Helping Aubie this week is Dr. Lisa Samuelson, Professor of Forest Biology and Ecology, in AU’s School of Forestry & Wildlife Sciences.

Dear Andrew,

What a good question!  Everyone enjoys leaf change in autumn and wonders why some years and places are better than others in producing color.  First, let’s answer the question of why leaves change color in the fall.  Chlorophyll is a green pigment inside the leaf that helps the leaf use light energy for photosynthesis.  Chlorophyll gives leaves their green color.  As temperatures get colder in the fall, leaves stop making chlorophyll and the existing chlorophyll fades.  The other pigments in the leaf that are blue, purple, red, yellow or orange in color are then revealed.  These other pigments also help in light absorption but are less abundant than chlorophyll so that is why you don’t see them in summer. 
Each tree species has a different combination of pigments and changes color at a unique rate so some are brighter than others in the fall.  The hickories, yellow-poplar, black walnut, the birches and ginkgo trees have pigments that give the leaves a yellow fall color.  The maples, sassafras, sumacs, scarlet oak, white oak, blackgum, sourwood, shadbush and euonymus trees have large amounts of red pigment mixed with yellow that result in stunning red or orange fall colors. 
Second, the right conditions are needed for the best fall colors. A decrease in night temperature to just above freezing favors the red colors but early frosts reduce fall color.  Bright sunny days and drought favor color and rainy cloudy days near peak coloration will decrease color intensity.  So, the best fall colors occur with clear, dry and cool weather.  

Thanks for your question,
Aubie and Dr. Samuelson