Autumn is when every tree is in bloom ~ Ilex Farrell
To be able to explain why tree leaves change their color in the fall, you must understand the basic physiology of the leaf itself.
Leaves are green because of chlorophylls that function by capturing the sun’s energy and to manufacture food for the plant or photosynthesis. All of this takes place in the plastids (specialized cells). During the growing season, the green color of these chlorophylls masks out all the other colors that may be present. So all you see is green.
As the growing season slows in autumn, chlorophyll production slows and the green-color dominance lowers to reveal the other colors of the leaf. Many influences such as amount of water, sunlight, temperature, and microclimate can manipulate the timing of the color changes. A couple of weeks of bright sunny days mixed with clear, cool nights seem to bring out the best fall colors.
There are two pigments responsible for fall color:
Carotenoids – provide the yellow, orange, and brown colors.
This one provides the coloring for carrots, corn, and daffodils. Just like chlorophyll, these carotenoids are found in the plastids of the leaf. Some trees that turn hues of yellow or orange are: hickory, beech, black maple, aspen, and birch.
Anthocyanins – responsible for the red and purple hues.
This pigment develops in late summer in the sap of the cells of the leaf. They are created by a response to bright light and too much plant sugars left in leaf cells. Anthocyanins also tint young leaves in spring and allow for the bright colors of red apples, blueberries, cherries, and strawberries. Trees that tend to be colored red to purple are: oaks, dogwoods, red maple, sourwood, and black tupelo.
Both pigments can vary due to many degrees in a leaf, along with outside influences, that cause color ranges that are endless.