Now that fall has officially arrived and the change of colors has begun, we found ourselves wondering about the science behind those glorious colors. We consulted Dan West, a Colorado State Forest Service entomologist who teaches at Colorado State University.
As days grow shorter with the approach of winter, trees stop making the chlorophyll that is responsible for photosynthesis, the process by which plants convert sunlight into chemical energy, West explained. Chlorophyll is responsible for the dominant green pigment in leaves. After chlorophyll production is shut off, the trees burn off the remaining chlorophyll in the leaves. As it dissipates, other pigments that were there all along — oranges called carotenoids and yellows called xanthophylls — become visible.
Fall stories
Meanwhile, some trees produce anthocyanins when fall comes, which leads to vibrant reds and purples.
“It is poorly understood why some trees produce more or less anthocyanins, but one thought is they may serve as a solar protectant or antifreeze,” West said. “Some species have way more anthocyanins than do others, but they are largely dependent on cool evenings turning the sugars into pigmentation. Obviously maples and other hardwoods in the northeast states have many more anthocyanins than do tree species in the west. The xanthophylls and carotenoids are present in the leaves throughout the growing season of deciduous trees.”
Some people believe the shorter length of days is the sole variable determining when the leaves will start to change, but there are other variables, such as temperature. That’s why the change this year is about a month later than last year.
“If it were only day length, the change would always be the same days, year to year,” West said. “Leaf change is not an exact science, due to the combination of cooler temperatures combined with day length. Day length triggers the trees to lay down an abscission layer, which in turn shuts down the chlorophyll generation. The rest is a function of cool evenings and sunny days to burn off the chlorophyll within each leaf.”
