The entire landscape shouts of passing time…every turning leaf and bare branch maps the progress of a season and the preparations of whole communities of unseen organisms for a winter many do not know. ~Peter Marchand
As we look across the forest, the greens are slowly giving way to the yellows and oranges, but it is the occasional red tree that really catches our attention. The yellow xanthophylls and orange carotenoids are accessory pigments harvesting a wider range of wavelengths of light than chlorophyll alone. They also serve as antioxidants protecting the chlorophyll from excess sunlight. So as the fragile green chlorophyll breaks down we see the background pigments that are already present in the leaf. But that still doesn’t answer where the red comes from.
nthocyanins are responsible for the reds, pinks and purples. Anthocyanin is yet another pigment found the leaf. It is colorless until a sugar molecule attaches to its structure. In the fall, sugar created in the leaf during the day through the process of photosynthesis, cannot escape from the leaf at night because chilly temperatures slow down the movement of sap. From the sugars trapped in the leaves, anthocyanin is transformed into brilliant flaming reds or striking purplish hues. We are seeing this transformation in the hobblebush at lower elevations and in the blueberries at higher elevations.
If the leaf contains carotene, as do the leaves of birch, it will change from green to bright yellow as the chlorophyll disappears. In some trees, as the concentration of sugar in the leaf increases, the sugar reacts to form anthocyanins. These pigments cause the yellowing leaves to turn red. Red maples and sumacs produce anthocyanins in abundance and display the brightest reds and purples in the autumn landscape.
The range and intensity of autumn colors is greatly influenced by the weather. Low temperatures destroy chlorophyll, and if they stay above freezing, promote the formation of anthocyanins. Bright sunshine also destroys chlorophyll and enhances anthocyanin production. Dry weather, by increasing sugar concentration in sap, also increases the amount of anthocyanin.
So the brightest autumn colors are produced when dry, sunny days are followed by cool, dry nights. If the first few days of September are any indication, we are on the way to a spectacular transformation as we march into the changing season.
If you want to see some early fall colors, venture to the higher elevations where the tundra landscape is beginning to transform. A fall visit to the higher huts is a perfect opportunity to experience the tundra in all its glory- “where every leaf is a flower”. Of course-be prepared for changing weather conditions. And if you are out hiking be sure to participate in AMC's Mountain Watch fall foliage survey
Photo credits: Mark Dindorf - Speckled Mtn 9/5/07, Nancy Ritger - Square Ledge 9/4/07, Mark Dindorf - Blueberry Ridge Trail 9/5/07, Mark Dindorf - Franconia Ridge 8/31/07.