The brilliantly colored leaves are beginning to emerge this time of year in the Sawtooths and at many of the surrounding Idaho mountain ranges. The science taking place behind Mother Nature’s yearly spectacle is easy to overlook. Here is a brief summer of the science behind our fabulous fall colors.
The green color comes from chlorophyll which the tree uses during photosynthesis- This is the process through which plants more or less eat. Trees break down carbon dioxide and water and turn them into oxygen and glucose.
Chlorophyll assists in bringing in sunlight for photosynthesis to occur, keeping the leaf vibrant and green. When the days become shorter the tree no longer wastes energy on the leaves -more or less, they are “let go” from the process.
This “letting go” process begins when a corky layer of cells known as the abscission layer forms between the tree and the individual leaves. This layer protects the branch when it becomes exposed to open air when the leaves begin to fall. The layers also disrupt the flow of nutrients bringing about the emerging colors.
There are two chemicals responsible for the colors we see on autumn foliage. Carotenoids (orange and yellow shades) and anthocyanins (red and purple hues). The carotenoids are present in the leaf from the beginning. When chlorophyll stops, the green fades and the orange and yellows begin to pop out. With anthocyanin, it forms from the glucose stored in the remaining, fading chlorophyll. The glucose becomes trapped and forms anthocyanin.
Each season is unique as the balance inside the leaf changes. The carotenoids, and the anthocyanins in combination with the remaining chlorophyll, change every year. These chemical processes are dependent on temperature, moisture, and sunlight. It explains why sometimes we see a leaf with red coloring on one side and more yellow on the other. A result of one side receiving more sunlight.
The final chemical to enter the leaf cycle is tannin. Tannins are found in the membranes of the cells within the leaves and they never fade. It is the final brown color present in late autumn.