Yet Another Why Leaves Change Color Article

Autumn Maple LeavesThere are tons of articles in print and on the web about why leaves change color in the autumn. There is some good information out there, but there is a lot of not so good information too that clouds the issue. I have heard many conflicting theories so I did a little research on my own and I’ll give you my impressions and some links to what I think are some of the better articles on the web.

Days get shorter and temperatures start to drop in the fall of the year. This triggers deciduous trees to begin preparing for the cold winter ahead. Chlorophyll, a rather complex molecule composed of carbon, hydrogen, oxygen, magnesium, and nitrogen has been working all summer long to convert sunlight into energy that the tree can use to change carbon dioxide and water into the nutrients that it needs. While Chlorophyll converts sunlight to energy it also is destroyed by bright sunlight so the tree needs to keep replacing it. Leaves are green because of all the Chlorophyll.

With falling light and temperatures the tree does not produce enough Chlorophyll to keep pace with the loses and the green color fades away. That is when other chemicals like carotene, there all the time but usually hidden by the intense green, come to the front. Leaves with lots of carotene appear yellow.

Some trees produce chemicals called anthocyanins in the fall. Anthocyanins give leaves a red or purple color and are also responsible for the red coloration of ripe apples. Why these compounds form in some trees and not in others is still a bit fuzzy as far as I can tell. Some say it is a kind of sun-screen to protect what little chlorophyll is left, some say it is just a happenstance of high concentrations of sugars in the tree sap and stray sunlight that is no longer absorbed by chlorophyll, some suggest that it helps trees reabsorb nitrogen. I guess some more research needs to be done.

In any event, all these colorful chemical compounds running around in the leaves provide us with the wonderful fall transformation. Varying concentrations produce the miriad of subtle hues from green to yellow to orange to red to purple to brown.

Weather conditions greatly influence the range of colors and the intensity. A warm autumn encourages chlorophyll to hang around and colors are late, pale, and short lived. The reds need sunlight to develop so cloudy days are not good, besides the overcast sky cuts contrast and doesn’t show off the colors to their best advantage. Frost too early will destroy the leaves before they have a chance to color-up.

The best weather for bright beautiful fall colors? Dry sunny days (even a little touch of drought in the fall is good) with cool, but not freezing temperatures.

So far this year in my area, the colors are not much to look at. A little bit of yellow here and there and some spotty red maples in swampy areas (soils conditions also influence color development, but we won’t get in to that whole deal now). Many trees are holding on to the green or have just started dropping some pale yellow/brown leaves. The weather has been cloudy, wet, and pretty warm. I’m really hoping for some sun before it is too late.

Here are some links to articles that I found to be useful and pretty authoritative on this subject.

ESF – Why Leaves Change Color

Chemical of the Week

Autumn Colors

Scarlet Oak

Pigment Dynamics



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