Among many Native American tribes it was once believed that every fall, as the days grew shorter and cooler the great hunters in heaven killed a bear to prepare for winter food. As the blood of the bear dripped down onto the earth it would stain many of the leaves on trees red. As they began to cook some of the meat the fat would drip down to the land leaving other leaves yellow in color. Just like the Native Americans, scientists have their own story as to why leaves change colors each fall. Scientifically, the tree is going through physical and chemical changes which render the splendid colors we look forward to each year.

Throughout most of the year (spring, summer and early fall) the leaves of a tree act as food making factories. The days are long and weather is warm, allowing for the chloroplasts in the leaves to perform photosynthesis, the process of converting light energy from the sun and carbon dioxide from the air into glucose, or food for the tree. Within each leaf are three pigment compounds; chlorophyll (green in color), carotenoid (orange), and anthocianins (red and purple). During the majority of the year, when the chloroplasts are busy making food, they are constantly producing chlorophyll, as it is necessary for photosynthesis. Thus, with so much chlorophyll hanging around, the leaves most often look green. As fall gains momentum, the days grow shorter and cooler. Trees begin to prepare themselves for a long dormant winter. The chlorophyll, which is not getting quite enough warmth or sunlight anymore, begins to breakdown. Without so much overpowering green pigment, the other pigments, which have been in the tree all along, are finally given a chance to shine.

While these physical changes are happening in the leaves the body of the tree is also gearing up for winter. The leaves on deciduous (or broadleaf) trees are not adapted to handle the very cold temperatures of winter. As water expands when it freezes, the water with each leaf cell would freeze and destroy the cell. Therefore, the trees drop their leaves soon after their color changing phase. As the tree recognizes that winter is coming is begins to form a thicker layer of cells at the base of the leaf stem. This layer slowly grows thicker and thicker until all nutrients are stopped from going in and out of the leaf. When this separation is complete, the leaf falls off the tree, only to continue its journey on the ground where it will decompose and provide important minerals to the soil and other plants. The tree is now ready for the cold. It has stored up enough food in its roots and stem to feed itself through the winter. Come spring, the tree will produce a new set of leaves and begin the process anew.

What specific colors a tree’s leaves turn before falling off is dependant on many factors. First, weather plays a large role in the general color patterns. A slow transition into winter will allow for a long dramatic change in color, where as a fast onset of very cold weather will shock many of the leaves into a brown color very quickly. According to most scientists the best conditions of a colorful fall are warm sunny days with cool (but not freezing) dry nights. Secondly, the type of tree can greatly determine which pigments dominate, for example birches, aspen and hickory tend to be more yellow (have more carotenoid, where as maples, sumac, and sassafras tend to be more red (anthocianins). Thirdly, positioning on a tree during the fall can change their coloring. If a leaf is more shaded it will tend to be yellow, where as longer exposure to sunlight will leave it red. Lastly, as the separation layer occurs between the leaves and body of the tree, sugars that have been produced by photosynthesis can get caught within the leaf, never making it to the tree body. The buildup of these sugars can make a leaf especially scarlet in color as the winter approaches.

The splendid colors of each fall have been enjoyed by humans for centuries. Whether you look to the great hunters in heaven or the lessening daily sunshine, it’s a beautiful thing to be entertained by and find joy within the natural cycles of the earth.

http://www.esf.edu/pubprog/brochure/leaves/leaves.htm
http://www.ces.ncsu.edu/nreos/forest/topics/leafco~1.html
http://scifun.chem.wisc.edu/chemweek/fallcolr/fallcolr.html
http://www.dnr.state.wi.us/org/caer/ce/eek/veg/trees/treestruecolor.htm
http://www.stormfax.com/leaves.htm