It’s beginning to feel a lot like autumn around the Bruce Museum. There are many things to enjoy about fall, from apple cider to pumpkin picking, and the rich reds and golds of the trees are a true delight. These color changes might be beautiful, but what they actually reflect is a series of biological and chemical processes that help prepare a tree for winter.
During most of the year, a tree’s leaves are a vibrant green. This color comes from the pigment chlorophyll, a vital part of the photosynthetic pathway. During photosynthesis, carbon dioxide and water are converted into food molecules like sugar. Chlorophyll absorbs sunlight, which provides the energy to power this process.
When days begin to shorten in fall, trees begin to break down and pack away their chlorophyll for the next season. Chlorophyll is very energy-expensive to produce, so it’s in a tree’s best interest to reuse as much as they can. They transfer the component molecules that make chlorophyll into permanent parts of the tree, ready to be put back together when new leaves are growing.
Though chlorophyll is the dominant pigment in most leaves, it is not the only one. Chlorophyll reflects light in the blue-green part of the spectrum, so plants use other pigments to absorb those wavelengths. One of those pigments is carotene, which helps create the color of carrots. When leaves are full of chlorophyll, the yellow of carotene isn’t visible. However, once chlorophyll begins to break down, the yellow of carotene begins to show.
Other leaf pigments bring more colors to the fall spectrum. Some trees, like maples, oaks, and dogwood, produce the red pigment anthocyanin. Unlike carotene, anthocyanin is only produced in autumn. The purpose of this pigment is unclear, but some scientists think it acts as sunscreen for the leaves, preventing damage and allowing the leaves to function a little longer. Different combinations of anthocyanin and other pigments can produce purples as well.
Eventually, even the beautiful colors of autumn begin to fade. Sunlight and early frost will damage pigments, eventually leaving behind only the durable brown pigment, tannin. As leaf cells break down and degrade, the leaves become dry and crunchy. Soon, the trees become bare, all their leaves littering the ground around them.
When it is time for leaves to fall, a release of hormones triggers the formation of a special layer of “abscission cells” in leaf stems. Abscission cells slowly cut off circulation between the leaf and the rest of the tree and push the leaf away from the main stem. Eventually the connection is so tenuous that the leaf can be pulled free by a gust of wind, or the vigorous shaking of a curious youngster.
For most trees, it is quite beneficial to shed leaves before winter. Winter has few hours of sunlight available for photosynthesis, and cold slows down photosynthesis even further. Freezing can kill leaves, so an oak that didn’t drop its leaves would probably lose them all anyways.
What about evergreen trees like pines and holly that keep their leaves throughout winter? They use a variety of strategies to protect their leaves from frost damage, but that subject is best saved for another article.
Kate Dzikiewicz, Paul Griswold Howes Fellow