"Science agrees on the mechanism of fall color, but there is debate as to what precedes it," said Peter J. Davies, Cornell University professor of plant physiology.  "Is it a wet summer or a dry summer that increases the brilliance?  Without a doubt, cool nights and bright days contribute quite a bit to fall color."

Davies explained that fall color comes from two main sources: pigments, such as yellow and orange carotenoids, and red anthocyanins.

Yellow and orange carotenoids are present in the leaves all the time but are masked by the green chlorophyll.  As the leaves become senescent (or age) at the end of the season, the green chlorophyll in certain tree species degrades, allowing us to see the oranges and yellows of the carotenoids, Davies said.  Senescence is triggered by the declining day lengths in the Northern Hemisphere at this time of year.

During the warm days of fall, the leaves can still make sugars by photosynthesis -- provided the leaves still possess chlorophyll, explained Davies.  When the night temperatures fall, the transport of the sugars from the leaves is slowed and these sugars are converted into the red anthocyanins.  This process also is enhanced if the plants are under stress, he said.

Only certain species develop fall color, said Davies.  "The propensity to do so is genetic and it is associated particularly with the sugar and red maples, as well as plants like sumac and white ash that are typical of this region," he explained.  In other areas, one or two species may show a strong yellow, but nothing like the trees in the Northeast.

Undoubtedly, Davies said, the weather at the time of fall color has the most influence.  The most color will develop under warm sunny days with cool (but not freezing) nights.  Cool, rainy days cause the leaves to fall without developing much color, as the rain and wind knock the leaves off more rapidly.

"If you look at trees at the edge of a woodland area, the trees exposed to sun are always more colored than those that are more shaded.  There are many opinions on the role of weather during the preceding summer.  I think most are anecdotal and I don't know if anyone has done a long-term study on the phenomenon," he added.  Even fertilizing a tree late in the season will decrease the fall color of the leaves, he said.

"It is my opinion that the more the tree is under (physiological) stress, the more color will be developed," Davies said.  "Thus a dry summer (leading to water stress or drought stress) will probably give more color the following fall than a moist, rainy one."

Note: If no author is given, the source is cited instead.

• Trees
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• Endangered Plants
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• Chlorophyll
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