Have your kids ever wondered why the leaves become so brilliant in the fall? The next time you're out leaf-watching, explain it to them!
A nice explanation is available at sciencemadesimple.com. In a nutshell, the reason leaves change color and drop off their host trees in the autumn is that they are no longer performing their primary function of producing energy. They are no longer needed by the tree.
During the growing season, most trees exhibit green leaves, the color of chlorophyll that's produced in the leaves by energy drawn from the sun. This chemical reaction provides energy to the tree. As long as there's enough sunlight and temperatures are warm enough, this energy supply line continues to churn: the sun feeds the leaves, the leaves feed the tree.
So long, sun!
However in the fall, the hours of daylight diminish and the nights grow cooler. That means the tree stops drawing energy from the leaves and instead begins to burn the energy it's stored inside its branches, trunk and roots. The leaves change colors as the last of the chlorophyll drains out of them.
The reds and oranges you see are colors that were always there, in many cases, but took a back seat to the more vigorous green sparked by the chlorophyll. In some cases, the colors are the result of sugars caught inside the leaves being alternately baked by the waning autumn sunlight and then cooled at night.
A dimmer outlook
By the way, this year's leaf display in the northeast might not be as bright as usual. Karl Niklas, a Cornell University professor of plant biology, says there's been too little rain and the summer temperatures have been too hot in our part of the country this year.
As a result, many trees are stressed and are shedding leaves early before they can get around to changing colors. "I wish I was wrong," Niklas says, "but I'm predicting that this year's autumn coloration will not be as grand as in years past."
Still, even in a less vibrant autumn, the leaves still turn enough to be admired---and you can still talk to your kids about why!
---David Murphy
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