Why Do Tree Leaves Change Colors in the Fall?

AutumnThe red, gold and purple tones of autumn leaves provide a once-a-year bounty for the eyes. But this phenomenon represents an interesting biological phenomenon, that provides lessons into tree biology, the interconnection between weather and trees and the ecological interactions between trees and their predators.

Unfortunately, the fall color here in the west is not as glorious as that in New England or along the east coast, but there is still plenty of gorgeous foliage to be seen. Aspens turn bright gold, a variety of shrubs and smaller trees produce oranges and other colors, and many of trees of eastern origin produce the prototypical fall fireworks you’d expect elsewhere.

And yes, before anyone asks, you can find pretty awesome fall color in California.

The Basic Process

Trees produce three basic pigments in their leaves:

  • Chlorophyll is a green pigment that is involved in the process of photosynthesis. It is continually created, used and degraded through the growing season, which causes the leaves to appear green.

 

  • Carotenoids are yellow to orange pigments that are also found inside leaves for much of the year. However, the chlorophyll generally overpowers these pigments, and mask their presence.

 

  • Anthocyanins are reddish pigments, which are typically not produced in the leaves until the fall.

Whereas chlorophyll and carotenoids are produced in small structures called chloroplasts, anthocyanins flow through the watery material inside the leaf’s cells.

Deciduous trees become dormant in the winter, which alleviates their need to manufacture food (photosynthesis). In fact, because water is often scarce during the winter, dormant trees tend to shed their leaves. This reduces the chances that the water evaporating from the leaves will exceed that which can be drawn from the ground.

So, as the days become shorter and the nights become longer, deciduous trees cease producing chlorophyll. As the chlorophyll breaks down, the underlying carotenoids (which have always been present) and the anthocyanins (which are produced around the same time that chlorophyll production ceases), become visible.

This yields the purple, yellow, red or orange leaves that characterize the season.

A short time later, as the tree prepares for its winter slumber, the leaves form something called an abscission zone at the base of the petiole (leaf stalk). This zone effectively blocks the transport of resources between the tree and the leaf, and eventually serves as an easy point of detachment. Once the abscission zone is complete, the leaf is normally blown off the tree a short while later.

Conditions that Improve Fall Color

You’ve probably noticed that fall color intensity varies from one year to the next, but it also varies from one location to the next – even when all other factors are the same.

But if you look carefully, you’ll notice that the yellow, gold and orange colors tend not to fluctuate from one year to the next; it is the red, maroon and purple colors that are dazzling one year and mediocre the next. Part of that is because the carotenoids which yield the yellows and oranges are present all year long and in similar quantities each year. But anthocyanins are only produced during a relatively small window of time, and the amount produced varies from one year to the next.

It turns out that weather is the primary factor that leads to the differing amounts of anthocyanins. When the days are sunny, the nights are cool and the weather dry, fall color is best. This collection of conditions helps encourage the production and retention of sugars in the leaves, which promotes the production of anthocyanins.

However, a number of other weather conditions can dampen the fall color for a given year. Summer droughts typically reduce the vibrancy of the fall colors, as do warm or wet autumns.