Why Do Leaves Change Color?

We all enjoy the colors of autumn leaves. The changing fall foliage never fails to surprise and delight us. Did you ever wonder how and why a fall leaf changes color? Why a maple leaf turns bright red? Where do the yellows and oranges come from? To answer those questions, we first have to understand what leaves are and what they do. Plants take water from the ground through their roots. They take carbon dioxide (CO2) from the air. Plants use sunlight to turn water and carbon dioxide into oxygen and glucose. Oxygen is released back into the air. Glucose is a kind of sugar. Plants use glucose as food for energy and as a building block for growing. Photosynthesis means "putting together with light" and it is the way that plants create sugar and oxygen from CO2 & H2O. A chemical called chlorophyll helps make photosynthesis happen. Chlorophyll is what gives plants their green color. As summer ends and autumn comes, the days get shorter and shorter. This is how the trees "know" to begin getting ready for winter. During winter, there is not enough light or water for photosynthesis. The trees will rest, and live off the food they stored during the summer. They begin to shut down their food-making factories. The green chlorophyll disappears from the leaves. As the bright green fades away, we begin to see yellow and orange colors. Small amounts of these colors have been in the leaves all along. We just can't see them in the summer, because they are covered up by the green chlorophyll. Since daylight wanes at a constant rate each fall, other factors, like soil moisture and weather, ensure that no two autumns are alike. Warm, sunny days mixed with cool, but above-freezing nights appear to produce the brilliant red hues associated with peak fall foliage. Leaf-peepers (people who annually follow the fall foliage) judge whether it has been a "good" or "bad" fall based on the proportion of red leaves—the more, the better. Other things being equal, that [ratio] changes more than anything else. The real question is: What's going on with these reds?" Native American lore explained the appearance of the magnificent colors as coming from above: After hunters killed the Great Bear in the sky, the story goes, its blood splashed down and turned the leaves red. And the yellow leaves? They get their tint from the bear's fat splashing out of the pot that it was being cooked in. The truth is the bright reds and purples we see in leaves are made mostly in the fall. In some trees, like maples, glucose is trapped in the leaves after photosynthesis stops. Sunlight and the cool nights of autumn cause the leaves to turn this glucose into a red color. The brown color of trees like oaks is made from wastes left in the leaves. Although scientists offer several different reasons for why some trees produce anthocyanins, they are responsible for the red colors. (The molecular structure of an anthocyanin includes a sugar, which is dependent on the availability of carbohydrates within a plant. Anthocyanin color changes with pH, so soil acidity affects leaf color. Anthocyanin production also requires light, so sunny days are needed for the brightest fall colors! The reason you'll see more vibrant reds during some years is that lots of sunlight and dry weather increase the sugar concentration in tree sap, triggering the tree to release more anthocyanins in a last-ditch effort to gather up energy to get through the winter. In addition, near-freezing weather, low nutrient levels and other plant stressors seem to trigger increased levels of anthocyanins. If it's been especially rainy and overcast, you won't see much red foliage. Without bright sunlight, the trees don't need the added protection that the red pigments provide, so they don't bother producing them. Here's a guide to the species of trees and the color they produce during autumn: Yellow: Green and black ash, basswood, beech, birches, butternut, and elm. In the maple species - boxelder, mountain, silver, striped and sugar. And don't forget mountain ash, poplar, serviceberry, willow, and witch hazel. Red and Scarlet: Red, mountain, and sugar maples; black, red, scarlet and white oak; hornbeam, sumac and tupelo. Brown: White and black oak Purple: White ash and witch hazel.