Why Leaves Turn Brilliant Colors in Autumn — The Science and the Mysteries

What causes the autumn color show — and does it have a purpose?

Every fall many of us pause to admire the reds, oranges and golds that blaze across temperate landscapes. Scientists can explain much of the chemistry and physiology behind those colors, but whether the colors evolved for a specific function remains debated.

How the colors appear

During the growing season leaves are packed with chlorophyll, the green pigment that captures light energy for photosynthesis. When days shorten and temperatures cool in autumn, chlorophyll production slows and eventually stops. As the dominant green pigment fades, other pigments already present in the leaf become visible:

• Carotenoids — yellow and orange pigments that are present all summer but hidden by chlorophyll.
• Anthocyanins — red and purple pigments that are often synthesized in autumn from sugars that accumulate in the leaf.

Two leading hypotheses about purpose

Researchers have proposed several adaptive explanations for why some trees produce anthocyanins in autumn. Two of the most discussed are:

Photoprotection

The photoprotection hypothesis suggests anthocyanins absorb excess light and protect leaf cells as chlorophyll declines. That protection could, in theory, allow the leaf to stay functional longer so the tree can recover more nutrients before the leaf falls.

Anti‑herbivore signaling

Another idea is that bright red coloration serves as a warning or deterrent to insects and other leaf feeders. Some experiments find fewer aphids or other pests on red leaves, suggesting a potential signaling role.

Why the explanations are complicated

A key complication in evaluating these hypotheses is timing. Early in leaf senescence a corky abscission layer forms at the leaf base, progressively isolating the leaf from the branch and limiting export of sugars and other compounds. Anthocyanin production is often triggered by a buildup of sugars that occurs once export is curtailed. That means many red leaves are already physiologically disconnected from the tree when anthocyanins appear.

Because of that sequence, both major hypotheses face challenges: if the leaf’s vascular connection is already blocked, anthocyanins can’t extend nutrient retrieval, and reduced insect feeding on red leaves may simply reflect reduced sap flow rather than a warning signal.

Mixed evidence and open questions

Studies across species, climates and methods produce mixed results. In some contexts anthocyanins appear to provide measurable photoprotection or influence herbivory; in others they do not. The variation suggests there may be multiple, context-dependent reasons for autumn pigmentation — or that in some cases pigmentation is a byproduct of other physiological processes rather than a direct adaptation.

Sometimes the simplest response is appropriate: the colors are beautiful and biologically interesting, even if a single universal purpose remains elusive.

Paul Cappiello is executive director at Yew Dell Botanical Gardens.

Originally appeared in the Louisville Courier Journal.