"Fieldnotes," October 6th & 7th, 2013: "Leaf Fall," by Peter Lesica, Anne Garde, Caroline Kurtz. http://www.montananaturalist.org/
"As autumn progresses, trees prepare for winter by stopping the production of chlorophyll, the green pigment that captures light. The leaves gradually change color as nutrients are withdrawn and transferred to roots and stems. At the same time, enzymes digest the cells at the base of the leaf stalk forming an abscission layer, or scar. When digestion is complete, the leaf falls off."