Above: Photograph by Marie Viljoen. Above: Zelkova trees in New York City. Photograph by Marie Viljoen. Above: Even in San Francisco, fall is starting to show its colors on a Japanese maple in my backyard. Photograph by Janet Hall. Above: Redbud trees turn a brilliant yellow in autumn. And 5 Other Burning Questions N. Join the conversation. The second chemical, anthocyanin, forms as a result of the glucose formed by the remaining, faded chlorophyll.
The glucose then becomes trapped in the leaf by the abscission layer, resulting in the formation of anthocyanin. The colors of a particular tree are a result of the carotenoids and the anthocyanins reacting to each other in different amounts, in combination with any chlorophyll left. The formation of these chemicals and the amount of each of them are dependent on temperature, moisture and sunlight, so every foliage season is unique, because every season the chemical balance found inside the leaf changes.
The anthocyanin formed in the right-hand part of this leaf because it was exposed to sunlight, but the left side was probably in shade, so the carotenoids caused it to stay yellow. Carotenoids and anthocyanins also break down after being exposed to sunlight. If a leaf manages to stay on the branch after the chemical processes have broken down, you would see the bright colors fade until it would finally be brown, a result of a final chemical, tannin. Tannins are found in the membranes of the cells that make up the leaves, so they never fade, which is why brown is the final color present in late autumn.
In these leaves, the tannins are interacting with the carotenoids and anthocyanins to create subtle, stunning combinations. 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. As the fall colors appear, other changes are taking place. At the point where the stem of the leaf is attached to the tree, a special layer of cells develops and gradually severs the tissues that support the leaf.
At the same time, the tree seals the cut, so that when the leaf is finally blown off by the wind or falls from its own weight, it leaves behind a leaf scar. Most of the broad-leaved trees in the North shed their leaves in the fall.
However, the dead brown leaves of the oaks and a few other species may stay on the tree until growth starts again in the spring. So, instead of trying to keep their leaves, some plants drop their leaves and seal the spots on their branches where the leaves had been attached.
Leaves are colored by molecules called pigments. The pigment that causes leaves to be green is chlorophyll. Chlorophyll is important for plants to make food using sunlight. During spring and summer when there is plenty of sunlight, plants make a lot of chlorophyll. In autumn when it starts to get cold, some plants stop making chlorophyll. Instead, those plants break down chlorophyll into smaller molecules.
As chlorophyll goes away, other pigments start to show their colors. This is why leaves turn yellow or red in fall. 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?
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