We live in a region of fall color.
Reddening leaves of dogwood (Cornus florida) and sourwood (Oxydendrum arboreum) often lead the way, although droughty conditions (not uncommon) may also encourage early leaf color in maples, probably as leaves have shut down production of chlorophyll early this year. My sister (then living in Central Texas) sent me an e-mail a number of years ago, asking about what really triggered the change in color in leaves—temperature, day length, 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’m now a volunteer one)? I had some fun reviewing the details and look forward to seeing how it will play out this season, with the warm and dry conditions forecast for this fall.
Why the Colors?
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 anthocyanins produce the purple and reds. Anthocyanins 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.
Diversity of Trees
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 sweet gums 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, sweet gum, as well as the maples. Hickories, river birch, redbud, tulip poplar, and sycamore turn yellow and gold, although the last two frequently turn brown and drop leaves early in droughty years like this one.
Conditions for Intensity
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 —from the anthocyanin pigments—at the same time chlorophyll production is declining. These are the “best” fall color years for bright red and orange hues.
Yellows are fairly consistent from year to year, since the carotenoids aren’t so affected by weather variations. Overly dry weather will produce more brownish leaves and early leaf drop, with washed-out colors in general.
So no two falls are alike!