Predicting aspen colors

Mountain Living

By Irene Shonle, Ph.D.

Every day brings more aspen leaves turning, but predicting peak color and how good the show will be is always an imperfect science. We know why the leaves change: there is a decrease in photosynthetic activity as the days get shorter in the fall. The yellow color that shines in September is always there, it is just masked by the green of chlorophyll all summer. As chlorophyll stops being produced, the yellow pigment from carotenoids is revealed. But the ‘when’ and the coloration are less predictable.

How brilliant the aspen stands are depend on recent local weather conditions and available moisture. Also, stand health is a critical factor for aspens to display strong colors and retain leaves later into the fall. Unhealthy aspen stands are less likely to have vibrant colors and have early leaf drop, whereas robust stands will have brighter colors. In 2015, there was widespread Marssonina blight, a fungus which caused the leaves to brown and fall off early. That was attributed to the wet spring of that year. It appears as though we will probably be seeing some stands with blight again this year, but it doesn’t seem as though it will be widespread.  Some aspen groves appear to be more susceptible than others.

And since everyone is always curious, why are some aspen red? The amount of red in a tree is dependent on two things: genetics and growing conditions. Some aspen are more prone to produce anthocyanins (the red color), which develops late in the summer in the when sugar gets trapped into leaves. The sugar is made in the leaves during the day, and cannot move out of the leaves when the nights are cool; the trapped sugars are transformed into anthocyanins by some trees. Thus, warm sunny days followed by nighttime temperatures below 45 degrees F produce the best conditions for red colors. Unfortunately, it seems as though the tree stands that are more likely to produce red colors also may be more susceptible to the blight. One particularly well-known red-prone stand just north of highway 46 on the Peak-to-Peak highway has never quite recovered from the 2015 blight.

As for timing, along the Peak-to-Peak highway, the best colors are usually around the Fall Equinox (Sept 22 this year), although this can be a week off in either direction. Given how the trees are starting to turn already, I would say that we are more likely to see an early fall than a late one. Higher elevation stands will turn a little earlier, and if you miss peak up here, go down to lower stands for a second chance. If you go out leaf-peeping, be prepared for a huge influx of distracted drivers and slow traffic conditions on peak weekends.

Irene Shonle is the Director of the CSU Extension in Gilpin County. The CSU Gilpin County Extension Office is located at the Exhibit Barn, 230 Norton Drive, Black Hawk, CO 80422, 303-582-9106, 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.

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