As daylight wanes and the hours under the stars stretch longer, Mother Nature colors the landscape with an unbe-leaf-able autumn palette.

Those jewel-toned pigments of rubies and purples, oranges and golds, are the result of a chemical process trees and their leaves undergo each year to prepare for winter.

Trees in Western Washington this week are displaying their peak fall colors, according to Explore Fall's foliage map. Ryan Garrison, a plant health specialist at the University of Washington's Botanic Gardens, sat down with The Seattle Times to discuss the annual autumn changes.

Here's the science behind fall colors, according to Garrison and the U.S. Forest Service.

What happens to trees and their leaves during the fall?

In the spring and summer, leaves soak up energy from the sun to produce food necessary for a tree's growth.

When fall arrives in the northern hemisphere, the drop in daylight signals to trees that it is time to stop growing and prepare to weather the winter.

Branches, stems, twigs and buds are equipped to survive winter and reawaken in the spring. But leaves are tender. To protect themselves against harsh winter weather, plants must toughen up their leaves or get rid of them.

In Western Washington, we're no stranger to coniferous trees, or evergreens, that keep their green pigment year-round (hence the name evergreen). A heavy wax coating protects their needle-like leaves during the fall, and a fluid within them helps resist freezing, allowing chlorophyll to stick around through the winter.

But deciduous trees with leaves -- usually broad and thin -- must shed their leaves to conserve energy and water to survive until spring, according to Pacific Science Center.

So, during the fall, trees begin to build a protective seal between each leaf and its connecting branch. This seal gradually severs the veins that carry nutrients from the leaves all the way down to a plant's roots. Nutrients like sugar become trapped in the leaves, which helps produce some of the pigments we see during fall, Garrison said (we'll get to that later).

Once a leaf is fully sealed off, it falls, ensuring the plant's survival until spring.

What causes different colors of leaves?

Chlorophyll is the chemical in plants responsible for converting light energy into nutrients, a process otherwise known as photosynthesis.

During the growing season, chlorophyll is abundant in leaves, giving them their signature green color.

As chlorophyll breaks down during the fall, the leaves unveil the pigments hiding underneath. If chlorophyll wasn't present in leaves at all, trees would show their autumn hues all year long, Garrison said.

Carotenoids produce yellow, orange and brown hues and are the same pigment that makes carrots orange, Garrison said. They're present in leaves all year long, just hiding beneath the chlorophyll.

Anthocyanins, the pigment that displays red and purple hues, are only produced in the fall as a result of the excess sugar trapped in leaves. Anthocyanins are also found in red and purple berries like cranberries and strawberries.

What trees display each color?

During the fall, oaks and dogwoods are likely to produce red colors, which can range from purplish to brown hues. Sourwood trees, which Garrison recommends Washingtonians see during the fall, also turn deep crimson.

Maple trees display different colors depending on the species. Red maples turn scarlet during the fall, while sugar maples turn orange. Black maples display yellow hues.

Hickory, aspen and larches turn golden.

Most of the trees native to Washington do not produce a lot of reds or oranges, Garrison said, so you'll likely spot a lot of yellow.

This time of year, evergreens shed their inner foliage, which you might notice turning brown before dropping to the ground.

What impacts color intensity?

The intensity of autumn's palette is dependent on the weather.

A period of "warm, sunny days and cool, but not freezing, temperatures at night generally produces the best colors," Garrison said.

The sunshine boosts the production of sugars, which get trapped in the leaves as the connective veins shrink. More sugar means more anthocyanin production, which means more bright red colors.

Since carotenoids are always present in leaves, the intensity of yellow hues remains fairly constant year to year.

Hard freezes "accelerate everything," weakening colors and causing leaves to drop quickly, Garrison said.

Fallen leaves, which soon turn to rotting leaves on the ground, are important sources of carbon and nutrients to be absorbed back into plants, Garrison said.

On the forest floor, decomposing leaves also become part of a spongy layer that absorbs and holds rainfall.