Why Do Leaves Change Color in the Fall?

For a large part of the year, leaves are a tree's workhorses, constantly converting carbon dioxide, water and sunlight into energy in a process called photosynthesis. The special ingredient for this process, the pigment chlorophyll, is w­hat gives leaves their bright, green color for much of the year. But while chlorophyll is the star of the show, it has some help in the form of the pigments carotene and xanthophyll. Xantho is Greek for "yellow," and carotene is what gives items like carrots and egg yolks their orangish color. These two pigments are always present in leaves and help absorb sunlight, which they transfer to chlorophyll for photosynthesis.

As summer nears its end and days get shorter, the increased amount of darkness incites trees to prepare for a sort of hibernation. Leaves won't be able to continue photosynthesizing during winter due to the dry air and lack of sunlight, so the tree does two things. First, it forms a separation layer made of corklike cells at the base of each leaf to seal it off from the tree. Second, it stops producing chlorophyll since it won't need this pigment until the days start to lengthen once again in the spring. With chlorophyll out of the picture, the yellow and orange pigments get a chance to shine.

The red hues, which come from pigments called anthocyanins, are slightly more complicated. Whereas all trees contain chlorophyll, carotene and xanthophyll, not all of them produce anthocyanins. Even the ones that do have anthocyanins only produce it under certain circumstances.

Remember that layer of cells at the base of the leaf? Its purpose is to protect the tree during the colder winter and prevent it from drying out. When the separation layer is complete, the leaves fall off in the tree's attempt to conserve energy. But before the leaves fall off and the tree closes up shop, it wants to pull in as much sugar and nutrients as possible from its leaves, which is where the anthocyanin comes in.

Although scientists offer several different reasons for why some trees produce anthocyanins and autumn leaves change color, the prevailing theory is that anthocyanins protect the leaves from excess sunlight and enable the trees to recover any last remaining nutrients. 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.

­So if autumn just isn't the same for you without the occasional splash of red, hopefully the weather will cooperate. If not, you'll just have to make due with the more reliable yellows and oranges. Or you could always join the leaf-peeping caravan in New England on its endless search for the ultimate display of color.