By Gooseberry Peter
Autumn’s vibrant hues may be fading fast, but before they disappear completely under a blanket of white snow, let us contemplate what has been another dynamic year for fall color in the Interior.
Many folks have been asking questions this year about anomalies they have observed in the “normal” fall color patterns. Broken down by color, the predominant inquiries that I have received are: Why was the yellow so faded and short-lived in the birches; Why was there more red in the birches and aspens; and Why are there some plants which normally die back that are still relatively green?
If you are a fall color enthusiast like me, you may feel somewhat disappointed in the lackluster showing given by the birch trees this year. Our autumn palette is normally dominated by bright yellow, much of which is contributed by the birch trees. This year, however, the birches began to turn earlier than normal and displayed a relatively faded yellow that went quickly (or immediately in some cases) to brown. What happened?
According to the U.S. Forest Service, an ongoing outbreak of two tiny insects is to blame. Larvae from the nonnative amber-marked birch leafminer and the late birch leaf edgeminer “mine” birch leaves during the summer; i.e. they live inside them and eat the insides of the leaves prior to emerging in the fall to pupate. Leaves damaged by the mining turn yellow-brown and fall off the tree prematurely.
This year, extensive damage due to leaf miner activity caused many birch leaves to turn and fall weeks ahead of their normal schedule, with a significant reduction to the quality and longevity of their autumn color. Despite the visual severity of the outbreak, the damage to the trees is usually mostly cosmetic and they should leaf out again just fine next year.
While yellow was greatly diminished, there was a noticeable increase in red tree foliage this autumn. Birch and aspen, both trees which normally exhibit yellow autumn leaves, may sometimes display red foliage when the growing conditions are right. Temperature and precipitation are both factors in determining leaf chemistry and therefore leaf color.
According to University of Alaska Fairbanks forest ecologist Jessie Young-Robertson, this autumn saw more red color in birches and aspens than in the previous 12 years. This color shift has been linked to warmer August air temperatures, reduced summer rainfall and more light reaching the leaves than normal (because less rain means fewer clouds, which means more available sunlight.) So, if it’s hot and dry again next August, we could expect to see more red birch and aspen leaves.
After most deciduous trees, shrubs and herbaceous plants have turned color and lost their leaves there are still a hardy few that retain their greenery. The fancy term for this condition is “delayed senescence” which basically means that it takes longer for some deciduous plants to lose their leaves or “die back” than others. This is especially true in some nonnative and invasive species where enjoying a longer growing season may convey a competitive advantage.
As of this writing, I can look out my window and see chokecherry trees that just started to turn color, bird vetch that is still green and perennial sowthistle that is still blooming even while the native plants around them have already settled down for their long winter’s nap.
If surveying the subtleties of fall color has sharpened your interest on any of the subjects mentioned herein, you can learn more by contacting your local Cooperative Extension office.
Want to contribute to the science of fall foliage? The U.S. Forest Service is collecting citizen science data regarding birch leaf miner damage through their Alaska Forest Health Observations Project, which can be found on the iNaturalist app. And if you find that delayed senescence makes the invasive plants in your neighborhood easier to spot, you can report them using the AK Invasives app.
Gooseberry Peter is the agriculture/integrated pest management program assistant for Cooperative Extension Service, a part of the University of Alaska Fairbanks, working in cooperation with the U.S. Department of Agriculture. For questions, he can be reached at gpeter3@alaska.edu or 907-474-6829.
