Have you ever wondered why the leaves on trees take on all sorts of colors in the autumn before they fall?
This will possibly surprise you…but we don’t really know the answer! Probably a dozen or so hypotheses have been advanced to explain the phenomenon, but two of them especially stand out and are actively debated from the scientific point of view: photoprotection and coevolution.
Before we explain these theories, it has to be remembered that autumn colors are caused by pigments present in the leaves. Carotenoids are responsible for the yellow to orange colors, while anthocyanins are responsible for the reds. These two types of pigments, however, have different behaviors. The yellow pigments, which are present year round, are masked by abundant green chlorophyll in the summer and revealed to our eyes once fall comes and the chlorophyll breaks down. The red pigments, meanwhile, are produced in the fall, just before the loss of leaves.
An anti-sun pigment
The photoprotection theory maintains that red and yellow pigments protect leaves from the harmful effects of the sun when temperatures are cooler by acting as a sunscreen. That protection would allow for a better recycling and storage of nutrients before the loss of leaves.
A warning signal
On the other side, the coevolution thesis suggests that red comprises a warning signal for insects such as aphids. These migrate to trees in the fall to lay their eggs before winter. The red color would indicate to the insects that the tree is not an appropriate host, because of a large amount of defense molecules, for example. Those trees, now grown unattractive, would suffer less damage by these herbivores once spring comes.
Despite a considerable number of studies, no hypothesis emerges as the winner. If the protective effect of red and yellow pigment has been demonstrated, we don’t know the real advantage they provide to the leaves. And the research on the capacity of red pigments to repel insects suggests that it doesn’t work all the time. In a word, to this day a certain mystery persists in the matter of fall colors. But not knowing the answer doesn’t stop us from fully enjoying this fabulous natural phenomenon!
To learn more:
- Archetti, M., et al. 2009. Unravelling the evolution of autumn colours: an interdisciplinary approach. Trends in Ecology & Evolution 24: 166–173.
- Pena-Novas, I., M. Archetti. 2020. Biogeography and evidence for adaptive explanations of autumn colors. New Phytologist 228: 809–813.
- Renner, S. S., C. M. Zohner. 2019. The occurrence of red and yellow autumn leaves explained by regional differences in insolation and temperature. New Phytologist 224: 1464–1471.