Trees and how the forest works

Microscope view of elongated root cells, in which small branched structures appear in dark blue. — An arbuscular mycorrhiza seen under the microscope: flax root cells containing arbuscules grouped in pairs. Msturmel, Public domain, source

The fungal networks that connect the roots of trees exist. This is not in dispute.

We must start there, because the rest of this page will correct a great many things, and it is important not to draw the wrong conclusion.

What is established

Underground, fungal filaments do indeed connect the roots of several plants — of the same species or of different species. This is called a common mycorrhizal network.¹

Their existence has been demonstrated. No one disputes it.

It is what is said about them that is the problem. The researchers put it this way: recent claims in the popular media about these networks are disconnected from evidence.²³

The three claims, and their three verdicts

In February 2023, three researchers — Justine Karst, Melanie Jones and Jason Hoeksema — published in a major journal a review of what is being said about these networks. They took three popular claims and looked at what supported them.⁴

First claim: these networks are widespread in forests.

Second claim: resources flow through them and improve the survival of seedlings.

For these two, the verdict is the same: insufficiently supported. The authors give three distinct reasons, which it is important to tell apart:

field results vary too much from one place to another;
the observations have other possible explanations;
the studies are too few to allow generalisation.⁵

These criticisms are not the same. The first says that the phenomenon is not consistent; the second, that the competing explanations have not been ruled out; the third, that not enough is known. All three can be remedied, but not in the same way.

Third claim: mature trees preferentially send resources and defence signals to their own offspring.⁶

Verdict: no peer-reviewed, published evidence.

None.

What makes this study exceptional

The authors did not simply say “it is exaggerated”. They went and counted.

They examined how the results of this research are cited by subsequent articles. Their finding: unsupported claims have doubled in twenty-five years.⁷

They also name the mechanism, and this is where the matter becomes instructive. What they describe is a positive citation bias: of all the results available, it is those that go in the expected direction that are cited most. The others exist, but they are taken up less often. No one is lying; the sorting happens by itself, article after article.⁸

This bias, they say, risks ultimately obscuring our understanding of how these networks really work.⁹

This point is worth dwelling on, because it matters for this whole site.

We have been saying from the start that a claim becomes distorted as it travels: nuance falls away, precision is lost. We have shown this some ten times, with examples to back it up.

This is the first time we have come across someone who has measured it.

What is a tree?

The question seems childish. It has no single answer, and that is what makes it interesting.

The main fact is this: “tree” is not a category in the classification of living things.¹⁰

It is not like “mammal” or “insect”, which denote groups of related organisms. There is no tree family. An oak is a much closer relative of a nettle than of a pine.

Four competing definitions

Depending on which one is adopted, the list of trees changes.

By form — a single, elongated stem bearing leaves and branches. This definition includes the banana plant and the papaya, which nevertheless have no wood.
By wood — a plant that produces wood as it thickens year after year. This definition excludes the banana plant.
By use — a plant that can be exploited as timber. This is an economic definition, and it changes with technology.
By size — above a certain height. But which one? The published minimum heights for a plant to be a tree range from 3 metres to more than 10 metres, more than a threefold difference.¹¹

What this means

Two serious botanists can count the number of tree species in a region and get different results without either of them being wrong.

They simply did not use the same dictionary.

And wood itself is a repeated invention

One last fact completes the demonstration that “tree” denotes a form and not a family.

Wood has appeared independently many times in the history of plants, in lineages with no close kinship. Herbaceous plants began to produce wood, separately, at different times.

On one archipelago, the number of times can even be counted.

On the Canary Islands, wood appeared at least thirty-eight times independently, from herbaceous ancestors. These thirty-eight transitions produced 220 woody species, spread across 34 genera and 15 families — about a fifth of the archipelago’s native flowering plants excluding monocots.¹²

Thirty-eight times the same solution, in the same place, by lineages that have nothing to do with one another.

There is even a tested explanation: drought. Woodier stems are better at resisting the gas bubbles that block the flow of water. Worldwide, species that have become woody are more numerous on islands exposed to drought. For 60 to 65% of the lineages studied on the Canaries, wood appeared within the last 3.2 million years, when summer droughts became established.¹³

A detail that sets things straight

All woody plants start out herbaceous. An oak three centimetres tall is a soft plant. The wood comes later.

Conversely: the potato produces wood in its roots.

“Tree” therefore describes what a plant does with its life, not what it is. It is a trade, not a family.

How does a forest work?

If the tree is not a taxonomic category, the forest is not a mere collection of trees either. It is a dynamic system of interactions and competition.

Far from the mythologised image of universal mutual aid, the forest works first and foremost through competition for two critical resources: light (above ground) and water or minerals (in the soil).

The dynamics of cycles and disturbances

A natural forest never stands still. Its functioning relies on regular disturbances (falling trees, fires, storms, insect attacks) that create gaps.

Regeneration — light reaching the ground activates dormant seeds or speeds up the growth of seedlings previously held back in the shade.
Ecological succession — fast-growing pioneer species establish themselves first, modify the microclimate and the structure of the soil, then gradually give way to climax species that grow more slowly.
Soil micro-biodiversity — the real engine for recycling dry matter (dead wood, leaves) lies in the soil fauna (bacteria, insects, worms and decomposer fungi), which is indispensable for releasing nutrients for the next generation.¹⁴

The safeguards: beware of false forests

Just as with the word “tree”, the definition of the word “forest” poses a major problem of measurement and of political interpretation.

The trap of the official definition

According to the FAO (Food and Agriculture Organization of the United Nations), a forest is an area of more than 0.5 hectares, with trees reaching more than 5 metres in height at maturity and a canopy cover of more than 10%.¹⁵

This strictly morphological definition leads to analytical distortions:

Field of trees vs ecosystem — in many assessments, an industrial monoculture of eucalyptus or maritime pine, planted in rows and felled every fifteen years, is counted in the same way as a natural forest.
Misleading reforestation figures — when a state announces millions of hectares of “forests gained”, these are often even-aged industrial plantations (trees of the same age), biologically poor and highly vulnerable to fires and diseases.¹⁶

A forest is not merely a carbon stock or a cover of trees: it is the simultaneous presence of structural complexity, temporal continuity of the soil and a network of ecological interactions.

What must above all not be concluded

We now come to the dangerous part of this page.

It must not be concluded that trees do not communicate. Nor that all this is false, invented, or that scientists talk nonsense.

The networks exist. The researcher who led the study says so herself: it is excellent that this research has sparked public interest in forest fungi.

The problem lies elsewhere. Many popular ideas have got ahead of the science.

That is not the same thing as being false. It means that they are presented as established when they are, at best, under examination.

How to read this kind of case

The method this site uses every time can be useful elsewhere.

Three questions, in this order.

What is established? Here: the networks exist.

What is being told? Here: mutual aid, signals, the nurturing mother that recognises her young.

What do we know about the gap between the two? Here, exceptionally: it has been measured. Unsupported claims have doubled in twenty-five years.

Most of the time, the third question remains unanswered. But simply asking it already changes everything: it prevents us from confusing what is demonstrated with what is repeated.

Note 1

A common mycorrhizal network (CMN) is formed when mycorrhizal fungal hyphae connect the roots of multiple plants of the same or different species belowground.

Karst, Justine, Jones, Melanie D. and Hoeksema, Jason D., “Positive Citation Bias and Overinterpreted Results Lead to Misinformation on Common Mycorrhizal Networks in Forests”, Nature Ecology & Evolution, vol. 7, 2023, p. 501-511