Why Do Leaves Turn Red in Fall?

Before the leaf falls

Aging leaves can return nutrients to the plant.

Autumn leaf aging, called senescence, is a period in which the plant can recover nutrients from its leaves. A leaf may be approaching the end of its life while still participating in that recovery.

One explanation for red pigments is that they help shield the remaining photosynthetic machinery from excessive light during this period. Researchers call this photoprotection: protection from damage caused by light.

Testing a light screen

Red-osier dogwood supplied an early test.

In a 2001 study, Taylor Feild, David Lee and N. Michele Holbrook examined how red and yellow aging leaves of red-osier dogwood responded to light. Red leaves absorbed more light across part of the visible spectrum and recovered better after a high-light stress treatment.

The researchers also tested blue and red light separately. Their results supported a screening role for anthocyanins: the pigments intercepted wavelengths that would otherwise reach the photosynthetic tissues.

The sunscreen comparison describes how the pigments screened incoming light. In this experiment, researchers tested visible wavelengths, extending the idea beyond protection from ultraviolet radiation.

From light to nutrient recovery

A 2003 experiment tested plants with and without red pigment.

William Hoch and colleagues compared ordinary plants with anthocyanin-deficient forms of three woody species. Their results supported the idea that the pigments could protect nutrient recovery under stressful light conditions.

Paper birch also performed well in the comparison, recovering nutrients without those red pigments. Other protective strategies can help plants recover nutrients before leaf fall.

Testing the wider explanation

Results across species complicate the picture.

Researchers also ask how the light-screening effect relates to the evolution of autumn redness across different tree species.

A 2021 comparison of 55 tree species found greater nitrogen-recovery efficiency in species with yellow autumn leaves. Anthocyanin presence was not associated with more efficient recovery, a result that challenged the proposed link between light protection and the evolution of red autumn leaves. The authors also acknowledged limitations of the comparison.

In a 2023 study of Norway maple, researchers compared green, red and yellow leaf sections. Under the conditions tested, the rate of damage to photosystem II, part of the machinery that captures light energy, did not differ significantly between red and yellow aging sections.

A different possible function

Red leaves may also affect insects choosing a host.

Another hypothesis concerns insects that select trees in autumn. If leaf color provides information about how suitable a tree is as a host, it could affect where those insects settle.

In a 2009 apple-tree study, Marco Archetti found fewer rosy apple aphids on varieties with red autumn leaves. Aphids tested the following spring also performed worse on trees whose leaves had been red in autumn. Those results supported the proposed signaling explanation for the apple varieties studied.

To test whether red leaves serve as a warning signal, researchers also have to account for insects' existing responses to color. Those preferences could influence host choice alongside any information the leaf color conveys about the tree. The apple findings concern the varieties studied and a possible role in insect behavior, while the light-screening experiments examine how pigment affects the leaf itself.