Autumnal Leaf Color
—Mary Gillick, Program Director
The progression of color change in autumn is predictable and allows us a way to chart the passage of the season. Some plants and trees are totally programmed by the length of day or photoperiod. Their lifecycle is governed by the interaction of light receptors and sunlight. This system provides a safety measure so the plant tissue is not adversely affected by freezing temperatures.
Deciduous woody plants in particular have a shutoff mechanism where a corky layer forms at the point of connection between each leaf and its branch. The water flow is cut off and the chlorophyll breaks down revealing the once hidden pigments remaining in the leaf.
The sequence of color tells us which species of trees are going dormant at that particular time. The burgundies and purples of the dogwoods start in September. These are garden trees in our area but they are also found in the native forest as understory plants. The red maples are next to change as the night temperature falls into the fifties. The bright oranges of the sugar maples follow and are stunning on trees that stand alone from other trees.
By mid-October the tulip trees have turned and the dropped leaves form a golden layer on the ground. Their shiny leaves have a shape like a cat's face. The hickories seem to get their orange-yellow color overnight. In a matter of days the compound leaves drop off especially when accompanied by wind and rain.
The sweet gums with their star-shaped leaves have the most variability in the color department. In the forest they are bright yellow as if the sun is shining on them. Those contain the carotene pigments. In sunnier places the leaves have burgundy and plum colors. Those pigments are called anthocyanins, which are sugars that form during the growing season. If the weather is overcast the colors are duller than during a season with dry, clear fall days.
The branch of science that documents the changes in the timing of floral and fauna lifecycles is called phenology. Observations collected over many decades are showing a later trend in the arrival of dormancy in the northeast forests. The third week of October used to be peak leaf color. In the last five years we are seeing peak color in the Rye forest as late as the second week in November.
For more information:
The National Arboretum's website: http://www.usna.usda.gov/PhotoGallery/FallFoliage/index.html
