Leaves are finally beginning to show signs of fall color (My picture is a few years ago, however). The early reds of the sourwoods seem to have been muted by drought, although the drought seems to have encouraged early leaf color in maples, probably as leaves have shut down production of chlorophyll early this year. My sister in Texas sent me an e-mail asking about what really triggered the change in color in leaves - temperature, daylength, moisture, or a combination. Her dog park group wanted to know!
Well, what are sisters for, after all, especially if she's a botanist and garden educator? I had some fun reviewing the details and look forward to seeing how it will play out here, with the severe and continuing drought, and until today, unreasonably hot (for October) weather.
Basically, our fall colors in the Eastern U.S. are revealed as chlorophyll production slows down, cued by the shortening days and lengthening nights. The interplay of pigments in leaves determines the fall colors of different species, with the temperature and moisture determining color intensities. As the chlorophyll (which provides the overriding green color of leaves) breaks down, the other pigments in the leaves become evident. The carotenoids produce the yellow and oranges and anthocynanins produce the purple and reds. Anthocynanins are actively produced as a reaction between sugars and proteins - in the watery vacuoles of leaf cells, and their colors are influenced by acidity. They start showing up as the chlorophyll breaks down, and corky deposits start blocking the downward flow of sugars between leaves and stems.
Different trees have different combinations of the basic pigments, and here in Eastern North America, we have the largest diversity of species of trees that exhibit fall color, so many of our natives are prized in Europe for fall color -- our sweetgums and tulip poplars for example.
Some of the trees that are shades of oranges, reds, and purples include the red, white, and scarlet oaks, persimmon, sassafras, dogwood, sweetgum, as well as the maples. Hickories, River Birch, Redbud, Tulip Poplar, and Sycamore turn yellow and gold.
Beech leaves also accumulate tannin, adding a bronze color to the underlying yellows. The fall weather plays a key factor in whether it's a particularly good year for color, especially in the reds and purples. Day and night temperature and general moisture levels are important. Warm sunny days (with lots of sugar production) with cool crisp nights produce the best reddish and purple colors – the anthocynanin pigments. at the time chlorophyll production is declining, generally affects how bright the colors are.
Yellows are fairly consistent from year to year, since the carotenoids Overly dry weather will produce more brownish leaves and early leaf drop, with washed-out colors in general.
So no two falls are alike!