The Times They Are a’Changin’… and So Are the Leaves

October is here, and the leaves of many trees are changing color. You may have learned in school that leaves are green due to the presence of chlorophyll, the plant chemical responsible for photosynthesis – the process plants use to make sugars from light, water, and carbon dioxide. Chlorophyll is dominant enough to hide the other compounds in the leaves, generally reds and yellows. As chlorophyll degrades in the fall, the other colors appear, and we see them as gloriously brilliant hues.

Plant scientists pretty much understand HOW leaves turn red in the fall. What is still unknown is WHY? Trees do not turn colors in order to boost tourism income from forest-visiting “leaf-peepers.” They don’t change just because it gives us pleasure. And why would leaves change color just before they stop functioning and fall? They change because there is adaptive value to the organism itself, that is, somehow changing leaf color benefits the tree.

There are two main hypotheses about why this happens. The first is that red pigments, anthocyanins, act as a sunscreen to shield leaf tissue against harmful effects of light in low temperatures. With cooler autumn weather, it becomes harder for leaves to capture and process CO2. At the same time, there’s more light shining on leaf surfaces because nearby trees are losing their leaves. The imbalance between too much light and not enough energy to perform photosynthesis creates “oxidative stress” – cellular damage that comes from overexposure to what are called ROS, or reactive oxygen species. You may have heard the term “free radicals.” In human bodies, oxidative stress is managed by antioxidants like glutathione. In many tree leaves, it’s likely managed by anthocyanins.

Why would a tree bother to try to protect a leaf that’s about to fall anyway? Because fully functional leaves absorb crucial nutrients like nitrogen and phosphorus better.

The second hypothesis about why leaves change color is the coevolution hypothesis. This idea suggests that red coloration presents a signal to insects that these leaves are not suitable hosts – that they have high levels of chemical protection, or won’t provide high nutritional value. Aphids, for instance, migrate from herbaceous plants in the summer to trees in the fall, to lay eggs on buds, so newly hatched aphids can start their lives in the spring with food nearby. It is in an aphid’s best interest to find the most suitable host – a healthy green leaf next to a bud – to migrate to. Aphids, like many insects, perceive color, and a red leaf could act as a signal that it is not the best place to raise the next aphid generation. A tree that is able to ward off insects by convincing spring herbivores to go elsewhere for the winter has saved itself future damage.

Right now, published research on both of these hypotheses is contradictory; we don’t yet know exactly why trees turn color. Plant physiologists, biochemists, and evolutionary ecologists are working together to understand how changing colors benefits a tree. Happily, you and I don’t have to understand all the details of the fall color show to enjoy it!

Article by Kerrie Kyde, FCFB, 10/1/2026