Thursday, September 22, 2011

Why Leaves Turn Colour in the Autumn

Not all leaves turn vivid colors in the fall. Only a few species of deciduous trees—notably maple, aspen, oak, and gum—produce grand performances for the annual autumn spectacular in North America.

Several factors contribute to fall color, but the main agent is light, or actually the lack of it. The amount of daylight relates to the timing of the autumnal equinox. (Tomorrow morning Sept. 23rd at 5:05 am EDT)

As the autumn days grow shorter, the reduced light triggers chemical changes in deciduous plants causing a corky wall to form between the twig and the leaf stalk.

This "abscission layer" eventually causes the leaf to drop off in the breeze. As the corky cells multiply, they seal off the vessels that supply the leaf with nutrients and water and also block the exit vessels, trapping simple sugars in the leaves. The combination of reduced light, lack of nutrients, and no water add up to the death of the pigment chlorophyll, the "green" in leaves.

Once the green is gone, two other pigments show their bright faces. These pigments, carotene (yellow) and anthocyanin (red), exist in the leaf all summer but are masked by the chlorophyll. (The browns in autumn leaves are the result of tannin, a chemical that exists in many leaves, especially oaks.)

Sugar trapped in autumn leaves by the abscission layer is largely responsible for the vivid color. Some additional anthocyanins are also manufactured by sunlight acting on the trapped sugar. This is why the foliage is so sparkling after several bright fall days and more pastel during rainy spells. In general, a dry fall produces the most-vibrant color.

Is this more than you wanted to know? I had a friend ask about leaves and their specific colours. I had to do some research to answer her question because it bothered me I didn't know. This above is what I learned. Do you go leaf peeping? Me, I do as I love the coolness of the weather and the vivid show of Nature's.

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