Leaves in the spring and summer are mostly a pretty uniform green color. For that, you can thank the chemical compound known as chlorophyll. Chlorophyll is responsible for the process of photosynthesis, which takes sunlight and converts it into the sugars needed to sustain the tree. There are a variety of other chemical compounds, with characteristic colors other than green, always present in the leaves; however, the green chlorophyll dominates and masks your ability to see them. Anthocyanin, carotene, and xanthophylls compounds are largely responsible for purple, red, orange and yellow colors in the leaves. Tannins are generally responsible for brown colors and only appear when the leaves are mostly dead. These compounds are generically referred to as “phytochemicals.”
In the Autumn, decreasing sunlight tells the tree that winter is on the way and the tree begins to go dormant and shuts down sugar production by the leaves. In anticipation of the leaves dying and falling, the tree forms a barrier where the leaf attaches to the tree called the “abssicion layer.”
When the last few weeks of summer are moist and mild, sugars build up in the leaves and get trapped on the leaf side of the abssicion layer. The chlorophyll in the leaves begins to die and the green color fades, letting the recessive colors of the other compounds shine. Some leaves are able to produce additional anthocyanin through the interaction of sunlight and the trapped sugars, enhancing red colors in the leaf. When the leaf truly dies, what is left behind is the brown color of tannin compounds. If you have ever noticed the runoff from a large pile of leaves, you may have noticed the water is a dirty brown color. This is also due to the tannins leaching out of the dead leaves.
The optimum conditions for display of leaf colors in the fall is a nicely most late summer followed by a period of cold nights with temperatures staying above freezing. Freezing temperatures will kill the leaves before the chlorophyll is gone, so the leaves will go quickly from chlorophyll green to tannin brown.
Trees have characteristic colors in the fall because of the different amounts and types of the various phytochemicals in them.
Oaks tend to turn to reds and browns; hickories are often golden bronze; yellow poplar have a golden yellow mien; dogwood is dressed in purplish red. Beech trees drape themselves in light tan; and sumac goes to a rich crimson. Maples have a variety of colors depending on species and are responsible for a lot of the variety of colors in the forest. Red maple turns brilliant scarlet; sugar maple, orange-red; and black maple, glowing yellow. Leaves of some species such as the elms simply shrivel up and fall, exhibiting little color other than drab brown. The evergreens (various species of pine, magnolia, etc.) maintain their vivid green color as counterpoint to their cousins bright coats.
Article by: W. Jerrold Samford, Troutman Sanders, LLP