DNA and weather are factors
By Irene Shonle
It’s been an unusual fall. It’s been so warm and dry that the cool damp of the spring is a distant memory. Yet it was those wet conditions that allowed for the development of Marssonina and ink spot leaf blights which in turn caused some susceptible stands of aspen to brown and drop leaves prematurely and left others to blaze in full glory.
It seems that every year, leaf peeping is becoming more and more popular; on peak weekends, our usually sleepy mountain highways are packed with people and their cameras as they look for that perfect picture to post on social media. One particular aspen stand by my office reliably turns red in the fall, and frequently causes a pile up of people and sometimes even accidents!
Yes, red aspen are a novelty, and I get lots of questions on why some aspen stands turn red rather than yellow. So, I am going to answer here, even though we will be on the tail end of the season when this is published. The short answer is that it is a combination of genes and the environment. Continue reading for the long answer.
First we need a quick review of why leaves turn color – in summer, trees produce chlorophyll (which is green) in order to be able to produce their own food via photosynthesis. As deciduous trees prepare to lose their leaves, they begin to resorb nutrients and to slow chlorophyll production, unmasking other pigments that have been present all along. Yellow and orange leaf colors are due to xanthophyll and carotene pigments.
Reds and purples are due to anthocyanins and these pigments are only produced in the fall (by some plants). It is still not entirely clear the role these red pigments play, and why a tree would spend energy to create them at a time when they are about to drop all their leaves. Theories range from reducing the risk of light-induced damage to leaf cells, protection from cold, and protection from insects, to helping leaves retain water. Some plants almost always produce red (in our area, red-twig dogwood and chokecherry are examples), but in some cases like aspen, only a few trees turn red. What makes them different?
In 1989, Kuo-Gin Chang and his lab, then at Colorado State University, analyzed the pigments of yellow and red aspen and determined that all aspen produced carotenoids, but anthocyanins were only found in red and orange aspens. The ability to produce anthocyanins appears to be a genetic trait that some trees have, but most don’t.
However, just because a tree can produce anthocyanins doesn’t mean that it always will. The researchers followed the trees they studies for five years. The trees that started off yellow stayed yellow, but some of the trees that started off red (i.e., they produced anthocyanins) were yellow in subsequent years. This demonstrates that both genes and weather cause the red. Years that produce the best reds have warm sunny fall days followed by cool, but not freezing, nights. I have been seeing the development of some nice reds this fall due to our current weather pattern.
The CSU Gilpin County Extension Office is located at the Exhibit Barn, 230 Norton Drive, Black Hawk, CO 80422, 303-582-9106, www.extension.colostate.edu/gilpin. Colorado State University Extension provides unbiased, research-based information about, horticulture, natural resources, and 4-H youth development. Colorado State University Extension is dedicated to serving all people on an equal and nondiscriminatory basis.