As Earth continues its journey around the Sun, the chill of northern climes creeps southward.

The air sweeps over us in waves, no longer from the summer Gulf, but from the center of the continent. Cool winds drive the rains. Your senses can see, smell, hear, taste and feel the coming of winter.

Trees sense winter's arrival, too.

For trees, autumn is a time long expected and prepared for. From the moment trees began to expand their leaves and make food in the spring sunlight, a biological timer has been running.

This timer is composed of sensors in buds, leaves and twigs that measure light and dark, day length and other environmental clues. The tree's genetic materials help define when and how a tree will begin to shut down in fall.

For many people, fall is a time for harvest celebrations and winter preparations. So it is with trees.

Trees stay on their ageless cycle. Summer prepares them for fall, and fall readies them for winter. Winter's resting time prepares trees for spring, and spring unfolds the biological glory of summer.

Fall is a time of reorganization. All of nature is making changes, preparing for the coming winter.

In trees, the topmost buds have sent a biological message that signals senescence. That's a big word for the ordered shutting-down of summer growth and conservation of valuable resources. Senescence brings the fall colors and leads to winter survival.

From soon-to-be-dead leaves, the tree withdraws many materials it has made or collected. It leaves waste materials behind.

The last bit of tree food is stockpiled in the living cells of the outer few annual growth rings. Twigs, branches and roots become the collection sites and warehouses of materials needed for another season to come.

Within the tree, biological doors and windows are being closed, locked and weatherproofed.

People can enjoy the colorful rites of autumn in the forest, across the landscape and with individual trees. The process of senescence presents amazing colors.

Some of the fall colors, such as the yellows and oranges, have just been revealed after having spent the summer wearing green cloaks. Other colors, such as some of the purples and reds, have been made just for autumn.

The pallet of colors in leaves is varied and rich. Individual leaves actually can shift and change colors over the senescence period.

Internal phosphorus and water contents, and external temperature and sunlight, help blend and mix leaf colors. From the first pale yellows of early fall through the bright oranges of October to the deep burgundies coloring the last oak leaf, autumn paints a feast for the eye.

The scene changes daily and accelerates as winter comes closer. The colors break over us in three waves. The first is composed of yellows and yellow-browns. Golden colors begin the second wave, which ends with oranges and light reds. The third wave is made of purples and dark reds.

These color waves sweep down from the higher elevations and farther north. Satellite images show the moving color fronts. A short ride northward, and up in elevation, is like taking a time machine farther into the fall.

We live among great forests and stands of trees. Autumn is a time to fully appreciate the values they bring. The trees' biological preparations are colorful warnings of the winter to come.