During the fall months, Ithaca — a city abundant in foliage — displays an autumnal grandeur of reds, oranges, yellows, browns, and even purples. It is this blazing medley of colors, which the northeast has much to offer at this time of the year, that creates the allure that fall has to observers.
So what causes the trees to transform from their summer viridescence, to a burning mélange? Over the past half year, food production processes have occurred inside the leaves of the trees. These processes produce the majority of food that is required for the tree to grow, and are located in cells that contain chlorophyll. Chlorophyll absorbs energy from sunlight, and is the source of a leaf’s green color.
When fall arrives, it means significant changes for the chemical composition of
the leaves. Professor Karl J. Niklas, a published scientist who researches plant evolution at Cornell University, explains: “What is happening in the early part of fall, is the days are getting shorter, the temperatures are getting cooler, and leaves of deciduous trees are getting stressed,” he said Once these physiological stresses reach a tipping point, the green chlorophyll molecules break down, and the leaves then cease their food production. This process is called plant senescence.
Depending on the amount of chlorophyll left in the leaves, a wide variety of colors can occur. Within the leaves, there are carotene and xanthophyll pigments, which possess orange and yellow colors, respectively. The loss of chlorophyll allows for these yellow and orange colors, which were previously obscured by high levels of the green chlorophyll, to reveal themselves for all to admire. Cadmium red and purple pigments can appear as well, which are a result of additional chemical changes in the leaves.
Outside factors that affect the color of the leaves — and the duration of their color — are light, temperature and water supply. An example of this is how the color of red maples is affected by the weather. As detailed in writings by the SUNY College of Environmental Science and Forestry, “Low temperatures above freezing will favor anthocyanin formation producing bright reds in maples. However, early frost will weaken the brilliant red color. Rainy or overcast days tend to increase the intensity of fall colors. The best time to enjoy the autumn color would be on a clear, dry, and cool (not freezing) day.”
While New York State is home to more than 125 different species of trees that are either native or naturalized within New York State, not all of them contribute to the coloration of the fall foliage, according to SUNY ESF professor Donald J. Leopold. Some are evergreens, trees which have adapted to continue the process of photosynthesis despite the formidable temperatures, and thus they remain green. Typically, these trees are conifers.
Ithaca residents, as well as those of the northeastern United States as a whole, witness nature’s spectacular transition from season to season each year, a beauty which one might argue is most apparent in fall’s brilliant assortment of colors.
“Our New England forests have a unique composition of tree species that are not found anywhere else in the world, so we have a very particular combination of tree species that are known for the vibrant autumnal coloration,” said Niklas. “We are very lucky in that respect. Tourists from around the world come to see our forest colors.”