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From Green to Brown
This page contains archived content and is no longer being updated. At the time of publication, it represented the best available science. However, more recent observations and studies may have rendered some content obsolete.
With temperatures dropping in the northern hemisphere, fall colors swept across the taiga forests of the Kamchatka Peninsula in September and October 2012. Within a span of 11 days, the forests of far eastern Siberia went from green—with only a slight hint of fall color—to a deep brown. Common tree species in the area include Erman's birch (Betula ermanii), Japanese stone pine (Pinus pumila), and Dahurian larch (Larix daurica).
In the fall, leaves change colors as they lose chlorophyll, the molecule that plants use to synthesize food. Chlorophyll makes plants appear green because it absorbs the red and blue light from sunlight as it strikes leaf surfaces. However, chlorophyll is not a stable compound and plants have to continuously synthesize it, a process that requires ample sunlight and warm temperatures. So when temperature drop and days shorten in autumn, levels of chlorophyll do as well.
As concentrations of chlorophyll drop, the green color of leaves fades away, presenting an opportunity for other pigments within leaves—carotenoids and anthocyanins—to show off their colors. Carotenoids absorb blue-green and blue light, so in the absence of chlorophyll, they cause leaves to appear yellow. Anthocyanins absorb blue, blue-green, and green light, so light reflecting off the pigment appears red.
The range and intensity of autumn colors is strongly affected by the weather. Both low temperatures and bright sunshine destroy chlorophyll. So if the weather stays above freezing, it is easier for anthocyanins to form. Dry weather, which increases the sugar concentration in sap, also increases the amount of anthocyanin. So the brightest autumn colors occur when dry, sunny days are followed by cool, dry nights.