October 12, 2001
The colors of autumn remind us of the summer sun, driving the carbon cycle with the photosynthetic intake of carbon dioxide and the production of sugars and biomass. University of Wisconsin-Madison scientists have a new theory about why autumn leaves turn scarlet and why the hues are more vibrant some years than others. They say that the red pigments -- called anthocyanins -- in plants such as maples, oaks, dogwoods and viburnums act like sunscreen. The pigments shade sensitive photosynthetic tissue in fall while trees reabsorb nutrients from their leaves. In addition to high light levels, other plant stressors such as near-freezing temperatures, drought and low nutrient levels trigger increased levels of the pigments. The researchers' theory agrees with the observation that autumn colors are best when the fall features dry weather with bright, sunny days, and cold nights.