It’s almost that time of year again. With the days getting colder as well as shorter, it’s only a matter of time until we find ourselves in the middle of fall. With the pumpkin spice lattes at Starbucks to the homecoming parade, cozy sweaters, and Halloween, it’s hard not to love the season. But something I’ve noticed that makes it especially magnificent here in State College is the changing of the leaves. With the hills, mountains and abundance of different trees it’s a sight to see when you look out amongst thousands of red, orange, and golden leaves. But what causes this beautiful and dramatic change?
Throughout most of the year, the trees as you might notice are either leafless or their green. Well this green color can be attributed to a large amount of chlorophyll within the lead. Although there are other pigmented colors within active leaves there is so much chlorophyll that it usually masks the other colors. What changes in fall, is the amount of light which is the factor that regulates chlorophyll production. So as the days grow shorter there is also less light which concludes in there being less chlorophyll produced. At this time, the decomposition rate of chlorophyll continues to remain constant causing the green color to begin to fade gradually from the leaves.
While this is all happening, the production of other sugar concentrations are responsible for the other colors being exposed. Anthocyanin pigments are responsible for turning the leaves that have an exceptionally high concentration of this pigment in them red. Another type of pigment that has some effect on the leaves are carotenoids. This pigment’s production does not depend on light, so the levels aren’t going to go down by the shortening of the days. Carotenoids can be orange, yellow, or red but the primary color that they turn is usually yellow. If the leaves have a good amount of anthocyanins and carotenoids, obviously that mixes red and yellow so that the leaves turn out to be some shade of orange. Similarly, leaves that contain mostly carotenoids but little/no anthocyanin is likely to be yellow. If neither of these pigments are in the leaf then there are also other things that can affect the color of the leaf such as tannins which can be recognized for being responsible for the brown color of some oak leaves.
From one source that I look at they suggested that temperature, light and water supply all have an influence of how long the leaves stay the colors that they are and how bright those colors are while they last. For example low temperatures are favorable for anthocyanin so that will richen the reds that we see. In constant, early frost will weaken these colors. According to chemistry.about.com “sunny autumn days are needed for the brightest color displays, since anthocyanins require light. Overcast days with lead to more yellows and browns.”
http://www.esf.edu/pubprog/brochure/leaves/leaves.htm
http://chemistry.about.com/od/howthingsworkfaqs/f/fallleafcolor.htm