At first it’s subtle. A sprinkle of yellow on the leaves, barely noticeable; almost like leftover sunshine. Then, it grows brighter, and soon whole leaves have changed to gold. The gold deepens; orange, brilliant and bold, lights up like fire on the treetops. Then, the orange fades into majestic royal reds. As a kid, I remember skipping down the yellow leaf littered ground and pretending it was a yellow brick road.
Sometimes people become so used to certain things occurring that they never wonder enough to find out why they happen. The changing fall leaves was a familiar experience for me that I simply took for granted; and so I have finally decided to do some research on the science behind this incredible feat of nature.
The color of the leaves is more than just a nice decorative touch — they are crucial to the growth and survival of the tree. A green chemical in the leaves, called chlorophyll, is where the tree synthesizes necessary nutrients. The process whereby this occurs is known as photosynthesis. When the sunlight is absorbed through the leaves, chlorophyll uses the energy to convert the carbon dioxide (CO2) taken from the air, and the water (H2O) taken from the soil, into sugar. Chlorophyll is continually being used up and replenished. When autumn begins there is not enough sunlight and water for photosynthesis to occur. And as the nights lengthen in time, the trees respond to the lessened hours of daylight by slowing down chlorophyll production to a halt. When winter comes, the tree will feed on already stored nutrients.
As the chlorophyll starts to disappear, the carotenoids in the leaves that had been hidden behind the green of the chlorophyll, become more apparent. Carotenoids, which produce a range of yellow, orange and brown colors, are also found in corn, carrots and bananas. The disappearance of the chlorophyll allows for the leaves to begin to change their appearance, with bright colors of yellow and orange becoming more prominent. The shorter days and lower temperatures trigger the veins of the leaves in the stem to slowly close up. This prevents further nutrients from being brought to the leaves. The brilliant red color sometimes seen on certain trees comes from this closing of the veins. Glucose then becomes trapped in the leaves, and with sunlight and a cool temperature, the glucose is broken down into anthocyanins, the pigment found in cranberries, cherries, red apples and strawberries. This beautiful red color comes out more often in lower temperatures and occurs more frequently to specific tree species.
The time of year the color arises depends on the genetics of the species and the latitude of the area. However, the intensity of the color depends on the weather before and during the arrival of autumn. A warm and wet spring helps bring more color to the fall. A drought in the summer can postpone color arrival. During the time of the chlorophyll diminishment, sunny, warm days as well as cool nights help improve the intensity of leaf color. Rainy days also promote leaf color.
Unlike deciduous trees, evergreen trees are able to maintain their leaves even during the winter. Their wax coating and antifreeze fluid in their leaves helps them survive cold winter days. Deciduous trees, on the other hand, grow a layer of cells at the base of the leaf stem, separating the leaf from the tree. These fallen leaves decompose and provide nutrients to the soil, thus working to maintain the ecosystem.
The slow coloring of the leaves creeps up on us, gently and deliberately, hinting to the changing of the seasons and the passage of time. In the comparison of the seasons to a lifetime, the autumn represents older age and approaching death. Yet the beautiful, brilliant golden colors suggest the beauty even at the end of life. Though they do not last long, their bright hues and many vibrant shades bring splashes of color to our outdoor walks before the cold days of winter. Make your way to Central Park and enjoy the delight. Soon the crisp crunch of leaves under our boots will tell of a bittersweet end to a season.