms belong to ectomycorrhizal groups, including boletes and chanterelles.

photo: Ramūnas Petrusevičius

Arbuscular mycorrhiza – common in tropical forests

The second major type is arbuscular mycorrhiza (AM).

Here, fungal hyphae grow into the root tissues and form microscopic branching structures called arbuscules inside root cells. These structures create a large surface for nutrient exchange between the fungus and its plant partner.

Arbuscular mycorrhizal trees are particularly common in tropical forests.

And this is where things become interesting.

Many temperate forests can contain large groups of trees belonging to the same species growing close together. Think of the extensive pine forests of southern Lithuania.

Tropical forests often look very different: tree species diversity is much higher, while trees of the same species are frequently more widely separated from one another.

Could fungi help shape these differences?

A large study published in Communications Biology analyzed forest plots around the world and found that mycorrhizal type is linked to patterns of tree diversity and how closely trees of the same species grow together.

The researchers compared trees associated with ectomycorrhizal fungi and those associated with arbuscular mycorrhizal fungi across 43 large forest plots worldwide.

They found that ectomycorrhizal tree species tended to show weaker negative effects from having adult trees of the same species nearby. In other words, young trees associated with ectomycorrhizal fungi were more likely to occur near adult trees of their own species than was the case for trees associated with arbuscular mycorrhizal fungi.

One explanation is that shared mycorrhizal partners may help young trees establish near adults of the same species, partly counteracting the effects of species-specific pathogens and other natural enemies.

By contrast, in many tropical forests dominated by arbuscular mycorrhizal trees, young trees of the same species are often less successful when growing close to adult trees of that species. This can help prevent one species from dominating and contribute to the extraordinary tree diversity typical of tropical forests.

So fungi are not simply underground helpers delivering nutrients to individual trees.

Their relationships with plants may help influence where trees grow, which species can live close together and, ultimately, the structure and diversity of entire forests.

Not bad for organisms whose most important work usually happens completely out of sight.

The more we learn about the thin fungal hyphae running through forest soils, the clearer it becomes that fungi are not just inhabitants of forests. They are part of the system that helps shape them.