It's fall again and the leaves in many places are already starting to change from shades of green to reds, oranges, yellows, purples, and everything in between. Fall is a wonderful time for a drive through the mountains, or a vacation to view the fall foliage. But no matter where you go, you may have some questions such as: why do some leaves change colors each year while other leaves simply turn brown and fall to the ground? Why do some leaves turn red and some yellow? Why are some years better to view than others? Where should I go to view the fall leave? This article will give some insight into some of these questions.

The Process

In order to grow, leaves require several things. The most important of which are sunlight, water, and carbon dioxide. Leaves are experts at using these three ingredients to make glucose through the process of photosynthesis. The chemical in the leaves that is responsible for this process, called chlorophyll, is also responsible for the green color of most leaves. During the spring in summer months, many leaves have small amounts of yellows or oranges contained in carotenoids and anthocyanins, but the green color of the chlorophyll is so strong that these colors are masked and all you see if the green. As the days shorten in the fall, there is less light available for photosynthesis and the leaves begin to lose their chlorophyll and green color and we are left with the oranges and yellows. In addition to this process, a second process can occur largely in maples where the change in the chlorophyll during the fall actually changes the color of the chlorophyll to reds and purples. The timing and intensity of fall colors are dependent on the exact pigment in the leaves (dependant on the species of the tree), the amount of sunlight (length of the day), and that years weather.

Timing and Intensity

The length of the day is the first factor to consider in the timing of the fall foliage. The length of day consistently shortens starting on the summer solstice in late June, resulting in less chlorophyll in the leaves. In the fall, the days are starting to get much shorter, and as this occurs, the leaves begin to shut down, meaning they close the veins that connect them to the tree, leading to color changes and the eventual fall to the ground.

The weather is another factor that affects the timing and intensity of the fall foliage. Of chief importance are the temperature and the amount of moisture. Although the exact relationship is still unclear, we know that warm days and cool (but not freezing) nights during the fall results in the longest and most brilliant colors. The amount of precipitation throughout the entire year also affects the fall foliage. A dry summer or spring may delay the fall foliage, while a wetter spring may increase the length of the fall foliage season. During severe drought, many of the leaves may actually fall off the tree before they turn color. The wind may also affect the amount of leaves on the trees, especially late in the season. The passage of a hurricane or other high wind event may greatly reduce the number leaves still on the trees.

If you are trying to make travel plans based on the fall foliage, your best bet is to assume that the leaves will begin to change in New England in late September or early October. This process may start slightly earlier in the mountains depending on the temperatures. If you are visiting further south, plan your trip for slightly later in the season. There is no exact way to predict the peak time for fall foliage, but part of the fun is trying to guess. If you want to find out where the foliage is out at the moment, you can check out the Foliage Network. This group uses a number of foliage 'spotters' who report on the changing leaves to give you an idea of what to expect as you head out for viewing. The site also has suggestion on places to stay. In addition to New England, the Southern Appalachian Mountains, the Rockies, and the Smoky Mountains all have beautiful fall displays. Driving on one of the 100 National Forest Scenic Byways is a great way to view these color displays. You can also hike or take a train or chair lift ride into the mountains for spectacular views.

The different Colors

In addition to the timing and intensity of fall foliage, the range of colors also changes with location and timing. For the most part, the color of the leaves is controlled by the specific species of tree. The timing that an individual tree will change color depends on the species as well as the exact location of that specific tree. Different species of trees have different amounts of the pigments in carotenoids, chlorophyll and anthocyanins and thus different colored leaves. Some trees, such as Elms, will simply turn brown and drop their leaves. The best fall foliage may be when a variety of tree species produce multiple colors across the same landscape.

Maple trees are responsible for the brilliant reds, crimsons, and oranges, although Black Maples produce bright yellow leaves. Sourwood, Dogwoods, Black Tupelo, and Oaks also have varying shades of red leaves. Hues of yellow and bronze leaves can be found on Aspens, Birches, Hickories, and Beach trees. The presence of these trees is what makes different locations ideal for viewing fall foliage. New England is home to a variety of tree, including the maples that produce the brilliant reds. The large amount of Aspen in parts of the Rockies means that this area is perfect for viewing a landscape of yellow. If you are hunting for fall foliage, you should avoid areas that are heavily populated with spruce, pine, or other needled trees since these tree do not chance color and usually remain green all year.

Fall is a beautiful time of year, so enjoy your leaf peeping!