I went hiking with some lab peeps a few weekends ago at Chatfield Hollow State Park. I was pretty stoked about frolicking in the New England foliage, but the bleak scenery we witnessed during the 30 minute drive didn’t seem too promising. When we arrived at the park, we were pretty bummed to see the leaves sporting a dull brown coat and surprised that many had already decided to retire, revealing bald spots to many trees’ chagrin. One friend would excitedly point out any glimpse of red that caught his eye, and we lauded these vibrant and persevering leaves with a chorus of oo’s and aa’s accompanied by a jig around the chosen tree.
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| Yuck, brown leaves. |
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| Oo, a glimpse of orange! |
Let’s take a moment here to explain how leaves change colors. As the tree grows throughout the spring and summer months, chlorophyll, which gives the leaves their green color, is constantly replaced in the leaves. During the summer, new stems grow from overwintering buds (those that passed through the winter season). Then, in the late summer/early fall, as the days get shorter and the nights longer, the cells near the juncture between the leaf and the stem divide rapidly but do not expand. This results in blocking the transport of carbohydrates (which include sugars, starches, and cellulose) from the leaf to the branch as well as the minerals from the roots into the leaves. The blockage at the juncture is known as the abscission layer. As a result, the production of chlorophyll slows down to almost a halt, allowing yellow (xanthophylls), orange (carotenoids), and red/purple (anthocyanins – produced from sugars trapped in the leaf) pigments to have their turn in the spot(sun) light and replace the verdant hues.
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| You can thank anthocyanins for the red pigments... |
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| ...and xanthophylls for the yellow ones. |
Why has the foliage been so gosh-darn disappointing this year? I’m just going to hop onto the “global warming is messing up our world” train to irk the Tea Partiers out there (they’re also messing up the world, but that’s another story). Basically, temperature, sunlight, and soil moisture majorly impact the quality of foliage. The growing season occurs during the summer, and ideal conditions for a dazzling foliage display involve ample rainfall in the spring, followed by a warm and fairly dry summer, and concluding with dry, warm sunny days and cool (but not freezing) nights in early autumn. The most important factor is the slow and steady onset of cool weather.
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| Foliage from two years ago, end of October. |
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| Foliage from this year, end of October. |
However, this year’s conditions in New England were less than ideal – there was ample rain in the spring, but hot and humid drought-like summer combined with heavy precipitation in the fall, which most likely disrupted the photosynthesis cycle and thus delayed the growth of the leaves. A drought during the growing season can cause the abscission layer to form early and cause the leaves to drop before they change color. In late August, Hurricane Irene trampled the Northeast, and the heavy winds and rain that accompanied her caused the leaves to fall before they fully developed color. Heavy rainfall near the end of the growing season does not help, as many of the junctures are blocked, so the water cannot circulate throughout the plant or get absorb properly from the soil via the roots. So, yes, blame the abnormal weather for the sepia-toned foliage this fall. And Sarah Palin. Ok, blame the political undertones on my recent science policy trip to D.C!
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| Aftermath of Irene. Many leaves were already blown off the trees by September. |
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| It snowed in late October - no hope for beautiful foliage. |
By the way, the reason for why leaves change colors in the fall is still not completely understood. Read Carl Zimmer’s post about speculations as to why trees even bother doing this.
Also, if you are interested in science policy and reforming science education, be sure to check out the Yale Science Diplomat's blog! And while you're there, read my post on our trip to D.C. to explore science career opportunities.
