Photosynthesis: Leaves Create Fall Ambience With Changing Colors

Let's face it, autumn activities such as apple picking, pumpkin carving, drinking cinnamon spiced lattes, eating cider donuts and wandering through endless hay mazes, wouldn't be half as fun if it weren't for the perfect ambience created by changing fall leaves.

During the summer, leaves tend to stay a boring shade of green. However, when September rolls around, the magic happens: Leaves change to beautiful shades of yellows, oranges, reds and deep maroons. Why does this happen, and why does it take place in the fall?

During the summer, chlorophyll, or pigments that give the leaves their green color, is absorbing higher amounts of natural sunlight. This energy is vital for photosynthesis, so during the summer the green pigments dominate and overwhelm the other (fall) colors. In the fall, days get shorter and temperatures start to cool, which slows down the production of chlorophyll. This is when the fall-colored pigments are finally allowed to shine through.

The abscission layer, or the separation zone, also contributes to this process. This area seals off water flow to leaves, which inevitably severs each leaf from its stem. This causes leaves to fall the forest floor or blow away. This adds another item to your fall to-do list: Jumping in leaf piles. 

Three pigments control leaves' fall coloring. Carotenoids control yellow and orange colors, while anthocyanins determine red and purple colors. However, like chlorophyll, these pigments often quickly break down and leave the leaves to turn brown.

This changing color performance is also impacted by certain weather conditions. In fact, in drought-ridden areas, the abscission layer may form sooner and cause the leaves to fall before ever changing color. According to the U.S. National Arboretum, the best conditions for fall coloration is "a growing season with ample moisture that is followed by a rather dry, cool, sunny autumn that is marked by warm days and cool but frostless nights." The leaves may also stay golden yellow, burnt orange and red longer if there are not a lot of heavy winds or rains during the fall.  

The U.S. National Arboretum also explained that, in addition to being a marvel to us, leaves that change color in the fall are preparing for the winter and next spring's growth. In the Northern Hemisphere, most trees are finished growing by late June. This is when they develop next year's buds, which will not open until they have endured long winter months and are warmed by spring temperatures.

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