There are things that you think you know until someone asks you to explain them and you suddenly realize you have no idea what you’re talking about. This happened to me recently when a student asked what causes the northern lights. “Well, they’re – they’re caused by – by the sun. And the Earth’s magnetic field. Interacting. Somehow.” While not inaccurate, this is a far from satisfying answer! So when I recently realized that I don’t really know why leaves change color in autumn (“It’s because… um… because the chlorophyll goes away!”), I thought I’d better find out.
Luckily for me, the U.S. Forest Service has a nice webpage devoted to answering this exact question. Turns out there are three different pigments that play a role in fall color.
- Chlorophyll, which, as everyone knows, is green and is essential to the tree’s survival because it facilitates photosynthesis.
- Carotenoids, the familiar yellow-orange pigments that produce the color of everything from carrots to cardinals.
- Anthocyanins, water-soluble pigments found in cranberries, blueberries, etc. – apparently a deeper red-purple color.
In autumn, deciduous trees respond to the lengthening nights by halting their production of chlorophyll. As the chlorophyll is broken down, the colors of the carotenoids, which are present in chloroplasts year-round, become more visible. Trees then clog the “veins” that transport fluids and sugars in and out of the leaves, causing a build-up of excess sugar that can trigger the production of anthocyanins, which are not present year-round. (I suppose this might explain why, even on an individual sugar maple tree, some leaves are solid yellow while some are much more pink – differing amounts of anthocyanins?) Once these connecting tissues have been sealed off all it takes is a puff of wind to dislodge the leaf from the tree.
Of course, this is all the proximate cause of leaf color change and fall. What’s the ultimate cause, the reason why it’s adaptive in the first place? While stems, buds, etc. are hardy enough to withstand the harsh conditions of winter, delicate leaf tissue is not. Since there’s not much sunlight in winter to power photosynthesis anyway, energetically it makes more sense for trees to just ditch their existing leaves in the fall and grow new ones when milder conditions return in the spring.
I am not a plant physiologist, so it’s possible I’ve oversimplified this by quite a bit. Feel free to set me straight in the comments. (Hey Dr. Wolverton, you’re a plant physiologist, do you still read this…?)