Fall Colors

Fall is closing in on us rapidly. The past two days have been less sunny, more rainy, and the trees have just begun tinge yellow and red. My friend Katie texted me today and asked "What makes the leaves change colors in the fall?" I pondered how I would answer this in less than 160 characters, then wrote this text:

"The trees are withdrawing nutrients from the leaves before they shed them. When green chlorophyll is broken down you can see other colored pigments!"*

*yes, I am one of those obnoxious people who refuse to abbreviate in text messages. I have a keyboard and goshdarnit, I'm going to use it.

There's a little bit more to it than that, as you might imagine, but not much more. When the day length begins to shorten, the light to weaken, and the temperatures to drop, deciduous trees (trees that shed their leaves in the fall) pass through an important transition to survive the upcoming winter. In the warmer months, leaves are the solar collectors and energy factories of the trees, but in winter, the trees will retreat into a quiescent state of tightly clenched buds and bare stems, and their leaves will be unnecessary to their long winter sleep. Leaves will only make the trees more susceptible to damage in the coming snowfall anyway (see photo here of results of early October snowfall in central Pennsylvania last year). But the leaves represent a lot of energetic investment on the part of the plant, particularly nitrogen, which the tree will need to survive the winter ahead. So as the trees bundle up for winter they pull nutrients out of the leaves to store up for the winter.



Here's how it happens. In the beginning of the fall, trees will gradually shut off the production of chlorophyll, their green pigment and secret to their photosynthetic magic. The remaining chlorophyll and its accompanying photosystems in the leaf tissue will slowly be digested and the nitrogen-rich pieces sucked back into the trunk and roots of the tree. (Imagine the roots of a tree are a basement for sealing up canned goods to be eaten over the winter.) The vibrant green of chlorophyll fades from the leaf to reveal the bright yellows and oranges of carotenoid pigments, accessory pigments that harvest light much like chlorophyll but at a different wavelength. Carotenoids also protect the delicate chlorophyll from UV and oxidative damage and thus are really good for you too in things like carrots (something your mom always told you but you never really wanted to listen to).

Carotenoids are also highly attractive and earn sleepy little wooded hillsides seasonal fame for their spectacular colors every autumn. But now I really digress. Anyway, the bright carotenoids have been there in the leaf all along, helping with photosynthesis and protecting chlorophyll. These pigments are just more stable than chlorophyll, and will persist longer in the leaf after resources have been withdrawn in the fall.

Now, that's yellow and orange leaves explained. Check. But what about red? If you were anything like me in my first fall in the Northeast, you probably searched the forest floor for the brightest, most cheery red-colored leaf you could find, held it tightly in your little gloved hand and marveled at just how very red that leaf shined against the grey days ahead.

Red leaves are more complicated. Maybe we can talk about them tomorrow.

Photo by Flickr user mmwm, licensed for non-commercial use by Creative Commons License.