Why Trees Change Color & Why They Don't
The fall makes me happy. The fall drives me nuts. And it's all the fault of the trees.
Fall makes me happy because it's the grand consummation of what people and plants have been laboring for all year. People harvest alfalfa, soybeans, sweet potatoes, and apples. Fall flowers bloom to set seed to beget a new generation next year. Trees and shrubs glow school bus-yellow, pumpkin-orange, and stoplight-red as they retrieve nutrients made in their leaves that summer.
But it's the same fall color that drives me nuts. Every year at Southern Living, we zoom all over the South, trying to capture beautiful images of fall foliage at its peak. (The image above isn't one of them. I just took it this morning as the full moon was setting behind some Bradford pears. Pretty artsy, huh?) And no one -- and I mean no one -- can tell you if the fall color will be good this year and when the peak will happen. There's no more sinking feeling than flying somewhere to photograph fall foliage, come in for the landing, look out the window, and see nothing but green, green, green.
(Ok, Ok, being told you bear a striking resemblance to Susan Boyle is pretty devastating too, but all I had to do fix this was pluck my eyebrows and change my hair.)
It's not the same when hunting flowers. I once did a story on a whole neighborhood in Chevy Chase, MD where every street was lined on both sides with 'Yoshino' flowering cherries. This is the same cherry planted around the Tidal Basin. Getting the story meant getting to Chevy Chase at peak bloom. But this wasn't that hard because there's a National Park Service website you can go to where an expert on cherry trees examines the flowers every day and tells you with great accuracy what percentage of flowers will be in full bloom on a given day.
There's no such site for fall color. Oh sure, some tourism sites will give you a week when they think fall color will be nice, but they really don't know. Any time you go looking for fall color, it's basically a crap shoot.
Why do trees change color anyway? Scientists say the color change for trees that turn yellow and orange is basically no change at all. The pigments for those colors are always in the leaves, only masked by green chlorophyll used for photosynthesis. As trees prepare to drop their leaves, chlorophyll breaks down, and voila -- yellow and orange fall foliage.
Red fall foliage is a different matter. The chemical compound responsible for red in the leaves -- anthocyanin -- isn't produced until late summer. How come? And why is it produced at all?
Here's the deal. Traits in plants not managed by people are passed along because they provide those plants with a competitive advantage. Traits like fast growth, extensive seed production, tolerance of dry or wet soil, and resistance to disease all make it more likely that a particular plant will survive to pass along these traits to offspring, while those lacking them will not.
So what advantage do trees like Japanese maple, dogwood, Bradford pear, and black gum gain by turning red in fall? They don't do it to impress potential mates. Last time I checked, trees were all blind and pollination was pretty much random (like going on eHarmony for the first time).
I've heard two theories. One says trees turn red to chase away egg-laying aphids, because aphids don't like red. This sounds highly dubious. If it were true, wouldn't yellow-leaved trees like hickories, ginkgoes, and birches be sucked dry by aphids? And if all aphids hate red, are there no aphids in Communist North Korea?
The other theory suggests anthocyanin acts like a sunscreen for leaves, protecting them from bright autumn sun as their formerly protective chlorophyll disintegrates. This sounds plausible, because the foliage of many trees emerges bronze or reddish in spring, before turning green. The chemicals causing this brief coloration protect the young leaves from sun damage.
What triggers the color change each fall? A number of things factor into it, including shortening days and cooling temperatures. But I think weather plays a huge role. A few years ago, Birmingham experienced a record drought. I predicted terrible fall color because of it. In fact, we enjoyed one of our most spectacular falls ever. This year, we've already received over 60 inches of rain. (Sorry, people in Texas.) Fall color sucks. I think it's because all the rain prolonged the production of chlorophyll. Trees were duped. A lot of leaves just fell off green. My sugar maple out front was half-nekkid before showing any color at all.
But this is just my theory. I could be totally wrong. Like I said, no one can predict whether fall color will rock or suck. And that drives me nuts.