2012 News items from Spearfish Canyon Society:
3. Fall Colors for 2011: The Society president, Jerry
Boyer, has been reporting canyon fall colors to media and public
Why autumn leaves? - For dummies like me:
In autumn, Nature's Call is comprised of three events: the days get darker, temperatures begin to cool, and the air becomes dry.
Leafy trees head Nature's Call and the chlorophyll in the leaves that once convert sunlight to food stops. The dark green leaves start to fade, and other color pigments show through. The trees drop their leaves to protect themselves from drying out in the winter.
For those who need chemistry - It's Greek to me:
In describing plants that decide to lose their leaves by sensing the length of nights, the word Photoperiodism comes from the Greek words for "a light/time system". For plants, the important thing is not that the amount of light has decreased, it's that the amount of dark has increased.
In describing leafy trees that drop their leaves in the autumn, the word Deciduous comes from Latin words meaning "to fall or cut".
Along with cold air in the autumn comes dry air. Cold air cannot hold as many water molecules, so, deciduous trees drop their leaves as a means to keep moisture inside--to keep from drying out.
Leaves become colorful because photosynthesis requires not only green chlorophyll, but also chemical interactions with a few other specialized pigments that provide our wonderful fall colors.
The process of a deciduous tree stopping its food-making process for the winter is chemically complicated. When the days get darker, plants have complex biochemistry that sets a series of events in motion. As the amount of light per day goes down, photoperiodic plants produce ethylene and a substance called abscisic acid. That leads to a change in the ratio of the hormones auxin and cytokinin. Scientists named these compounds by their functions. Auxin is the hormone that makes cells elongate or grow, similar to the word augment. Cytokinin comes from words that mean "put division in motion." When cells divide, they're growing. As the ratios of these specialized plant hormones change, trees stop making the famous energy-producing chemical chlorophyll. They pull in all the nutrients they can from the leaves, and cut the leaves off from their main stems.