Autumn Colors, Abscission Layers, and How Trees Grow Leaves

"The Science of Color in Autumn Leaves"
The United States National Arboretum

"As the days get cooler and frost is in the air, deciduous trees and shrubs put on an autumn show in all shades of red, yellow, purple, and brown. The splendor of crisp fall days and vibrant leaves brings to mind memories for nearly everyone who lives in an area where deciduous forests are the dominant vegetation. In many parts of the country, autumn leaves are an important factor in tourism.

"Many think that cool weather or frost cause the leaves to change color. While temperature may dictate the color and its intensity, it is only one of many environmental factors that play a part in painting deciduous woodlands in glorious fall colors.

"To understand the whole process, it is important to understand the growth cycle of deciduous trees and shrubs. Most have a relatively short period of annual growth. New stems begin to grow from overwintering buds when the days become long enough and the weather is warm enough to support growth. For most trees, growth is usually completed by late June in the Northern Hemisphere. Next year's leaf buds are set at this time and will not open until they experience the chill and short days of winter followed by the warmth and longer days of spring...."

The page isn't the fastest read online, and has some fairly technical terms: like "abscission layer." On the up - or down - side, those generally have handy - or annoying - popups that explain what they mean. An "abscission layer," for example, is "an area at the base of a leaf stalk, fruit stalk, or a branch in which a layer of loose cells that are poorly attached to each other develops; the abscission layer causes a leaf, fruit, flower, or other plant part to fall away from a plant"

There's a decent selection of photos - and a pretty good discussion of why the fall colors of New England and northeast Asia are a short, rather striking display; while southern Appalachian regions have a longer but (generally) less spectacular fall foliage season.