For many people, the autumn season welcomes a pleasurable change in the weather. We notice, even during a warm fall day, the air gets cooler and the brisk breeze forces out the fleece. There are a couple of questions to be asked every year at about this time. Why do leaves change color and drop their leaves when fall weather appears? Much of it has to do with day-length and temperature. The important thing is not that the amount of sunlight has decreased but rather the amount of dark has increased. The plants we’re talking about in this case are the deciduous trees, which are the trees producing the vibrant colors and those that lose their leaves annually. These woody plants “sense” the days are getting shorter during late September as winter slowly creeps in on us. Things like pigment, light, weather conditions; plant species, soil type and location all play important roles in the fall party and colorful confetti trees create for us to enjoy.
When daylight hours are less and temperatures are cooler, photosynthesis slows down and there is less chlorophyll production. This reduction reveals yellow or orange pigment called carotenoids and are usually hidden by the abundance of chlorophyll present in leaves during the growing season.
Unlike chlorophyll and carotenoids, which are present in leaf cells throughout the growing season, anthocyanins are produced mainly in the fall. These natural chemicals give color to familiar fruits such as cranberries, red apples, cherries, and plums. These complex compounds in leaf cells react with excess stored plant sugars and exposure to sunlight creating vivid pink, red, and purple leaves. A mixture of red anthocyanin pigment and beta carotene often results in the bright orange color seen in some leaves.
The cool fall temperatures cause the closing of leaf veins and prevent sugars from moving out which prolongs fall color. Thus a succession of warm sunny days and cool crisp nights can create quite a display.
Soil moisture levels have an impact on the ability to produce good fall color. A prolonged drought can delay color change for a few weeks. The ideal conditions for producing the best colors are good summer weather, with timely rainfall, and sunny fall days with the cool night temperatures.
Tree species vary in their ability to provide fall color, as some trees just don’t produce anything noteworthy. Color depends on the nutrient levels of iron, magnesium, phosphorous, or sodium in the tree. Some tree species displaying yellow foliage are ash, birch, beech, elm, hickory, poplar, and aspen. Red leaves are seen most often in dogwood, sweet gum, sumac, and tupelo trees. Some oaks and maples present orange leaves while others range in color from red to yellow, depending on the species.
Even with these facts the timing, location, and intensity of autumn color are not completely predictable. To truly experience the colorful display you must be tuned in to your trees and your weather. Get outside and enjoy the party our woodlands are providing and enjoy nature’s beautiful confetti.
It’s Fall, but why are the leaves still green? article and video, WLFI.com
It’s late October and many leaves on trees are still green. What’s up with that?, IndyStar.com
Why Leaves Change Color, The Education Store, Purdue Extension
Why Leaves Change Color, USDA Forest Service, Northeastern Area
Fifty Trees of the Midwest App for the iPhone, The Education Store
Native Trees of the Midwest, The Education Store
Trees and Storms, The Education Store
Tree Installation: Process and Practices, The Education Store
Lindsey Purcell, Urban Forestry Specialist
Purdue Forestry and Natural Resources