The many hues of autumn leaves are determined by light, temperature, and chemistry.
Window On Nature
By Lowell and Kaye Christie, F47246
September, October, or November? Exactly when it happens depends on your location, but if you’re in the Northern Hemisphere, you soon will experience the most colorful of seasons. Winter combines white snow with bare bark on much of the continent, as a season of grays on white. Spring is the green season, with grass sprouting from barren ground and ever-changing shades of verdant foliage appearing. Summer heat mutes these colors and the natural world slows down, seeking shade from the overpowering sun.
But then, with the arrival of cooler weather, brilliant colors in massive displays remind photographers that even expensive camera equipment really can’t capture the feeling of fall, as leaves begin to drop and the cycle repeats.
Each year at this time we feel an almost gravitational pull toward New England, where we know the colors will be at their brightest. Because of a specific combination of temperature, light, and vegetation that occurs nowhere else, these eastern states have unmatched displays of fall foliage.
Everyone knows that most leaves are green, and the reason is chlorophyll, a green pigment vital for photosynthesis. During that process, sunlight destroys the chlorophyll, so the plant must continuously produce more, which requires a constant supply of water.
As deciduous trees prepare for winter, they shut down the water transportation system to their leaves and form a barrier at the base of the leaf stems. During this process, the tree actually withdraws a large portion of the leaves’ nitrogen back into its woody tissue, where it will act as antifreeze during the coming winter months.
With the decrease in water to the leaf, production of chlorophyll ceases, and the greens of spring and summer are replaced by the yellows and oranges of autumn. The yellow color is caused by the pigment xanthophyll, the same pigment that creates yellow in an egg yolk. Orange comes from the pigment carotene, which is also found in carrots. Either or both of these are present in the leaves of most plants, but normally hidden by the stronger green color of chlorophyll. As the chlorophyll is gradually depleted, the leaf usually takes on a yellow hue. The color moves progressively from the leaf margins and on to the leaf veins, which are the last to turn from green to yellow.
In western North America, this leads to mountains of gold as fields of aspen change their colors. But western foliage can’t compare with that in the multicolored East, so there must be more to the story.
While autumn yellows and oranges appear in foliage by the subtraction of green, the reds and purples and blues are something new added to the leaf. These more vivid colors are created by a special water-soluble pigment called anthocyanin, carried in the tree sap. This is the same pigment that produces most of the blues and reds in spring wildflowers.
The amount of acid in the actual tree sap controls the color produced by the anthocyanin. Perhaps you’re familiar with the common backyard plant, the hydrangea. Depending upon the acidity of the soil, hydrangea flowers can be blue, red, pink, or purple. By adjusting the pH of the soil, you actually can alter next year’s color. In the same way, if the leaf sap of a tree is acidic, the autumn leaf will turn red. If the sap is alkaline, the leaf will turn blue or purple. But to get the best, most vivid colors, the amount of sunlight is also critical.
The amount of anthocyanin that is created in leaves depends upon the amount of sunlight they receive. If September and October are mostly cloudy, very little of the pigment is produced, and though the leaves will still turn color, they tend to go from green to yellow. Fortunately, the New England states usually have sunny days and cool nights, increasing the amount of anthocyanin in the leaves. That, in turn, leads to brilliant reds, oranges, and purples.
There’s one more factor that makes New England the place to be in the fall: the presence of the sugar maple tree. If you’ve ever eaten maple syrup, you know the sap from a sugar maple is exceedingly sweet. And in order for an autumn leaf to have the brightest and most vibrant shades of red, it must have a high concentration of sugar.
During the summer, the maple tree’s leaves create extra sugar, which is transported to the tree’s root area for use the following spring. But as the tree cuts off the flow of liquids through the leaf stem, sugar is trapped in the leaves at the same time that the chlorophyll-producing areas are no longer receiving water. Low temperatures accelerate this process, so New England’s autumn nights with temperatures consistently below 45 degrees produce the perfect environment for exceptional fall colors.
Sugar maple leaves, unlike many other trees, contain all three of the major color-producing pigments “” xanthophyll, carotene, and anthocyanin. Because of the effect of sunlight, maple leaves that are in the shade tend to be yellow. Those in the direct sunlight will be red. As you watch a tree during the peak of the foliage season, you can see the red color move downward from the top of the tree and inward from the edges. And the more sugar that has been trapped in the leaves, the more brilliant the reds.
We’ve always spent a lot of time in autumn watching the fall colors, but since we grew up in the West, we assumed that the beautiful pictures taken in the East were a good representation of what fall foliage was like. Then, in our first year of full-time RV travel, we spent a month in autumn based in Jaffrey, New Hampshire.
You can’t capture it adequately on film. Every day we would walk and drive through the changing scenery. That first time happened to be an exceptional year, but we’ve returned often. Even after a cloudy season when the colors are less brilliant, it’s been well worth the trip.
Watch the trees for the next few months. This time of year, they are spectacular almost everywhere. But, at least once, make a trip to New England for the fall foliage. You’ll never forget it.