Why do leaves change different colors in the fall?

Three factors influence autumn leaf color- 1) leaf pigments, 2) length of night, and 3) weather. The timing of color change and leaf fall are primarily regulated by the increasing length of night.

Where do fall colors come from?

During winter, there is not enough light or water for photosynthesis. The trees will rest, and live off the food they stored during the summer. They begin to shut down their food-making factories. The green chlorophyll disappears from the leaves, and the chemical carotenoids who were always present in the leaves become more visible. In the fall, when the leaves are nearing the end of their life cycle, the chlorophyll breaks down, and the yellow-orange is revealed.

Most trees have also evolved to produce anthocyanins, when it’s bright and cold in autumn. These have a reddish tint. They’re also sometimes made in newly sprouting leaves, which explains their sometimes reddish tint. Where chlorophyll and anthocyanins coexist, the color of a leaf may look bronze, as in ash trees. At high enough concentrations, anthocyanins will make a leaf look almost purple, as in Japanese maples.

How does weather affect autumn color?

A succession of warm, sunny days and cool, crisp but not freezing nights seems to bring about the most spectacular color displays. During these days, lots of sugars are produced in the leaf but the cool nights and the gradual closing of veins going into the leaf prevent these sugars from moving out. These conditions spurs the production of the brilliant anthocyanin pigments, which tint reds, purples, and crimson. Because carotenoids are always present in leaves, the yellow and gold colors remain fairly constant from year to year.

The amount of moisture in the soil also affects autumn colors. Like the weather, soil moisture varies from year to year. A late spring, or summer drought, can delay the onset of fall color by a few weeks. A warm period during fall will also lower the intensity of autumn colors. A warm wet spring, favorable summer weather, and warm sunny fall days with cool nights should produce the most brilliant autumn colors.

What colors can I expect to see in my tree?

Oaks: red, brown or russet

Hickories: golden bronze

Birch: bright yellow

Poplar: golden yellow

Maple trees show a whole range of colors, oranges, reds, purples & yellows

What triggers leaf fall?

The veins that carry fluids into and out of the leaf gradually close off as a layer of cells forms at the base of each leaf. These clogged veins trap sugars in the leaf and promote production of anthocyanins. Once this separation layer is complete and the connecting tissues are sealed off, the leaf is ready to fall.

 

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