The days cool and shorten. The smell of fall is in the air. A bright sun starts many changes in the landscape. One change is the color within tree leaves.

The color of summer leaves is the green of chlorophyll. As chlorophyll fades in fall, other pigments are revealed or produced.

As different pigments are fading, being produced or changing inside leaves, dynamic color changes become visible. Together, the various color agents can yield almost infinite combinations of leaf colors.

Photo by J. Cannon, UGA College of Agricultural and Environmental Sciences -- email for high-res photo

COOL SHADES of fall actually protect the aging leaves on trees, somthing like sunglasses do. Anthcyanins block out blue light, allowing the leaf to continue producing nitrogen, sulfur and phosphorus until it finally dies.

In the fall we can most easily see one special type of pigment. These pigments help protect trees from sunlight. Trees have a number of other pigments, but this type is unique. Its role in leaves is to block ultraviolet light and shade the living cells.

Trees use these blue-blocking shades to protect fast-aging leaves. The longer the declining leaves survive, the more growth materials can be recovered before they fall. These UV-blocking pigments, called anthocyanins, are the tree's sunglasses.

Anthocyanins are dissolved in the leaf cell water. These watercolor pigments produce a myriad of hues ranging across reds, pinks, purples and blues. They are the pigments found in bronzed or dark-leaved trees. Besides fall colors, anthocyanins also color some tree flowers, fruits and new tissues.

These pigments make cherries, cranberries, apples and beets reddish while making grapes, blueberries and plums blue. The more common forms of anthocyanins yield orange-red, purple-red, bluish-purple, rosy-red and a host of purple colors.

Anthocyanins are actually a large group of different-colored pigments that change as the leaf cell environment changes. Each pigment does not have a specific color. It ranges widely in appearance based on leaf conditions.

Dissolved in the leaf cell water, anthocyanins are always changing the reflected light from leaves. They're not stable in the cells and must be continually manufactured by the leaf.

As the inside of leaf cells becomes more acidic with age, the anthocyanins become more red. More basic or higher pHs, as in early fall cells, generate more blue colors.

The amount and form of iron and aluminum in leaf cells also change anthocyanin colors. As chlorophyll fades, bright sunlight stimulates anthocyanin production.

It is not only the failing leaves of fall that benefit from anthocyanins. Many young tree tissues are protected with anthocyanins until they have a full complement of pigments and can properly function. The colored blush of new spring growth in many trees is from anthocyanin pigments.

Anthocyanins are welded onto sugar molecules within each cell. Sugar is required for anthocyanin production and presentation. In the fall, as leaf sugars are mobilized for shipment out of the leaf, low temperatures and poor transport slow the movement of the sugar. The result is leaf sugar enrichment and anthocyanin production.

The tree is trying to remove valuable nitrogen, sulfur and phosphorus from the leaves, too, before they fall. As the leaves begin to have too little of these elements, they generate protective anthocyanins to shelter a failing chlorophyll system.

Fall colors herald the coming winter and the following spring. The trees produce these pigments for other valuable jobs. We get to see great colors as a by-product of a leaf's decline. Go out and enjoy the cool colors the fall sun has made.