Dear Miss Floribunda,
The drought seems to be over, and autumn is finally here, with trees in full color. I am hearing different opinions about whether dry weather or wet weather makes for more beautiful autumn foliage. Also, some are arguing about whether the drought has made autumn late this year. I’m hearing a lot of opinions without real explanations, so I’m just baffled. What do you think, and why?
Whether-Beaten on Street
Forgive me for whethering you further, but whether or not the recent drought strained the health of our trees, it won’t have a bad effect on general foliage color. Although the drought has left some trees with dead branches, the leaves on the remaining branches have changed to the bright colors we expect in autumn. The most drought-damaged trees actually changed leaf color early, which is a tree’s reaction to stress. However, I have consulted my trusted friend Fiona Feuille-Morte, of the National Arboretum, for confirmation. She informed me that dry weather in spring is harmful to developing leaves and can cause them to fall prematurely. Substantial rain in the summer will ensure that the leaves in fall remain on the trees longer after they change color, but too much rain in early fall actually mutes the colors of the leaves. Dry weather in the fall enhances color. Light rain is acceptable, but, of course, torrential rain with high winds will just rip the leaves off before they reach peak color. The best conditions for a beautiful autumn, she says, are a rainy summer followed by a dry autumn with calm, sunny days and cool, windless nights.
Because you want explanations for opinions, let’s review what it is, exactly, that determines leaf color and what causes it to change in the autumn. Almost everybody knows that tree leaves are green because of something called chlorophyll. Chlorophyll is a molecule that enables photosynthesis, the interaction of light with carbon dioxide and moisture within a plant’s leaves, to create the sugars and oxygen needed for survival. Along with chlorophyll molecules, leaves have molecules for yellow and orange pigments called xanthophylls and carotenoids, respectively. These recondite colors remain invisible until autumn, when the green chlorophyll that dominates and masks them subsides with cooler temperatures and diminishing sunlight.
The trees begin to prepare for winter in a way that reminds me of the animals that hibernate. Like the bears that stop eating and go to sleep till spring, there is a shutting down of the nutritional process in trees. The all-important photosynthesis stops when the seasonal formation of a cellular membrane called the abscission layer blocks the flow of nutrients into leaves and prevents the formation of chlorophyll. In the absence of the dominant green chlorophyll, the xanthophylls and carotenoids emerge, and leaves turn yellow or orange. In addition to these, red and purple anthocyanin pigments are created from trapped sugars in trees like maples and dogwoods, widening the spectrum of autumn’s display. These gorgeous pigments are the ones most dependent on the dry weather and crisp nights that close the veins going into the leaf, preventing sugars from moving out. When many colors blend, you get the rich browns that characterize oak leaves. After a hard freeze, any leaves still clinging to tree branches turn the duller brown color of the remaining tannins.
Dr. Meriwether, my meteorologist mentor at the National Oceanic and Atmospheric Association (NOAA), assures me that the drought has had nothing to do with the timing of our autumn, but that longer nights are responsible for when fall comes. And temperature has a lot to do with the change in leaf color. As a result of climate change, the onset of cool weather and leaf change has been getting later every year. Drought and other severe weather abnormalities are an effect of climate change, not a cause. More severe droughts than our own, notably in Africa, are dire results of this trend.
To give you an example of what has happened in our own area, the U.S. Department of Agriculture’s Plant Hardiness Zone Map listed Hyattsville in zone 6b in 1990, with a winter low temperature of -5 degrees Fahrenheit. In the latest map (2012), Hyattsville was placed in the warmer zone 7a, with its lowest temperature being 0 degrees Fahrenheit. When a new map comes out, I’m sure our zone will be changed to a still warmer one.
To discuss these and other ecological concerns, please come to the next meeting of the Hyattsville Horticultural Society on Saturday, Nov.16 at the lovely home of Heather Olsen, 4915 42nd Avenue. The meeting begins at 10 a.m.