Forests stay mostly green through autumn and winter in the Pacific Northwest, but the leaves of a few trees and shrubs in this region lose their chlorophyll to reveal fiery colors, which beautifully contrast with the emerald background.
Why do leaves change color in autumn?
North America lies within the latitudes of the north temperate zone, where, in general, cool winters are followed by warm summers. In summer, plants that live in the temperate zone get plenty of sunlight and are not at risk of being damaged by freezing temperatures. Winters, on the other hand, can be relatively dark and cold. Different groups of plants have evolved unique strategies for surviving harsh winter conditions.
Some plants simply produce seeds and then die before winter arrives. Others, like the cone-bearing pines and firs, have small, waxy leaves– what we usually call needles– that are highly resistant to freezing and shed snow easily. These plants keep their leaves for several years and can still use the dim winter sunlight to drive photosynthesis. Many broad-leaved trees and shrubs get rid of their foliage all together and then go dormant for the winter. Large leaves containing lots of water are great for capturing sunlight and for high-output photosynthesis, but are very vulnerable to freezing and drying. Deciduous plants (the ones that lose their foliage seasonally) are vigorous and productive in the summer, but must hunker down in their bare branches for the winter, quietly awaiting the return of warmth and sunlight. The changing colors of leaves that we see on deciduous plants are the result of chemical processes that happen before the leaves die and fall off. More about that below.
How do leaves change color?
Even though deciduousness is an adaptation to freezing temperatures, temperature isn’t the most important cue used by plants to begin their preparations for winter. Instead, most deciduous plants that lose their leaves in autumn are responding to the length of nighttime darkness. The length of night increases through autumn in the north temperate zone and is much more reliable than temperature as a cue that winter is on the way.
So what about the color? Chlorophyll is the chemical in leaves that is largely responsible for capturing the energy from sunlight and converting it into chemical energy in the form of sugars. This is the process of photosynthesis. Chlorophyll absorbs certain wavelengths of light and reflects others, especially green. In the growing season when temperatures are warm and the sun is bright, leaves produce large amounts of chlorophyll to keep up high rates of photosynthesis. As autumn and winter approach, the leaves stop producing chlorophyll and their sugars are sucked down into the branches and roots. As the green chlorophyll breaks down and disappears, other colors are revealed: orange, red, yellow, brown, and purple. This is also when we humans start to say, “Ooooh…” and, “Wow!” while walking around in the forest.
The brilliant colors that we see in autumn come from other chemical pigments in the leaves that are normally masked by millions of green chlorophyll molecules. Carotenoids are pigments that help in photosynthesis and give leaves yellow and orange tones. Anthocyanins are red, purple, and blue pigments that are produced by leaves after about half the chlorophyll has been broken down. Eventually, the leaves are depleted of their sugars, their water and food supplies are pinched off where they attach to the branch, and they fall off. The nutrients in the leaves are recycled as they decay on the forest floor.
Which trees and shrubs change color in the Pacific Northwest?
Washington is the “Evergreen State” and, really, so is Oregon. The landscapes of these states are dominated by coniferous trees that do not drop their leaves all at once in the autumn. Green is a year-round experience here. However, there are a handful of deciduous tree and shrub species that radiate beautiful orange, yellow, and red colors in October and November. These colors are particularly striking because they glow against a contrasting, dark evergreen backdrop.
Vine maple (Acer circinatum)
The most vibrant fall foliage in the Pacific Northwest is that of vine maple, which grows in the understory beneath Douglas-fir and western hemlock, from sea level to about 3,000 feet. Vine maple is often a brilliant vermilion red in October.
Huckleberry (Vaccinium species)
An assortment of huckleberry species live in the mountains of the Pacific Northwest. Large swaths of these shrubs blanket many mountain basins and provide a spectacular show as their leaves turn bright orange or dark red in autumn.
- Big-leaf maple (Acer macrophyllum)
- Oregon ash (Fraxinus latifolia)
- Cottonwood (Populus trichocarpa)
- Quaking aspen (Populus tremuloides)
- Willows (Salix species)
- Western larch (Larix occidentalis)
Places to go in the Pacific Northwest to see showy fall foliage
- Cascade Range: Mount Baker-Snoqualmie National Forest, Highways 542 and 20.
- Cascade Range: Wenatchee National Forest, Highway 2.
- Cascade Range: Gifford Pinchot National Forest.
- Mount Saint Helens National Volcanic Monument.
- Columbia Gorge, Highway 14.
- List of Fall Color Hikes from Washington Trails Association.
- Columbia Gorge, Highway 84.
- Cascade Range: McKenzie Pass-Santiam Pass Scenic Byway, Highway 242.
- Cascade Range: McKenzie River, Highway 126.
- Oregon Fall Foliage blog.