When winter approaches, our part of Earth receives less sunlight, and the air grows colder, this season is commonly known as “Fall”. When these changes happen, trees prepare for winter. People believe that leaves die on the tree and the wind blows them off the tree. According to Peter Raven, president of the Missouri Botanical Garden and a renowned botanist, “the wind doesn’t simply pull leaves off trees. Trees are more proactive than that. They throw their leaves off.” Deciduous trees have cells in them that act like scissors. These cells build up a thin bumpy line that push the leaf, bit by bit, away from the stem. You can’t see this without a microscope, says Peter Raven. The tree will then seal the spots where the leaves were attached and bunker down for the winter months.
The falling of these leaves on a tree actually helps the tree to survive the cold, dry air of winter. In the warm seasons, leaves use sunlight, water, and air to make the tree’s food, in a process called photosynthesis. In that process, the tree loses a lot of water through tiny holes in the leaves. In winter, the tree does not get enough water to replace what it would lose through the leaves. If the tree did not seal the spots where the leaves grow, it would die. When spring brings warm air and fresh water, the tree will sprout new leaves and start growing again.
Leaf color comes from pigments. Pigments are natural substances produced by leaf cells. The three pigments that color leaves are:
chlorophyll (which produces the green color)
carotenoid(produces yellow, orange, and brown colors)
anthocyanin (produces a red color)
Chlorophyll is the most important of the three. Leaves contain chlorophyll in order to use the sunlight to produce its own food through the process of photosynthesis.
Carotenoids are organic pigments that are found in the chloroplasts and chromoplasts of plants and some other photosynthetic organisms. Carotenoids create bright yellows and oranges in familiar fruits and vegetables. For example, corn, carrots, and bananas to name a few.
Anthocyanins are pigments that may appear red, purple, or blue depending on the pH, they add the color red to plants, including cranberries, red apples, cherries, strawberries and others.
In the fall, because of changes in the length of daylight and changes in temperature, the leaves stop their food-making process. The chlorophyll breaks down, the green color disappears, and the yellow to orange colors become visible. Most anthocyanins are produced only in autumn, and only under certain conditions. Not all trees can make anthocyanin, but sugar maples seem to have the easiest time in doing so.
Resources: State of New York: College of Environmental Science and Forestry: http://www.esf.edu/
Dnr.wi.gov- Environmental Education for Kids: http://dnr.wi.gov/org/caer/ce/eek/veg/trees/treestruecolor.htm