Bright Days, Cool Nights Help Create Autumnal Splendor, Says Cornell Plant Physiologist

"Science agrees on the mechanism of fall color, but there is debate as towhat precedes it," said Peter J. Davies, Cornell University professor ofplant physiology.  "Is it a wet summer or a dry summer that increases thebrilliance?  Without a doubt, cool nights and bright days contribute quitea 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 butare masked by the green chlorophyll.  As the leaves become senescent (orage) at the end of the season, the green chlorophyll in certain treespecies degrades, allowing us to see the oranges and yellows of thecarotenoids, Davies said.  Senescence is triggered by the declining daylengths in the Northern Hemisphere at this time of year.

During the warm days of fall, the leaves can still make sugars byphotosynthesis -- provided the leaves still possess chlorophyll, explainedDavies.  When the night temperatures fall, the transport of the sugars fromthe leaves is slowed and these sugars are converted into the redanthocyanins.  This process also is enhanced if the plants are understress, he said.

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

Undoubtedly, Davies said, the weather at the time of fall color has themost influence.  The most color will develop under warm sunny days withcool (but not freezing) nights.  Cool, rainy days cause the leaves to fallwithout developing much color, as the rain and wind knock the leaves offmore rapidly.

"If you look at trees at the edge of a woodland area, the trees exposed tosun are always more colored than those that are more shaded.  There aremany opinions on the role of weather during the preceding summer.  I thinkmost are anecdotal and I don't know if anyone has done a long-term study onthe phenomenon," he added.  Even fertilizing a tree late in the season willdecrease 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 colorthe following fall than a moist, rainy one."