Nirja Basawa
Have you ever walked through a park, or gone on a drive, or just looked out the window and noticed that the trees aren’t green? Instead, they were beautiful colors of red, yellow, and orange. Most of us know that the trees changing color are a sign of fall coming, and soon they will fall off and create those crunchy heaps on the ground to jump in. But how does this happen? Why do some trees change colors in the fall only to shed their leaves in winter? The answer comes back to science.
Leaves hold small chemicals in them called pigments, which can be different colors based on what pigment it is. In spring and summer, the leaves hold pigments called chlorophyll, which makes the leaves green. Chlorophyll is very important to the leaves, since it helps them make energy from sunlight through a process called photosynthesis. As summer fades and goes into fall, the days become shorter, meaning there is less sunlight for the trees. This is a sign for the leaves to stop doing photosynthesis and start preparing for winter. So, in fall, chlorophyll starts to fade from the leaf, letting other pigments like xanthophylls (yellow), carotenoids (orange), and anthocyanins (red) show through, creating the beautiful blend of yellow, orange, and red leaves on the trees. The tree then absorbs the chlorophyll and the stems of the leaf become weak, causing them to fall off. This way, the tree can use the chlorophyll again next spring and save energy.
Changing amounts of sunlight isn’t the only thing that can cause leaves to change their color. Trees are very sensitive to changes in their environment, and weather plays an important role in the changing colors. Temperatures drop during the fall, and there is more rain and snow. So, a very early frost or drought could cause leaves to fall off the tree before they even change colors.
There are some trees that don’t seem to change colors during the fall, known as evergreen trees. If you’ve ever gone on a drive through the mountains and seen those tall Christmas trees along the slope, those are evergreens! Those huge pine trees in the forest you’re hiking in? Also evergreen. Gone to the beach and seen a palm tree? Surprise, that’s also an evergreen. What makes these trees so special is that their leaves don’t turn amazing shades of red, orange, and yellow and fall to the ground. Or do they? Evergreen trees do have leaves that change color and fall, they just don’t do it all at once like other trees. Instead, they shed their leaves and needles how we shed our hair: in small groups, always being replaced by a new group rather than falling out all at once. So evergreen trees do shed, but the only reason they keep their bright green color and leaves is because they do it quite slowly, and are mostly in “hibernation”.
So the next time you’re strolling through your neighborhood and see the beautiful colors on leaves, remember the magical science that’s taking place inside at microscopic levels and enjoy the beauty of nature!
Bibliography
“Why Do Leaves Change Color?” NOAA SciJinks – All About Weather, 9 Nov. 2020, scijinks.gov/leaves-color/.
Simpson, Richard K. “Why Do Leaves Change Color in the Fall Season?” Ask A Biologist, National Science Foundation, 16 May 2017, askabiologist.asu.edu/questions/why-do-leaves-change-color.
Bloom, Molly. “Here's How Evergreen Trees Stay Green All Year.” MPR News, MPR News, 1 June 2015, www.mprnews.org/story/2015/06/01/how-evergreen-trees-stay-green.
