It’s the last day of 2020, and the leaves have already thinned in most of the tree canopy on our property in Acadiana. In Michigan and Vermont, Mother Nature’s big swaths of color inspired foliage tours, tracking apps, calendars and jigsaw puzzles. But here in Louisiana, this comparatively anemic spurt of red stopped me in my tracks to check it out. I felt like a hummingbird.
The generic fall color story is getting around: leaves change from green to yellow and orange when production of the main photosynthetic pigment chlorophyll shuts down, leaving behind other pigments. The carotenoid molecules were there all along, slotted among the chlorophyll molecules all summer. When the green molecules break down, the plant does not replace them, leaving only the yellow and orange that shine through the otherwise clear leaf cells. Is there something feeble about chlorophyll, or something stronger about the carotenoids? No. This fall color shift happens “on purpose” as an evolutionary adaptation. The change happens in response to shorter day length, mediated by light sensitive hormones that change gene expression. But why? The evolutionary explanation is that the angle of the sunlight is lower on the horizon, especially up north. This light has fewer of the wavelengths that can excite chlorophyll, so it’s not worth keeping chlorophylls around for the little work they’re doing. Better to stop making it until the wavelengths are more suitable. In the meantime, it’s worth keeping the yellow and orange pigments that trap other wavelengths that help the leaves fix carbon dioxide and make sugar as long as they can. But as days get even shorter, even the leaves aren’t worth keeping. Leaves have millions of tiny holes (stomata) for gas exchange that are also good at pulling up water from the roots by evaporation (transpiration). If there is little liquid water in the soil because it’s all frozen as snow, the plant is at risk of dehydrating through their leaves. For most species, it’s better to drop the leaves, conserve water, and grow some more in spring. That means that falling leaves were killed off “on purpose”, not a weakness or sign of impending death. What about Christmas trees and other evergreens? Plants with needle-like leaves also have those tiny holes, but the needle shape has less surface area for dehydration, along with nice-smelling waxes and fats to prevent drying out even in the dark, cold winter.
That explains the green to yellow/orange transition. But what about the red pigments in my red oaks today? Those are anthocyanin molecules that are not part of the light-harvesting pigments in the chloroplast. They are made elsewhere in the cell from leftover trapped sugar after the veins shut off water and sugar transport. The red color does not help with photosynthesis. In fact, we aren’t sure it has any adaptive value in fall leaves. It may just be a by-product of end-of-season chemistry. Anthocyanins are more famous (and adaptive) as flower petal coloring that attracts insect pollinators. In flower petals, anthocyanins are made “on purpose”.
So by now you might feel better about Louisiana’s wimpy fall colors. Our days never get as short or long as days up north, so there is not a mad rush in a few weeks in October to cram in more photosynthesis. The sun is not as low on the horizon, so chlorophyll persists in more species. The temperatures are warmer and soil is wetter, so plants are less likely to drop their leaves to save water. And there is less likelihood that the veins will close all at once during a sunny cold snap, so we don’t get as much of the red blast. That makes our red fall color special. Not wimpy. Not feeble. It’s just like it’s meant to be.
A short video summary is at https://www.scientificamerican.com/video/why-do-autumn-leaves-change-color-2013-10-03/