Biology 103
2001 Second Web Report
On Serendip

In the last few weeks, you might have (like me) walked around in a blissful daze, eyes fixed on the magnificent view above your head. That's right, it's fall, and the leaves are once again incredible. Glorious fall foliage like we have here sure makes you feel lucky to be alive, doesn't it? (If it doesn't, well, you're just a horrible person.) Well, you're luckier than you might think. While driving back to school with my mom one weekend, we were ooh-ing and ah-ing over the spectacular leaves when I received a shock - my mom said something to the effect that brilliant foliage like the kind bordering the highway existed only in North America - oh, other parts of the world had deciduous trees, of course, and their leaves changed colors in the fall, but half-heartedly, nothing like what we see every year. I was flabbergasted - and yet also intrigued. Is it true? I wondered, and if so, then why? So I set out on my quest for enlightenment, bravely surfing the uncharted regions of the internet. And this is what I found…

First of all, my mom was not entirely correct (big surprise, you probably think, but it really is amazing how much moms know - it's in the job description). Besides the U.S.A. and Canada, Japan apparently has some pretty impressive fall foliage, which draws a lot of tourism every fall, and so do certain areas of China. Furthermore, not all of North America has super-cool foliage. Obviously it doesn't exist in the deserts or prairies or the frozen tundras - there are no trees there - and equally obviously, not in coniferous forests. But an area can have plenty of nice deciduous trees and still have nonexistent or sub-par fall colors. The question remained: why should autumn be beautiful and awe-inspiring in some areas and not in others?

First, let's go back to second grade. Remember how every year in elementary school, they'd teach you why leaves changed colors in the fall, and every year you would forget it after about ten hours? I had forgotten what it was that made leaves change color in the first place, although I seemed to remember that it had something to do with chlorophyll. So I set out to refresh my memory. First, I reviewed the basics of photosynthesis. As you probably already know, photosynthesis is the process whereby plants use sunlight to convert carbon dioxide and water into oxygen and carbohydrates (which have a “general formula” of Cx(H2O)y). The formula for photosynthesis is:

And how do these plants magically capture the sunlight needed for photosynthesis? Why, with the help of chlorophyll, of course! Chlorophyll absorbs red and blue light from the sun's rays which hit the leaves of trees, and reflects the color green. The absorbed light is used in photosynthesis, as seen above, and the reflection of green light is the reason why leaves appear green. The formula for chlorophyll molecules is C55H70MgN4O6 - quite large, and they attach to the membranes of chloroplasts within the cell (the process of photosynthesis takes place within the chloroplasts).(2). Chlorophyll is not stable; bright sunlight causes it to decompose. Thus, chlorophyll must constantly be synthesized - an effort which requires sunlight and warm temperature.(2). Autumn, with its waning daylight and falling temperatures, obviously presents a challenge.

At a certain point, the effort required to maintain chlorophyll is just too taxing for even the sturdiest of deciduous trees. In winter, there is not enough water or light for photosynthesis. The trees shut down their “food-making factories” (they stop producing chlorophyll)(1) and live off food they stored during the summer. At this time, then, the green color disappears from the leaves. However, chlorophyll is not the only chemical found in tree leaves. Many trees also contain the compound carotene in their chloroplasts.(1) Carotene (C40H36) acts as an accessory light absorber - while chlorophyll absorbed red and blue light, carotene absorbs blue-green and blue light - and appears yellow. Carotene is much more stable than chlorophyll; thus it remains visible in autumn leaves when chlorophyll is gone.(2) Anthocyanins absorb blue, blue-green, and green light, and appear red. They are responsible for the deep reds of some maple leaves, as well as for the red skin of apples and the purple skin of grapes.(2). Anthocyanins are not attached to cell membranes, but are rather dissolved in cell sap. The appearance of anthocyanins is also quite variable, as it is very sensitive to the pH of its cell sap - more acidic sap produces a red color, less acidic sap creates a purple shade.(2)

So I found out all this stuff about why and how leaves change color, but I still had no clue as to why we get a good color-show and other regions don't. Well, as I continued my research, it soon became apparent that the range and intensity of fall foliage is influenced by the weather. For instance, dry weather increases sugar concentration in sap and therefore increases anthocyanins.(2) Low temperatures also destroy chlorophyll and enhance anthocyanins, so it follows that cold weather is conducive to having cool fall colors. However, there is a catch - the temperatures must be cold but not freezing. Frost and super-cold temperatures can kill or injure the leaves before their pigments can properly develop.(3) So the weather has to be dry and cold (but not freezing) and also sunny. You see, even as the amount of chlorophyll is declining, some photosynthesis still occurs in the leaves during the fall. If the weather is too cloudy or rainy in the fall, photosynthesis is limited and the intensity of the leaves' colors is diminished.(3). Ideal conditions for eye-popping, show-stopping fall foliage are warm and dry sunny days followed by nights which are dry and below 45° Fahrenheit, but not freezing. And there's still the matter of just having the right kind of plants - nice color-changing broad-leafed deciduous trees.

Northeastern America is extraordinarily lucky in this regard - England, for instance, has many of the same trees we have, but their autumn colors are often muted and dull - nothing like what we enjoy every year.(3). New England is not the only region to enjoy such autumnal good luck of course - southeastern Canada, parts of the mountainous West, Seattle, Portland, San Francisco, Japan, and regions of China also boast brilliant fall displays.

It turns out I'm ten times luckier than I ever thought I was - besides being alive and not living on the streets with no food and only a cardboard box for shelter, and living in a free country where it's illegal for people to stone me to death or burn down my house because they don't like my religion, etc. etc. and so forth…I get to enjoy beautiful autumn leaves! So get out there and enjoy the beauty of nature while the extraordinary, impressive, gorgeous leaves are still there! Have fun! See ya outside.

WWW Sources

2)The Chemistry of Autumn Colors, Featured "chemical of the week." The title pretty much explains it.

3)Fantasy, Facts and Fall Color, Intelligent article written by a professor of "Tree Physiology." Explores mythology behind changing fall colors and explains in detail the various factors that affect the "prettiness" of autumn colors.

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