I love fall. The cool weather, the large comfy sweaters and the hot apple cider are just a few of my favorite things about this time of year. The leaves, however, have to be the best part. The colors are spectacular on a hike, and the crunch of leaves under your boots is a pleasant song to walk to. This sound came rather early this year due to the very dry summer, and it made me wonder why leaves change color and fall in the first place.
Well to answer this question, I had to go back to science class. At some point in grade school, we were all taught about this fantastic plant process called photosynthesis. Photosynthesis means “put together with light.” The leaves of a plant absorb sunlight and breath in carbon dioxide from the atmosphere. Using chlorophyll, a natural chemical that gives the leaves their green color, and the water absorbed through the tree’s roots, the leaf creates oxygen, which it releases back into the atmosphere, and glucose (sugar) to store as food.
When the days become shorter, and the tree sees less and less sunlight each day, the tree starts to prepare for winter. Trees hibernate through the winter and live off the glucose they have stored from the summer. With the food-making process stopped, the chlorophyll is no longer needed and the tree stops producing it. The absence of chlorophyll reveals the natural yellow and orange color of the leaf’s carotenoid that is normally hidden by the sugar-making chemical. Brown leaves are a result of the absence of chlorophyll and the presence of waste in the leaf. Reds and purples are created by leftover sugar in the leaf that reacts to the sunlight and changes its color. So, the warmer yellows and oranges come out as the leaf stops making food, and the deeper and richer reds and purples come from the leftover food in the leaf.
The leaves then fall off the tree because they are no longer being used, and the tree needs to cover up its plant tissue to survive the winter. This process started happening early this year, because the lack of water affected the food-making process. With little water, the leaves could not create glucose and were not being used. The trees went into survival mode and dropped the leaves, sealing off its plant tissue to preserve its sap.
Regardless of how the leaves fall, they continue to nurture the tree. As the dead leaves decompose, they act as mulch for the soil around the tree's base. For tips about getting the most nutrients out of your dead leaves, read our fall garden clean up blog.
