Why do leaves change color in the fall?

Every year, from the first days of fall, the landscape is transformed. Forests turn shades of gold, orange and red before the leaves gradually drop to the ground. As beautiful as it is, this spectacle is no mere whim of nature: it is a remarkable survival strategy that trees use to face the winter.

Trees see winter coming

Unlike animals, trees cannot move to escape the cold. They have to adapt to the changing seasons.

As temperatures fall and the days grow shorter, trees sense these changes through the dwindling light and shifts in temperature. These signals trigger a whole series of biological reactions that prepare the tree for the cold season.

One of the main strategies is to cut down on exchanges with the outside world. Leaves, which make up the largest surface in contact with the air, are also responsible for significant water loss through evaporation. In winter, when the ground is frozen, roots have a much harder time absorbing water. Keeping their foliage would then become a real handicap.

Deciduous trees therefore opt for a radical solution: shedding their leaves until spring returns.

Ethylene, the messenger of fall

This transformation is orchestrated by a plant hormone called ethylene.

As fall conditions set in, trees gradually step up production of this molecule, which acts as a true signal to go dormant.

Under its influence, a separation layer, often likened to a tiny cork plug, forms at the base of the petiole, the point where the leaf is attached to the branch.

Little by little, this layer blocks the flow of sap between the branch and the leaf. Deprived of water and nutrients, the leaf then enters a phase called senescence, a natural, programmed aging process.

Before letting the leaf go, however, the tree recovers much of the precious elements it contains, including nitrogen, phosphorus and some sugars, which are stored in the trunk and roots to be reused the following spring.

Why do leaves turn yellow, red or orange?

The green color of leaves comes from chlorophyll, the pigment essential to photosynthesis.

Throughout the growing season, chlorophyll is produced continuously. Its high concentration masks the other pigments naturally present in the leaf’s cells.

When senescence begins, chlorophyll production gradually stops and the existing molecules break down.

The green then slowly fades, revealing other pigments that had been invisible until then.

Carotenoids, present in leaves all year round, bring out yellow and orange hues. These are the same pigments found in carrots, squash and some varieties of corn.

Anthocyanins, on the other hand, are often produced during the fall. They create shades ranging from bright red to deep purple, like those seen on maples, grapevines and some oaks.

Why don’t all trees turn the same colors?

Each species has its own fall palette.

Birches and poplars usually turn golden yellow, while beeches take on coppery tones. Maples are famous for the fiery reds that have made Canada’s forests so renowned.

The weather also plays a key role.

Sunny days followed by cool nights boost anthocyanin production and intensify the reds. A mild, rainy or very cloudy fall, on the other hand, will often produce more muted colors.

That is why fall landscapes are never exactly the same from one year to the next.

Why do leaves eventually turn brown?

As the leaf ages, all its pigments eventually break down.

All that remains are the compounds in the cell walls and dead tissue, which give leaves their characteristic brown color.

The separation layer at the base of the petiole is now fully formed.

A simple gust of wind or the weight of the rain is then enough for the leaf to break off for good and fall to the ground.

A cycle essential to the life of the forest

Once on the ground, leaves do not become waste.

Broken down by fungi, bacteria, insects and earthworms, they gradually enrich the soil with humus, a fertile, nutrient-rich layer.

These nutrients will then be absorbed by the trees’ roots and help new leaves grow in the spring.

So the magnificent colors of fall are not just a feast for the eyes. They mark the beginning of a vast natural recycling cycle, essential to the health of forests and to the renewal of plant life year after year.