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What causes the leaves to change?

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Tree Talk with Joanna Angle

Although the phenomenon of leaves changing color is still not completely understood, we know that it is primarily influenced by three things.

The least variable of these factors is the increasing length of night.

As nights grow longer and cooler, biochemical processes are triggered in leaves.
The second contributor is the pigmentation present. Chlorophyll is the dominant pigment that makes leaves green.

It absorbs sunlight to produce sugars for food.

Carotenoids are pigments responsible for yellow, orange and brown colors in fruits, vegetables and flowers.
They are also present in tree leaves during the spring and summer, but are masked by the green of chlorophyll.

Anthocyanins are plant pigments that produce red, blue and purple shades.

Most anthocyanins are produced within tree leaf cells in autumn, in response to bright light and excess plant sugars.

As days grow shorter chlorophyll production slows, then stops.

 Once the green of chlorophyll is eliminated from leaves the carotenoids and anthocyanins present are revealed to show their respective colors.
The USDA’s Forest Service explains that tree species also plays a role.
“Oaks turn red, brown or russet; hickories, golden bronze; aspen and yellow-poplar, golden yellow; dogwood, purplish red; beech, light tan; and sourwood and black tupelo, crimson. Maples differ species by species — red maple turns brilliant scarlet; sugar maple, orange-red; and black maple, glowing yellow. Striped maple becomes almost colorless. Leaves of some species such as the elms simply shrivel up and fall, exhibiting little color other than drab brown.”
Timing of color change also varies by species.
The third influence on fall foliage is the weather conditions, especially temperature and moisture present before and during the diminishing production of chlorophyll.
Years that have seen the most spectacular fall color displays typically have had a warm, wet spring and temperate summer followed by a series of warm, sunny autumn days with cool but frost-free nights.