From the tip of Brittany to the edges of the Urals, a broadleaf forest covers nearly a third of Europe. It sheds its leaves in autumn, falls asleep in winter and comes back to life in spring. At its heart reigns a 450-year-old oak, a fortress where thousands of species make their homes, love and fight. Following it from one season to the next means discovering a forest within the forest, and what is lost when it is replaced by fields of trees.
From the time of the Celts, the oak, “the king of the forest”, has symbolised the link between the spirits of the sky and those of the earth.
Hornbeams, chestnuts, beeches, birches: a patchwork of 72 million hectares, and in France a forest that is two-thirds broadleaf.In the Jura, the cabinetmaker Romaric Gauthier chooses his wood “by the right moon”, when the sap goes down to the roots: the wood dries faster and resists fungi without chemical treatment. “Do we trust the elders? I, for one, want to believe it.” Above all, he wonders which tree to take so as to disturb as little as possible an ecosystem where connections are everywhere. Our oak, for its part, is a sanctuary. In its million leaves, insects lay their eggs and feed; mosses, lichens and fungi nestle in its bark; the jay and the squirrel squabble over its acorns, wood mice build their city among its roots. At the very top, the great spotted woodpecker has hollowed out its nest hole in wood softened by fungi, and will hand it over to others, bats among them. In the Tronçais forest, the ONF biologist Laurent Tillon tracks by radio transmitter Bechstein’s bat, which devours the insects that ravage the oak. A single colony can occupy 300 to 350 roosts in a year: where large trees have been kept, the colonies are doing very well.
Romaric Gauthier accepts knots and twisted branches: a wood that is harder to work, “but really an extremely expressive wood, very alive”.
The fungi that soften the wood are xylophagous; between the bird, the bat and the tree, says Laurent Tillon, there forms “a kind of mutual agreement”.
Antenna in hand, Laurent Tillon follows the signal from the transmitter fitted the day before on a female, at 150.620 MHz, to trace her back to her colony.In autumn, the old oak drops its ripe acorns and invites the forest to a feast. The squirrel buries its stores and will not find them all: from one of them, an oak may grow. But the great sower is the jay, which moves nearly 4,600 acorns a year, sometimes very far. The tree goes dormant and leaves several hundred kilos of leaves on the ground, which an army of recyclers and decomposers returns to the forest. The mycologist Francis Martin, who has been exploring the symbioses between trees and fungi for more than forty years, points out that here we are walking “on a carpet of living beings”. On its own, the oak could not draw elements from the soil. With the milkcap, the bolete or the truffle, it forms a chimeric root, the mycorrhiza, the site of a real barter: the fungus’s filaments bring it water and nutrients, and it gives them nearly a quarter of the sugars made by its leaves. For one metre of root, nearly a thousand metres of filaments: a fine example, he says, of the role of cooperation in evolution.
The jay would like to keep the old tree’s acorns all to itself; by scattering them, it gives the oak the hope of conquering new horizons.
Before winter, the oak stores its nutrients in the base of its trunk and in its roots, then sheds its leaves to save its energy.
The little milkcap is specific to the oak. Francis Martin, for his part, would like “two or three lives” to decipher these barely revealed mechanisms.When winter comes, the bare oak faces the storms without the suppleness of the willow. Its roots anchor it like the foundations of a cathedral, and it has formed swellings on its branches, weak points that give way first: it offers the wind a part of itself to save the rest. The real danger lies elsewhere. Of the millions of acorns it has sown, not one has yet produced an adult tree: the young shoots are devoured, especially in winter. Where lynx and wolves are lacking, roe deer are up to ten times more numerous than fifty years ago. In an area where the lynx is present, the ecologist Sonia Saïd discovers the animal on the images from a camera trap, she who has never seen one in the flesh. Colleagues have observed it in Poland: when the lynx is around, the time the roe deer spends keeping watch is time it does not spend eating. At the foot of a small oak, the bramble has been browsed, the buds are intact. “As long as there is bramble, the oak is left in peace.”
Beyond twenty centimetres of snow, roe deer struggle to move: for herbivores, winter is “really a complicated time”.
Walkers, dogs and a fox pass in front of the camera trap, then the lynx. “I think they’ve already seen me several times,” says Sonia Saïd.
At the end of winter, when brambles and ivy run short, the roe deer attack the buds and compromise the oak’s growth.In spring, life explodes on every level, and death is on the prowl. Small wasps, the cynipids, cause galls to grow on the oak, a wound for the tree and a home for the insect. Caterpillars swoop down in their thousands; each leaf that is eaten alerts its neighbours, which load up with tannins. In Lausanne, the researcher Edward Farmer follows, on a plant made fluorescent, the electrical signals that run along the veins and trigger the defences: “most of the time, and the oak is a good example, it’s the trees that win”. In 56 million years, oaks have gone from one species to 435. But human upheavals move faster than the forest: towns, roads and crops break it up, and fields of trees are replanted where biodiversity is being eroded. On the Landes plateau, Sylvain Angerand, founder of the association Canopée, defends old pedunculate oaks that have become rare and shows a forest cleared illegally. In the plantations, not a single birdsong: “a wooded desert”. “A young stand of maritime pine is a box of matches.” The forest, he says, is at a crossroads: simplify the ecosystem, or protect what remains to be protected.
Globular outgrowths of plant tissue, galls are a kind of compromise between the tree and the insect, on the road to coexistence.
The electrical signals trigger the synthesis of a hormone, jasmonate; the insects, for their part, spare the veins and quickly change leaves so as not to raise the alarm.
According to Sylvain Angerand, 40% of plantations were failing in 2022, with 80% or more of their trees dead; mixed with oaks, stands hold up better.“It’s almost a pity that we can’t see everything that goes on in a forest, because if we could, we would be totally dazzled.”Edward Farmer, researcher
“L’arbre qui cache la forêt is fascinating. […] The series goes beyond the trees themselves and the forests it depicts, to remind us of the complexity and importance of these ecosystems.”Mònica Planas Callol, Ara (Barcelona)translated from Catalan · July 2026
“Sumptuous images, a dreamlike voice-over, a hypnotic soundtrack […]. Beneath the trees, we marvel at the intertwining of the infinitely small and the infinitely large. […] By celebrating the majesty of the setting, L’arbre qui cache la forêt urges us to preserve it.”Ouest-FranceJune 2024