Tell Me Why the Leaves Change Color
Simone (left), age 4: Once there was a tree with a lot of leaves in it. The wind blew the leaves down to the ground — which is brown — and we had to rake them up. There were no purple leaves because they were changed to yellow. Then they turned purple. My favorite color is purple.
The Parent's Answer
Jonathan (left, below), Simone's dad: I thought that the leaves changed color due to the temperature. They're dying as part of the change of seasons.
The Scientist's Answer
Our most basic assumption about foliage is wrong, says Bob Edmonds, who heads up the forestry and wildlife program at the University of New Hampshire Cooperative Extension. "The bright colors don't arrive in the fall. They're there already." They show up when photosynthesis stops. Remember high school biology? Photosynthesis is when trees use sunlight and their own chlorophyll to convert carbon dioxide and water into the sugar that fuels their growth. Without enough sunlight, there's no photosynthesis, and no chlorophyll, which is what lends leaves their lush green pigment. In spring and summer, when the sun is brightest, chlorophyll is so plentiful that it overshadows the ever-present chemicals that create autumn's rich hues: red and purple (anthocyanin); orange (carotene); yellow (xanthophyll); and brown (tannins).
How to Explain It to Kids
Like us, trees need to eat to grow. In fact, they like to eat sugar — a type of sugar they make with the help of the sun and a green chemical called chlorophyll. We can tell they're eating because chlorophyll is making the leaves green. Trees eat (and grow) the most during spring and summer, when there is the most sun. In autumn, trees slow down to rest, essentially hibernating. Luckily, they've made and stored enough food to stop their work. Since they are no longer eating, the green goes away, revealing the fall colors that had been there all along.