During a process called acclimation, trees and shrubs prepare for winter and its low temperatures by ceasing growth and becoming dormant. This process actually starts in late summer in response to decreasing day length and is rarely noticed by our human eyes. But from September through November, as day length continues to decrease and temperatures drop, we can’t help but notice and appreciate the process of acclimation as deciduous trees and shrubs burst into vivid hues of red, orange, and yellow. These colors signal that trees and shrubs are reducing their photosynthesis levels and moving nutrients from leaves to stems and roots that serve as storage locations in winter. The variety of fall colors we see are created by the interaction of four pigments – chlorophyll, carotenoids, anthocyanins, and tannins – and variations in temperature, light levels, and water supply. Each of these pigments is uniquely effective at absorbing particular wavelengths of light. The foliage colors we see are dependent on the wavelengths not absorbed by pigments which are instead reflected to our eyes.
Absorption spectra of chlorophyll and carotenoids
A sugar maple high in carotenoids
Source: K. Zuzek, UMN Extension
Carotenoids are accessory pigments found in plants. Like chlorophyll, they are also bound to membranes in chloroplasts and are present throughout the growing season. Carotenoids are also found in chromoplasts, organelles similar to chloroplasts except that they lack chlorophyll. They absorb blue and blue-green wavelengths and pass this harvested light energy on to chlorophyll for use in photosynthesis. Carotenoids are much more stable than chlorophyll and have another important role in plants. They help to protect against excessive light and by doing this, extend the life of the more fragile chlorophyll pigment. Chlorophyll and carotenoids working together during the growing season remove violet, blue, red, and orange wavelengths of light, leaving bright green as the reflected color that our eyes see in leaves. But in autumn when chlorophyll production stops and only carotenoids are left, yellow, orange, red, and some green wavelengths are reflected and the human eye sees the yellow and orange colors of willows, honeylocusts, lindens, birches, aspen, maples, elms, ginkgos, and hickories.
A sugar maple high in anthocyanins
Source: K. Zuzek, UMN Extension
that resemble tiny water balloons. Anthocyanins absorb blue, blue-green, and green light.
A fourth group of pigments that impact fall color is the tannins. Tannins are found in vacuoles and in cell walls and play an important role in plants’ defense mechanisms against pathogens, insect feeding, wildlife browsing, and environmental stresses. Like carotenoids, they are always present but are only visible when chlorophyll levels decrease. Tannins are responsible for the brown fall color seen in oaks and other species. They also mute the brilliance of carotenoids and anthocyanin pigments, resulting in the deep and rich burgundies, copper, and golds we see during autumn on oaks.
The rich tapestry of fall color we enjoy in Minnesota is impacted by genetics, weather, and climatic factors. Genetics determine whether a tree or shrub can produce only some or all of the four pigments involved in fall color. Genetic variability between individual plants creates subtle differences in color from tree to tree within any one species. Decreasing day length and temperatures in fall trigger a reduction in chlorophyll production. As this happens, yellow, orange, and brown fall colors appear as the carotenoids and tannins that are always present are unmasked. If weather conditions are favorable for anthocyanin production - lots of sunny days to drive the process of anthocyanin production; the absence of high winds or freezing temperature that remove or damage leaves before peak fall color; cooling daytime temperatures that favor sugar production; and cool (but not freezing) night temperatures that slow the movement of sugars out of leaves - anthocyanin-producing plants will provide us with the brilliant orange and red fall colors that result in a good year of fall color. If weather conditions are not so favorable to anthocyanin production – cloudy days, excessively high or low temperatures, early frost, high winds - we have mediocre or poor years of fall color.
The Minnesota Department of Natural Resources produces an interactive map called the Fall Color Finder that can direct you to regions of the state displaying best fall color.