European forests seem to lack many species that produce good color, especially the reds. Some trees produce their colors earlier than others, and the mix of species may affect the peak color production. Summer drought conditions stress trees. They may thus lose their leaves prematurely or start color production prematurely.
The result is a reduction of color during the peak of the season. Adequate summer rains promote good tree health, leaf retention and, therefore, color production during the autumn. The right weather during the autumn can promote more intense color production. In fall, plants break down and reabsorb chlorophyll, letting the colors of other pigments show through. Image by Sander van der Wel. The color change usually happens before the leaves fall off of the tree.
Why might that be? It takes a lot of energy to make chlorophyll. If the plants break down the chlorophyll and move it out of their leaves before the leaves fall, plants save energy. The plants can reabsorb the molecules that make up chlorophyll. Then, when it's warm and sunny enough to grow again, the plants can use those molecules to remake the chlorophyll. That way the plants don't have to make chlorophyll from scratch.
There are other pigments in leaves called carotenoids. Carotenoids are yellow and orange. Anthocyanins are other plant pigments that are only made in the fall. These pigments cause red, pink, or purple colors. Along with the green pigment are yellow to orange pigments, carotenes and xanthophyll pigments which, for example, give the orange color to a carrot.
Most of the year these colors are masked by great amounts of green coloring. But in the fall, because of changes in the length of daylight and changes in temperature, the leaves stop their food-making process.
The chlorophyll breaks down, the green color disappears, and the yellow to orange colors become visible and give the leaves part of their fall splendor. At the same time other chemical changes may occur, which form additional colors through the development of red anthocyanin pigments. Some mixtures give rise to the reddish and purplish fall colors of trees such as dogwoods and sumacs, while others give the sugar maple its brilliant orange. The autumn foliage of some trees show only yellow colors.
Others, like many oaks, display mostly browns. All these colors are due to the mixing of varying amounts of the chlorophyll residue and other pigments in the leaf during the fall season. An early frost weakens the color b y destroying the creation of anthocyanins , however.
Drought can also cause leaves to fall off without changing color. Where just a few tree species dominate , like in New England and Northeast Asia, color displays are intense but short. Diverse forests mean a longer display. Cloudy and warm falls like those in Europe cause dull colors. Join the ZME newsletter for amazing science news, features, and exclusive scoops. More than 40, subscribers can't be wrong. Where the stem of the leaf attaches to the tree, a layer of cells forms that eventually cuts the tissue that attaches the leaf to the tree.
There is a closed scar on the branch where the leaf was attached ; t he leaf is then free to fall when prompted by wind, gravity, rain , and so on. When the leaves die and the chloroplasts are completely broken down, leaves turn a boring brown. And that is the science behind why the leaves that fall in the autumn are everything from red and yellow to orange and bronze to, finally, brown.
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