Shorter days and chillier temperatures have ushered in the first of our fall foliage. Along the trails the hobblebush has begun to don a purplish hue. Red maple trees blush in the sunnier spots along roads and in clearings. Soon enough the beech leaves will add bronze to the color palette and birch will highlight the forest with a luminescent yellow.
Why all the vivid colors? In the summer most leaves appear green, due to the green pigment chlorophyll. All throughout the summer these dark, green chlorophyll pigments cover up other pigments in the leaves- such as the orange carotenoids and the yellow xanthophyll pigments. However, shorter days and cooler weather indicate the onslaught of winter and signal the trees to stop producing chlorophyll. As old chlorophyll is used up, no replacements arrive to fill in. Slowly, the green color fades allowing the other pigments to shine through.
So, the yellows and oranges are there the whole time, just waiting for their time to shine. But why? Just as chlorphyll helps the tree to make sugar, so do the other pigments have a role in the trees success. The job of the carotenoids and xanthophylls is to assist the chlorophyll in harvesting light during photosynthesis. They are able to absorb a wider range of wavelengths of light than chlorophyll alone, which enables the plant to capture more energy. They also serve as antioxidants, protecting the chloroplasts from damage from excess sunlight.
The bright red color, like that displayed by red maple trees, can be attributed to a third pigment- anthocyanin. Anthocyanin is found in the sap of cells. Sugar is created in the leaf during the day, but 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 formed. Scientists have not pinned down how anthocyanins contribute to the success of a plant. Some findings suggest, however, that they may help protect leaves either as a sunblock or as a way to convert UV radiation into heat. What they do know is that bright, cool days and chilly, but not freezing, nights cause the brightest colorations.
Whatever the reasons, I'm inclined to just sit back and enjoy the show.