Tuesday, September 30, 2014

North Country Fall Color ‘Hoax’ Revealed

By PAUL HETZLER   reposted with permission
The sunny days and cool nights we’ve had recently are creating an especially intense “art show” across our hardwood forests. We’re fortunate in New York—the northeastern U.S. is one of the few places on the Planet Earth where trees produce such a phantasmagoria of color. You’d have to go to northern China or Japan to see anything close to what we have here.
The autumn foliage in northern New York is approaching peak color, and it’s time once again to explore just how this explosion of pigment came to be. Very few people alive today remember, but years ago we only had orange fall color—thanks to carotenoids in the leaves—with no yellows or reds, which are caused by xanthophylls and anthocyanins, respectively.
Then in the 1930s the Hoover Administration rolled out new leaf-color enhancement legislation to boost tourism in the northeast as a response to the Great Depression. It was called the Hoover Omnibus Anthocyanin and Xanthophyll bill, or HOAX.

OK, it’s unlikely it happened that way, but hey, I wasn’t alive then, so who knows? We know more about fall color today than when I was a kid, but we still don’t understand it all.
Most of us were taught in school that during the growing season, dark green chlorophyll masks the lighter yellow and orange compounds that naturally occur in leaves. Each fall, trees actively seal the vascular connections between twigs and leaves with a waxy abscission layer. This kills off chlorophyll and reveals yellow and orange.
Every tree species has its own paint palette. Birches turn brilliant yellow; serviceberry and many poplars often give us a mix of oranges and yellow. But whence come red leaves? This is the mystery. We know that relatively few tree species create red fall colors. Red and sugar maples are renowned for their ruddy foliage. Some oak species produce a deep scarlet, and our native white ash can make an intense red-purple hue.
The chemicals responsible for the red and purple range are called anthocyanins. These are large, complex organic molecules that take a lot of energy to create. Many plants invest in them in springtime to protect their tender emerging leaves from UV damage. Once the foliage hardens off, the plant stops making this “expensive” compound, and the anthocyanins break down. That early-season outlay makes sense, but what about in autumn?
Chlorophyll is more vulnerable to UV damage at cold temperatures, and you may read that anthocyanin is produced in the fall to protect chlorophyll from UV rays. Sorry, but I don’t buy that.
Renowned as frugal and pragmatic creatures, trees don’t expend energy without a dang good reason. It seems far-fetched they’d use precious stored energy to protect dying chlorophyll at the same time they’re busy making the abscission layers that are killing said chlorophyll. If the “fall suntan lotion” explanation is correct, maples would turn red at roughly the same time, with all leaves coloring evenly through the crown, and in all weather conditions.
One thought is that it’s a strategy to change soil conditions under its drip line to favor its species. Certain natural chemicals (including anthocyanins) made by plants can inhibit the growth, or the seed germination, of other species. This is known as allelopathy. Perhaps maples and oaks are trying to limit competition from their neighbors. The problem is that anthocyanins are weakly allelopathic. Also, maple seeds are intended to disperse on the wind; acorns via animals. Leaves fall close to the parent tree, where they don’t want loads of their progeny.
Someone told me that one of the most important phrases for an educator to learn is “I don’t know.” Well, I don’t know why foliage turns red in addition to yellow and orange each fall. But whether the reason is a conspiracy or just a mystery, I’m sure glad it does.
Paul Hetzler is a forester and Cornell Cooperative Extension of St. Lawrence County horticulture and natural resources educator.