Color Change Begins With Cool Weather
Shedding leaves is a tree's protective mechanism. Trees such as evergreens have evolved to survive winter by producing needles whose cells contain an antifreeze-type substance. But deciduous trees prepare for winter by toughening and sealing off areas where sap flows, and in doing so they shed tender leaves that could not survive freezing weather. The process by which autumn leaves change color and fall from the tree begins in the spring when the leaves are formed, according to Loralie Cox, horticulturist with the Utah State University Cooperative Extension. At the point where the leaf meets the tree is a layer of cells called the abscission, or separation layer. Tiny veins in the abscission are responsible for the passage of food from the leaves to the tree. In turn, the tree sends water to the leaves. In response to shorter daylight hours in autumn, the cells in the abscission layer swell, gradually cutting off the the exchange of water and food between the tree and the leaf. This ends the leaves' production of chlorophyll, causing their green color to fade away. Colors that were in the leaves all along, but were masked by the green chlorophyll, begin to appear.
Pigments in Leaves
To understand why leaves change color, it is first necessary to understand that leaves are the food factory for a tree, using the process of photosynthesis to create glucose. Chlorophyll not only gives leaves their green color, but is a necessary part of photosynthesis. When trees prepare to shed their leaves, chlorophyll production is stopped, and other pigments in the leaves begin to appear.
The many beautiful colors we see in fall leaves are from the pigments commonly found in plants and flowers. Yellow pigments, such as what we see in birch, hickory and ginkgo trees, come from xanthophyll, also found in bananas, corn and daffodils. Carotene (the same substance that imparts color to carrots) creates the brilliant orange seen in some maples. Red and purple leaf colors, such as what we see in dogwoods and sumacs, come from anthocyanin pigments, which also give color to apples, cranberries, grapes and blueberries. The brown fall leaf coloring of trees like the oak is caused by tannin, a waste product of certain trees.
Other Factors Affecting Leaf Color
The amount of moisture and the temperature during the period when leaves are slowing their chlorophyll production can affect the vibrancy of fall leaf color. A combination of warm sunny days and cool (but not freezing) nights encourages the most sugar production, creating brilliant crimson and purple leaf colors.
If trees are affected by a late spring or summer drought, their leaf color change might be delayed by several weeks. Colors might be muted by a string of wet days or especially warm weather in the autumn. The optimal conditions for spectacular fall leaf color is when a warm, wet spring is followed by a hot summer, and then autumn brings warm sunny days and crisp, cool nights.