Why do leaves change colors in the fall?

Beyond The Forecast

(WFRV) – In Northeast Wisconsin, many spots are nearing the peak of the fall colors. Colors of leaves are ranging from yellow, orange, and red. But why do the colors change from their typical green color seen through most of the year?

The answer lies in the numerous cells that contain chlorophyll which gives leaves their green color in the spring and summer. Chlorophyll absorbs sunlight and transforms carbon dioxide and water into sugars and starch which provides food for the plants.

During the fall, sunlight is shortened and cooler temperatures stop plants from producing chlorophyll and the food-making process breaks down.

This causes the leaves to change color. The different colors of the leaves are correlated to the amount of chlorophyll residue in the leaf. The true pigment colors of the leaves are revealed when there is a lack of chlorophyll.

Other chemical factors, such as the stem of the leaf weakening, cause the leaves to be more vulnerable to fall from the trees.

According to the Harvard Forest:

  • Summer droughts can cause trees to prematurely lose their leaves
  • The red leaves are enhanced by cold (not freezing) and sunny days
  • Rainy and windy autumn days can knock off leaves prematurely before peak color

Try it Yourself Experiment:

Materials: Three leaves from the same tree, mason jar, coffee filter, plastic wrap, bowl of hot water and rubbing alcohol

Steps:

  1. Break leaves up into small pieces
  2. Pour rubbing alcohol into the mason jar until it just covers the leaves
  3. Stir and mash the leaves until alcohol turns barely green
  4. Cover the jar in plastic wrap and place in a bowl filled with an inch high of hot water
  5. Let stand for 30-45 minutes until the alcohol is dark green and then place a strip of the coffee filter in until it is just touching rubbing alcohol
  6. Leave for another hour or until the colors separate up the coffee filter

The final product on the coffee filter will separate different colors from the filter. The combination of hot water and rubbing alcohol acts to take away the chlorophyll or green color. What is left is the color of the leaf on the filter without the chlorophyll, which can range from yellow to red.

This is the process that happens in the fall when the cooler temperatures and lack of sunlight limit chlorophyll.

Fall foliage in Northeast Wisconsin will reach peak in most places the second week of October. Have awesome foliage pictures? Share it with us:

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