Q. Why do trees change color in the fall and what determines if we have a good display on a given year?
A. Those magnificent colors you see in the fall are actually there all summer -- it's just that you can't see them because of the green chlorophyll in the leaves. As our days get shorter and the temperatures cool down, trees cease green chlorophyll production, causing the yellow chlorophyll to show. Any sugars trapped in the leaf react with each other in the presence of sunlight to form the reds and oranges -- thus, the more sun, the more brilliant the colors. The best weather conditions are the same ones we enjoy in the fall -- bright, cool days and chilly but not freezing nights. The slightest change -- too hot, too cold, too wet or too dry -- can slow the process or cause trees to lose their leaves before changing color.
-- Linda Radimecky, Fort Snelling State Park naturalist
