Seasonal Cycles: Autumnal Tints

“Nature herself holds her annual fair in October, not only in the streets, but in every hollow and on every hill-side.” As Thoreau eloquently describes, fall in New England is marked by the transformation of green leaves to vibrant displays of reds, oranges, and yellows. This color change marks the preparation of trees for the winter ahead and occurs in response to a mixture of factors including cooling temperature, shortening days, and declining water availability. Chlorophyll, the pigment responsible for photosynthesis and the green color of leaves, begins to break down, and trees reabsorb valuable nutrients from the leaves before the leaves eventually fall from the branches.

Although fall phenology has not been documented or studied nearly as well as seasonal events in spring, there is evidence that this shift into winter is occurring later now than in the past as a response to climate warming. Because leaf color and drop are less sensitive to temperature than budburst and flowering, the delay of autumn seems to be less pronounced than the early arrival of spring.