Unusually mild weather and an abundance of rain may slow the color change in aspen in the lowest elevations of Rocky Mountain National Park.
What causes this shift of colors as the weather cools and days shorten? Chlorophyll is the green pigment responsible for the green color in plants and for most of the photosynthesis or food production in plants. As temperatures cool at night, plants start to break down chlorophyll and pull its building blocks back into the trunks and roots, conserving these vital resources for the future. However, other "accessory" pigments in the leaves are not broken down and stored. Accessory pigments are masked by chlorophyll in spring and summer, but remain to color the leaves in the fall. In aspen, the main accessory pigments are yellow and orange carotenes - the family of pigments that give carrots their distinctive orange color. In other plants such as wild geraniums, anthocyanins - red pigments - function as accessory pigments and provide the distinctive red fall colors. Accessory pigments capture light from other parts of the spectrum than that captured by chlorophyll, and thus make plants more efficient at making their food.
You may wonder why the aspen trees you see in suburban yards seem to turn colors independent of one another, while whole clumps of aspens in the park seem to turn color at once, while other clumps turn earlier or later. In suburban yards, each aspen tree is usually an independent plant put in place by the home owner. Aspens in the wild reproduce by sending out underground shoots more often than by seed. These shoots eventually pop up some distance away from the original aspen tree that produced them. This forms something we think of as a separate tree, but in fact, they are just stems of one plant. All these connected aspen stems, known more accurately as a clone, turn color and lose leaves in the fall in unison because they are one plant.
The brief and brilliant season of fall is caused by processes as varied as its vibrant colors.