Deciduous Dormancy: Why Trees Drop Their Leaves

By Anouk Beaumont · October 11, 2026

The Trigger: Shorter Days

While cool temperatures accelerate the process, the primary signal for dormancy is photoperiod — decreasing day length. Trees sense shorter days through photoreceptors (phytochromes) in their leaves and buds, initiating a cascade of hormonal changes weeks before visible color change.

Chlorophyll Breakdown and Color Revelation

Chlorophyll (green pigment) is continuously produced during the growing season. As production slows in fall, existing chlorophyll degrades, revealing pigments that were present all along:

Carotenoids (yellow, orange): Present year-round, masked by chlorophyll
Anthocyanins (red, purple): Newly synthesized in fall from trapped sugars in some species
Tannins (brown): Waste products that remain after all other pigments degrade

The Abscission Zone

A specialized layer of cells (abscission zone) forms at the base of each leaf stem. These cells produce enzymes that dissolve the cell walls connecting leaf to branch. Once fully separated, a protective cork layer seals the wound to prevent water loss and pathogen entry.

Why Drop Leaves at All

Leaves are vulnerable to freezing damage and desiccation. By dropping them, the tree eliminates its most water-losing surfaces and protects the valuable vascular tissue in branches. The tree recovers some nutrients (nitrogen, phosphorus) from leaves before they fall — the brown color of fully senesced leaves indicates most nutrients have been reclaimed.

Deciduous Dormancy: Why Trees Drop Their Leaves

The Trigger: Shorter Days

Chlorophyll Breakdown and Color Revelation

The Abscission Zone

Why Drop Leaves at All

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