UMass Amherst Researcher Explains Fall Foliage

AMHERST, Mass. - While leaf peepers trek to the region to enjoy New England''s brilliant fall foliage, University of Massachusetts biology professor Bernard Rubinstein sees the season not just with an aesthetic eye, but from a scientific perspective, as well. A plant physiologist, he studies the science behind the bright fall colors that awe local residents and tourists alike.

"The colors that are especially notable to us are the luminous reds and yellows and their various combinations," says Rubinstein. "These hues are due to particular molecules, called pigments, that absorb certain colors of light, and reflect or transmit others." The yellow colors are due to a family of chemical pigments called carotenoids, he says. These pigments are always present in the leaves, but they only appear in the fall when the green chlorophyll, which is responsible for photosynthesis, begins to disappear. The red colors are largely due to a class of pigments called anthocyanins. Unlike carotenoids, these pigments are not present in the leaves during the rest of the year, but only appear during the fall.

Contrary to common belief, the major trigger that initiates the change of leaf color is not temperature, Rubinstein says, but changes in day length, in particular to the shorter days and longer nights of fall. However, temperature also plays a role: "Certainly, colder temperatures may hinder some processes and warmer ones may speed them up, but perhaps the most striking effect of temperature is seen during a combination of warm days and cool nights," he said.

Other variables may affect foliage in a given year. This year, the most pronounced factor may be the fungus disease that caused enlarging circles to appear on the leaves of several species during the wet months of April and May, Rubinstein says. "If the leaf stays on the tree, it will probably turn the same color as the healthy leaves, but the areas killed by the pathogen would not change color," he said. However, the extent of the damage could have caused leaves to fall prematurely. "Then there would be fewer leaves per tree and, of course, the display would be correspondingly less." There has also been some "scorching" and curling of leaves in some areas due to the drought.

People often wonder why plants and trees display different colors during foliage season, Rubinstein notes. "Plants with disparate autumnal foliage may be different species," he says. "Thus, the swamp maple is bright red in the fall, while the sugar maple is a bright orange, the poplar is yellow, and many species of oak are dark purple." Also, plants of the same species appear to have different colors, depending on the condition of the plants. If, for example, one tree has more water, or was attacked by insects, or was in a cooler location, any of these factors will affect the intensity and even the types of coloration, according to Rubinstein.