As the shorter, cooler days of autumn arrive, they usher in a pageant of fall leaf colors. Why does the autumn leaf display seem especially brilliant some years? The answer lies in the chemistry of certain plant species, and in the way their leaves react to sunlight and air temperature.
Only some tree species have weather-dependent leaf displays. The yellow hues of aspen, cottonwood, and many other plants come from chemicals that are present in the leaves throughout the growing season.
The bright green of chlorophyll masks this yellow, but fades as chlorophyll breaks down in autumn. The yellow color changes little from year to year, because weather doesn’t affect the chemicals that produce it.
Red and purple hues—found in maple, sumac, dogwood, and other plants—are different. The chemicals that form these colors, called anthocyanins, are also present in leaves throughout the growing season.
In some plant species, however, their production ramps up in autumn when conditions are right. Those key conditions include warm, sunny days and cool, frost-free nights.
During sunny autumn days, leaves produce plenty of sugar. Cool nights hinder the usual flow of sugar from leaf to tree branch. The sugar then builds up in the leaves and boosts the production of anthocyanin.
The result? Dazzling splashes of fuchsia, crimson, and burgundy across the autumn landscape. But if autumn weather brings a string of cloudy days or nights that are too frosty, the leaves produce fewer anthocyanins, and their colors aren’t as flashy.
This Earth Note originally aired in 2011. It was written by Alexandra Murphy and produced by KNAU and the Sustainable Communities Program at Northern Arizona University.
