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Solving The Mystery Of Autumn

The curtain is rising on an annual show – one that’s particularly brilliant in our part of the world. It’s deciduous trees’ annual show of color. Mother Nature gets the credit for producing this show, but did you ever wonder what really makes it happen?

The answer is quite simple yet quite complex. A combination of leaf pigments, light, weather conditions, plant species and geography all work together to create fall color.

Leaves’ natural color is yellow, a pigment in the (ka’rotn-oid) family, and it’s always present. However, green chlorophyll, which is necessary for the manufacture of food during photosynthesis, masks the yellow.

The sun provides the energy for photosynthesis. So, photosynthesis slows down and then ceases as the amount of daylight dwindles and temperatures plummet. As this happens, less and less chlorophyll is produced until the leaves’ natural color becomes visible.

Some leaves turn red in the fall. This is caused by anthocyanins (an-tho-cy-a-nins), a pigment produced only during the autumn months. These complex, water soluable compounds in leaf cells react with excess stored plant sugars and exposure to sunlight, resulting in brilliant pink, red and purple leaves. A mixture of red anthocyanin pigment and yellow carotenoids often results in the bright orange color we see in some leaves. There appears to be more orange leaves this year than usual.

Weather conditions that occur before and during the decline of chlorophyll production can affect the color that leaves display. Carotenoids are always present so the yellow and gold colors are the least affected by weather. The red tones, created by anthocyanin, are most affected by weather.

Lots of sugar is produced in leaves on warm, sunny days. Trees exposed to brighter sunlight generate the reaction between anthocyanins and the excess sugar, creating the bright red hue. Sharp changes in climate can paint the most spectacular display of color. Cooler temperatures cause the veins in the leaves to gradually close, preventing sugars from moving out, which preserves the red tones. The lush tones of fall we see all around us are caused by warm, sunny days followed by crisp, cool nights.

Soil moisture can also affect autumn color. A particularly dry summer can delay the onset of color change by weeks. A warm, wet spring, favorable summer weather, and sunny fall days with cooler temperatures at night are ideal conditions for producing the most radiant colors.

Tree genetics and species determine what color leaves will turn. Color also depends on the levels of iron, magnesium, phosphorous and sodium in the tree and the acidity of the chemicals in the leaves.

When the show of color is over, the curtain of leaves falls. As photosynthesis ceases, the base of the leaf, known as the petiole, closes up since no food is leaving. No water and nutrients flows in, either. Meanwhile, next year’s leaf bud, positioned below the petiole, has formed and grows until it pushes the leaf, disconnecting the tissue that holds it to the branch, and the old leaf falls.

Just like the theater, it’s now time to clean up the litter with your leaf rake or blower.

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