In late September, within a span of just 11 days, the forests of far eastern Siberia dramatically changed from green—with only a slight hint of fall color—to a deep brown. The MODIS sensor on NASA’s Aqua satellite captured the fall transformation in the accompanying pair of images.
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 isn't a stable compound. Plants have to continuously synthesize it, a process that requires ample sunlight and warm temperatures. So when temperature drop and days shorten in autumn, chlorophyll levels 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.
Images courtesy of NASA.