Tis the season for scenic drives and rambling hikes. For pumpkin spice and apple flavored everything. For harvesting the fruits of the growing season. But, in this region dominated by eastern deciduous forests, nothing represents fall more so then the trees. The spectacular color changes of the trees are enough to create a tourist season. Our hillsides decked out in red, yellow, purple, and orange are worth traveling for.
While the sights of fall are spectacular, the science behind the changing colors is as fascinating. To understand the fall, you have to understand leaves. Leaves are the food-producing factory for the plant. They collect sunlight and carbon dioxide. Along with water brought up from the roots, the cells in the leaves produce glucose for the plant, which feeds the plant’s growth. But plants take up much more water than they need for food production. So they have to get rid of it, through a process called transpiration. It is through little holes in the leaves, called stomata, that the extra water escapes. Scientists estimate that a large oak tree can transpire 40,000 gallons of water a year. The water transpired by plants makes up about 10% of the water in the atmosphere.
This system is successful during the warm, bright growing season but as days get shorter and temperatures drop, there is not enough sunlight for food production. As the ground freezes, trees would lose more water through their leaves than they would be able to take up. So deciduous trees have evolved to drop their leaves for the winter months. A calculated strategy. They miss out on any food production on bright, warm winter days. And they have to rebuild their food factories in spring from energy stored in their roots. But anyone who has seen the damage to trees in an early snow storm, while their leaves are still on can understand that the wide leaves of deciduous trees don’t handle heavy snow loads very well. What served them well in the warmer months is now a liability. As days get shorter and temperatures drop, the tree senses this and produces cells that essentially cut the leaves off from where they are attached to the branch; and lets them go.
That explains why trees lose their leaves but not why they change color. The fall colors are so beautiful, wondering if the trees do it just for joy is both entertaining and excusable. But nature is practical. Producing different color pigments takes energy and nature seldom wastes. So the colors serve a purpose.
Most know the green chlorophyll pigment captures the light that fuels photosynthesis. It is assisted with this light capturing by other pigments including yellow xanthophyll and orange carotene. The yellow and orange colors are present all the time. They are just covered up by the green of the chlorophyll and not visible in spring and summer. When the tree starts to shut down, it stops producing chlorophyll. The dominant color starts to fade and other colors are visible.
There is a way to see these pigments in the leaves through a simple experiment. I came across what was billed as a “great science fair experiment” on several websites while searching to confirm my understanding and spelling of leaf pigments. (Xanthophyll is not an everyday word for me.) To satiate my curious mind I gave it a try.
Tree leaves are crushed up into rubbing alcohol to release their pigments. To see all the pigment colors that may be in the leaf, place one end of a strip of white coffee filter or paper towel in the alcohol and let sit for at least an hour or more. Slowly the pigments creep up the paper and separate into different colors. For a full description of the experiment, visit www.scientificamerican.com/article/bring-science-home-leaf-colors/
I tried it with several different leaves. With green leaves, the green chlorophyll was visible but so was the yellow xanthophyll pigment. The experiment demonstrates that yellow is there all along. Just hidden. So the fall color is more of a revealing then a changing. But not for all colors.
The red color, caused by a pigment called anthocyanin is a little different. It is only produced in some trees as the leaf is shutting down production. That seems like a big expense for the tree that is closing up shop. So why does a tree do it? It turns out that we don’t really know. Scientists think that the red pigment may help the tree gather sunlight to produce the last of the sugar before winter and that the red pigment may help protect the leaf from sun damage.
That rings true for me. I recall watching a young oak tree in the yard put on new growth in early summer. After a few days of fresh, new green growth, the leaves turned red for a few days. They turned green again and appear to be just fine. It was curious. Leaves only change color in fall, right? Maybe not. Was the anthocyanin protecting these tender leaves?
Deciduous trees showcase one strategy for survival of the cold winter months. They embrace the change around them and adapt to it. They drop what no longer does them any good. They hold close what matters most. But first they seem to celebrate. In the dark and cold, the best thing they can do to keep on living is to rest. Not to quit completely but just to take a break before picking up where they left off.
There is so much to learn from the natural world. Through observation, questioning and experimenting we learn how it works. And perhaps how we work too.
Audubon Community Nature Center builds and nurtures connections between people and nature. ACNC is located just east of Route 62 between Warren and Jamestown. The trails are open from dawn to dusk as is Liberty, the Bald Eagle. The Nature Center is open from 10 a.m. until 4:30 p.m. daily except Sunday when it opens at 1 p.m. More information can be found online at auduboncnc.org or by calling (716) 569-2345.
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