According to Nancy Gray, a park ranger at Great Smoky Mountains National Park, more than 1.2 million people visited the 800-square-mile sylvan paradise in the southern Appalachians in October 2000, "mostly on weekends and in the last two weeks of the month" when leaf color peaks. "We have more than 100 species of native trees, most of which change color," Gray says. "It's pretty spectacular." Combine leaf color with blue skies and clear vistas, and "people do come to the mountains now." If a million people visited Great Smoky in October, a lot more turned out nationwide.
Gray says the demographics of fall visitors also changes compared to summer tourists: she sees fewer kids. Oohing and ahhing over leaves is something adults like to do, without distractions from bored children whose idea of a weekend away centers on water slides, fast food, and noise.
Explaining How is Easy
One of the ways plants protect themselves and conserve water in winter is to jettison tender leaves, whose high surface to volume ratio makes for good photosynthesis but renders leaves sensitive to freezing and desiccation. So, as night length increases in late summer and fall, leaves senesce. The chloroplast bound system of photosynthetic proteins and pigments disorganizes and degrades, leading to the eventual disappearance of green chlorophyll. In the final coup de grace, a specialized layer of cells called an abscission zone forms across the stem-like base of the leaf, or petiole. The zone acts like a weak link; in response to wind and gravity, the leaf breaks off and falls.
Look out over a fall forest and you'll see two basic mechanisms of color change. In oaks, hickories, and tulip trees, the receding green chlorophyll unmasks yellow and orange accessory pigments called carotenoids and xanthophylls, which normally function as antennae to funnel light energy to photosynthetic reaction centers, and to draw off excess energy that could damage the system. The pigments are lipid soluble and reside in degrading chloroplasts.
Other trees and shrubs, including dogwood and sugar maple, turn red and purple due to synthesis of water soluble pigments housed in another organelle, the vacuole. Plants make leaf anthocyanins, belonging to the flavonoid family of pigments, from sugar synthesized during bright autumn days.
Deciphering Why is Harder
A team consisting of graduate student Taylor Feild, David Lee, and Michele Holbrook wondered whether anthocyanins do anything for red-osier dogwood, a shrubby species related to the common woodland tree, Cornus florida. Lee, a professor of biological sciences at Florida International, has been interested in anthocyanins for a long time and decided to pursue the matter during a research leave in Holbrook's lab at Harvard.
The researchers took advantage of the fact that open grown red-osier dogwood leaves turn reddish purple in the fall, while those shaded by a canopy of overlying trees turn yellow, with little or no anthocyanin. Leaves of the red type accumulate anthocyanin in the upper layers of photosynthetic cells, called palisade (turn a red leaf over and it still looks green). In other words, the two kinds of leaves comprise a natural test of anthocyanin function.
When the experiments were done in mid-August to late September, as anthocyanin accumulated (but prior to actual chloroplast breakdown), both kinds of leaves had about 70 percent of the normal chlorophyll content seen during summer (July) and showed no discernible structural differences. However, when the researchers applied light to the top surface of the leaves, the photosynthetic cells below the palisade showed 50 percent greater levels of activity in red-turning leaves than in yellow. When bright light was turned off, photosynthesis in the red form recovered but that in the yellow type remained low, indicating permanent damage.
Since anthocyanins preferentially absorb blue light, the Harvard team then applied blue vs. red light to both leaves. Again, blue damaged the yellow form more than the red form. On the other hand, red light, which passes through the anthocyanin-enriched layer, reduced photosynthesis in both forms and recovery lagged equally.
Lee and co-workers think open grown red-osier leaves accumulate anthocyanins in advance of chloroplast senescence so the pigments will be in place to prevent oxidative damage brought on by light later on. Destruction of leaf photosystems liberates lots of free chlorophyll, which if excited, produces reactive oxygen species including free radicals and peroxide. By filtering out blue and green light, anthocyanins prevent chlorophyll excitation, thereby avoiding toxic reactive oxygen.
Why is that important? Species such as dogwood recycle much of the nitrogen and other constituents of leaves before they fall, importing them back into the stems and roots. The research team hypothesizes that reactive oxygen species could poison the recovery process. In other words, anthocyanins act as sunscreen to block light induced damage that would upset retrieval of nutrients important for next year's growth.
There's More to Learn
Holbrook also admits that what happens in red-osier dogwood might not be true for other anthocyanin accumulating plants. "I wouldn't want to generalize yet," she says. "Many of the maples are different--anthocyanin comes on as the chloroplasts degenerate, not before." Lee agrees: "There's not a lot of literature in this area, so we have to be cautious." Still, ongoing studies seem to support their hypothesis. In a survey of 18 forest species, most produce anthocyanins even if a red color isn't visible to the unaided eye. And almost all of them show the same pigment distribution, with anthocyanin in the palisade layer. "It's a pretty widespread phenomenon," enthuses Lee. The work will be submitted for publication soon.
Lee and Holbrook haven't ruled out the possibility that anthocyanins play more than one role in senescing leaves. Lee says that anthocyanins directly scavenge free radicals in vitro and in vivo, citing recent work by Kevin Gould in New Zealand, so they may mop up any reactive oxygen that does manage to form. The real test will be determining whether anthocyanin production correlates with leaf tissue nutrient levels. Lee predicts lower levels of nitrogen in senescing red leaves compared to yellow ones.
While researchers worked on fall color biochemistry and function in Cambridge, millions streamed to the mountains to enjoy the end product of the process. Whether they were viewing sugar maples in Vermont or sourwoods in North Carolina, people may not know why leaves are so beautiful, but does it really matter? Everyone's in awe of nature's majesty nonetheless.