Why Trees Change Colors

As New England's foliage season begins, we are often asked why trees change color.

It was once thought that autumn colors were simply a byproduct of dying leaves. In other words, fall colors were a tree's gray hair.

Modern research suggests that fall colors may have a larger role in the life of trees. It may be that brilliant autumn colors may constitute a visual warning to insects.

In the fall, certain insects choose trees on which to lay their eggs. In the spring, the eggs hatch and the juvenile insects feast on the host tree. Could it be that the production of bright colors is meant to announce the tree's lethality to insects? Through natural selection, insects may have come to avoid these colored leaves as trees evolved to produce even brighter foliage.

This 'leaf-signal' hypothesis proved so provocative that multiple worldwide studies were conducted. Early results found that trees with strong fall colors tend to suffer less insect damage.

Other research argues a different explanation: that changing colors serve as a leaf sunscreen. The interior of an autumn leaf is a frenzy of activity as the chlorophyll and other molecules necessary for photosynthesis are broken down. Therefore, the leaves are less efficient. Autumn leaves cannot capture all the sunlight hitting them and the excess can cause damage. Red and orange colors, produced by pigments called anthocyanins, appear to protect the leaves by blocking sunlight.

Some trees, like birches, produce no anthocyanins. Their yellow leaves are produced by a class of pigments called carotenoids which help chlorophyll absorb sunlight but do not shield the leaves in the fall.

The ongoing debate is more proof of the complexity of tree physiology.