CLEVELAND - Yes, the summer was dry - very dry in many spots. From mid-summer on, I noticed many trees around northern Ohio succumbing to the intense heat and dryness. Those trees, most likely already weakened by other factors, dropped their leaves months early. Many of these trees will not survive to see another summer.
Those trees that did hold their leaves are now ready for the transition to fall, when leaves change colors and eventually fall off the tree.
First, a quick primer on the chemicals in the leaves. Chlorophyll is the most important chemical in the leaves as it is responsible for photosynthesis. That's when plants use sunlight to change carbon dioxide from the air and water from the ground into sugars and oxygen. Chlorophyll is green. That's why leaves appear green during the growing season.
Carotenoids are present, along with chlorophyll, in the chloroplasts of the leaf cells. Carotenoids produce the yellow, orange, and brown colors in such things as corn, carrots, and daffodils, as well as rutabagas, and bananas.
Anthocyanins are substances produced by the leaf as the growing season stops, due to an abundance of sunshine and leftover sugars in the leaf. Anthocyanins give grapes, blueberries, strawberries, and cherries their red or purple color.
During the growing season, chlorophyll is continually produced and broken down. As daylight decreases in September and October, chlorophyll production stops and eventually all the chlorophyll is destroyed. Hence the green color in each leaf disappears. What's left are the carotenoids and anthocyanins. It's their turn to show off their colors.
Now, back to the weather. Drought can affect fall leaf displays by causing premature leaf drop. A stressed tree will drop its leaves before fall. Those leaves aren't usually very colorful. A drought can also delay leaves from turning colors in the fall. A hot period during fall will also lower the intensity of autumn colors.
Here in Ohio, peak fall color occurs during the second and third weeks of October. From mid-September on, weather conditions are crucial to produce the best fall leaf colors. Temperature and moisture are the main influences.
A week or two of warm, sunny days and cool nights can result in the most spectacular color displays. During these days, lots of sugars are produced in the leaf. These sugars are normally carriers out of the leaf to the roots. But the cool nights and the gradual closing of veins going into the leaf prevent these sugars from moving out. Lots of sugars and lots of sunlight foster production of the brilliant anthocyanin pigments, which tint reds, purples, and crimson. Because carotenoids are always present in leaves, the yellow and gold colors remain fairly constant from year to year.
The amount of moisture in the soil also affects autumn colors. Like the weather, soil moisture varies greatly from year to year. The countless combinations of these two highly variable factors assure that no two autumns can be exactly alike. The bottom line: despite the drought this year, if fall days are warm & sunny with cool nights in between, then fall should put on quite a show this year.
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