Fall Color
A late-season kayak trip reveals fall’s changing colors while exploring the science of chlorophyll, carotenoids, anthocyanins, and leaf loss.
As I push my kayak away from shore, I immediately notice how different the river looks from a few weeks ago. The trees that were once mostly green are now yellow, orange, red, and brown. As I paddle downstream, I begin to notice that these colors are not random. Each one tells a story about what is happening inside the leaves.
Throughout spring and summer, leaves are green because they contain a pigment called chlorophyll. Chlorophyll absorbs sunlight and allows trees to carry out photosynthesis. During this process, trees use water and carbon dioxide, along with energy from sunlight, to produce glucose and oxygen. This provides the tree with the energy it needs to grow.In fall, shorter days and cooler temperatures cause deciduous trees to stop producing chlorophyll. As the green pigment fades, other pigments become visible. Yellow and orange pigments called carotenoids are present in leaves throughout the growing season but are masked by chlorophyll. As I paddle past birches and aspens, their yellow leaves are evidence of these pigments finally being revealed.
Farther down the river, I see bright red maples. Their color comes from pigments called anthocyanins. Unlike carotenoids, anthocyanins are not present in the leaves throughout the growing season. Instead, trees such as maples and dogwoods synthesize them in the fall. The red leaves reflected in the water make the landscape feel especially vivid, even though the color is the result of a precise biological process.
At the same time, the trees are preparing to lose their leaves. As temperatures fall, most deciduous trees form a protective structure called an abscission layer between the leaves and branches. This layer cuts off the flow of fluids to the leaves, allowing them to separate and eventually fall. The leaves floating around my kayak are therefore part of the tree's natural preparation for winter.
Not every tree produces bright fall colors. Beeches and oaks turn rusty brown because their leaves contain neither the carotenoids responsible for yellow and orange nor the
anthocyanins responsible for red. When chlorophyll disappears, leftover compounds such as iron and tannins become visible. I also notice that many brown oak and beech leaves remain attached to their branches. These trees do not form the abscission layer in the same way as many other deciduous trees, allowing their leaves to cling through winter.
As I paddle back toward shore, I begin to see the fall landscape differently. The changing colors are more than a beautiful seasonal display. Green represents active photosynthesis, yellow and orange reveal carotenoids, red shows the production of anthocyanins, and brown reveals compounds left behind as chlorophyll disappears. Falling leaves mark the final stage of the process.
Fall can feel quiet and even a little bittersweet because the trees are preparing to let go. Yet what looks like an ending is actually part of a yearly cycle of survival.
