October brings shorter days, chilly morning temperatures and the annual autumn extravaganza we call "fall colors." Everyone enjoys them, but the biology that sets the stage is equally fascinating.
In September, sumac thickets turn flaming orange. Then browns and yellows mottle raspberry leaves, and Virginia creeper, red maples, sassafras and flowering dogwood join the scarlet brigade.
By late October, elms, oaks and hickories add their earthy tones to the autumnal landscape.
Why and how do leaves change colors each fall? It's all about chemistry.
Chemical pigments in plant cells give leaves their color. During the growing season, leaves are green because chlorophyll, which reflects green light, is present in large quantities. Other pigments are present in smaller amounts, but chlorophyll masks their effects.
Chlorophyll is the chemical that drives all life on earth. Every chlorophyll molecule in every leaf captures the sun's solar energy and chemically converts it into sugar, the basic source of energy for all plants and animals. Without chlorophyll, life as we know it would cease to exist.
But chlorophyll persists only during the growing season. As fall days grow shorter, chlorophyll breaks down and previously hidden pigments reveal themselves. The fall colors we see in different trees reflect the relative proportions of yellow, red, orange and brown pigments in their leaves.
Trees' biological clocks are set by photoperiod (day length) and fine-tuned by temperature, elevation and latitude. For each tree species, there is a critical day length during which chlorophyll breakdown begins and other pigments reveal themselves.
After a few showy days, leaves begin to fall to the ground. Hormonal changes cause a cork-like layer of cells to form at the base of the leaf stalk. The connection between the stalk and stem weakens, and wind and rain break the leaf free from its base.
The leaf scar left on the stem protects the tree from water loss during the winter. When water is locked up as ice and snow, drought-like conditions prevail. By dropping their leaves, deciduous trees protect themselves from the winter "drought."
For more information: www.dcnr.state.pa.us/forestry. Click on "Pennsylvania Fall Foliage." Click "Weekly Fall Foliage Reports" for autumn updates.
Biologist, author and broadcaster Scott Shalaway can be heard 9-11 a.m. Saturdays on 1370 AM WVLY (Wheeling) and noon-2 p.m. Sundays on 1360 AM WMNY (Pittsburgh). He can be reached at scottshalaway.googlepages.com or 2222 Fish Ridge Road, Cameron, WV 26033.