Science Talk: What Makes the Leaves Turn?

By Christopher Pouch

Published October 1, 2019


(Habib Affinih / ASP)

The Capital Region is beginning to see the first reds and yellows of autumn.


In a season filled with cool weather, apple picking, and the return of football, the bright colors might be the best part of all.


Why, exactly, do the leaves change color?


According to Dr. George Robinson, a plant ecology expert and retired UAlbany professor of biological sciences, the process is caused by a reduction in chlorophyll brought on by colder and shorter days.


“Trees have developed ways to determine when the season is changing,” Robinson says. “Sunlight coming in at an acute angle, and the temperature, give a signal that is transmitted chemically.”


Chlorophyll, a chemical found in all plants, is responsible for absorbing sunlight and turning it into energy. To maintain production of the chemical in leaves during winter, Robinson says, would be inefficient.


As a result, chlorophyll production wanes and the chemical begins to break down within leaves before they fall.


According to a report by the SUNY College of Environmental Science and Forestry, chlorophyll is also a pigment, responsible for the green color of most leaves during the summer months.


It is not the only pigment present, though.


Chemicals that cause shades of red, orange, and yellow are also present within leaves, but most of the time they are eclipsed by green. Only when chlorophyll production wanes do these pigments become visible, creating classic fall foliage colors.


When chlorophyll begins to break down in a leaf, though, it does not simply become waste to a tree.


According to a report from Appalachian State University’s (ASU) Department of Biology, it is reabsorbed by the tree in the form of smaller molecules to be reused in the spring.


During this process, the other pigments present in leaves may serve to protect the molecules being reabsorbed.


“As chlorophyll degrades in the fall, light energy impinging on the leaf can cause injury to the internal biochemical machinery,” according to the ASU report. “Especially the parts responsible for withdrawing nutrients back into the leaf. The presence of carotenoids may help the leaf dissipate this excess energy.”


While the colorful scenery only lasts for a few weeks in any given place, the change occurs at different times in different regions.


Albany is just beginning to see the turn, while the Adirondack Mountains upstate are enjoying peak colors. Further South in New York City, the best colors will likely be in mid to late October.


With summer having come to an official end last week, the leaves will begin to turn crisp reds, oranges, and yellows.


This is the time to grab a camera, a warm scarf, and anything pumpkin spice. Fall foliage is in season!

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