How are Colorado’s aspens faring in this hotter world?

By Sue Ballenski, Fort Collins chapter

Imagine being in a grove of aspen in the summer. Above you, the green leaves block the sun and flutter in the cool breeze. Around you is an astounding variety of plants, columbines, wild roses and grasses. Nearby you can hear squirrels or mice moving through the underbrush and in the distance you can see elk grazing. Aspens create a powerhouse of life and biodiversity.

The tree itself is remarkable. They are one of the few deciduous trees in the Rockies and have the largest geographic distribution of any other tree species. In Colorado, they grow anywhere from 5,000 to 10,000 feet in elevation, preferring sunny, moister areas, often forming islands within a conifer forest. They reproduce mainly by suckering from their roots, such that an entire grove of aspen is really only one organism. Aspen trees themselves are relatively short-lived at 100 years, but the clone can exist for a thousand or more by continuously sprouting new trees. Aspens sequester more carbon and capture more moisture in their extensive root system than other trees. They also tend to be more fire resistant and sprout new saplings quickly afterward.

Yet they are declining at a landscape scale. First noticed in southern Colorado around 2004, foresters have named the phenomenon Sudden Aspen Decline or appropriately, SAD. The initial decline could be traced to severe droughts a couple years earlier. Although the droughts were bad, aspen had historically survived worse. The difference was a combination increasingly common with climate change in the western U.S.: low precipitation/snowpack with higher temperatures. The higher temperatures evaporate whatever little moisture may be available or in the plants themselves.

Trees live by pulling water up from their roots. The drier it is, the more water tension is necessary to suck to get the moisture needed for photosynthesis in its leaves. When this tension is broken, an air bubble or “embolism” forms. Enough of these embolisms and the tree’s health is compromised, inviting a variety of insects and disease. Aspen are particularly vulnerable since they pull most of their water from the upper part of the soil, which in drought is drier. Climate models show a reduction in aspens’ range, moving the trees up-slope to cooler, moister environments, and onto more north-facing (less sun) slopes.

Generally, SAD trees exhibit a progressive loss of branches and leaf area, and drop leaves earlier in the fall. An impacted clone will have an open canopy, with few to no sprouts or saplings. The decline and mortality is often within 2-5 years on a landscape scale.

However, the good news is that there are ways to mitigate and restore stands. After a fire or coppice cutting (selectively cutting down trees), aspens will aggressively sprout suckers to grow new trees. This successfully regenerates stands that have experienced low to moderate SAD mortality. Coppice cutting also provides the opportunity to diversify the ages of trees in a stand, giving the stand more resiliency generally. Both methods require quick action to evaluate the stand, the degree of mortality, the age of the trees, etc. Coppice cutting is an option for private land owners seeking to protect their aspen forests. These methods can be applied proactively to any stand to improve overall health. Federal land managers and Colorado State Forest Service can also promote aspen in areas more conducive to its future growth.

REFERENCES

Anderegg, W., Berry, J.A. et. al. The roles of hydraulic and carbon stress in a widespread climate-induced forest die-off. Proceedings of the National Academy of Sciences, 2011. DOI. 10.1073/pnas.1107891109

Joyce, Christopher. An Arbor Embolism? Why Trees Die in Drought. Nov. 23, 2012. National Public Radio. https://www.npr.org/2012/11/23/165667600/an-arbor-embolism-why-trees-die-in-drought

Kitchen, Stanley G.; Behrens, Patrick N.; et. al. 2019. Guidelines for aspen restoration in Utah with applicability to the Intermountain West. Gen. Tech. Rep. RMRS-GTR-390. Fort Collins CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station.

Oleniacz, Laura. Colorado’s Famous aspens Expected to Decline Due to Climate Change. Sept. 16, 2020. NC State University. https://news.ncsu.edu/2020/09/colorados-aspens-expected-to-decline-due-to-climate/

Shepperd, Wayne D.; Smith, Frederick W.; Pelz, Kristen A. 2015. Group clearfell harvest can promote regeneration of aspen forests affected by sudden aspen decline in western Colorado. Forest Science. 61: 932-937.

Singer, J.A., Turnbull, R., et. al. Sudden Aspen Decline: A review of Pattern and Process in a Changing Climate. Aug. 9, 2019. Utah State University. https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=8822&context=aspen_bib#:~:text=In%20North%20America%2C%20sudden%20aspen,the%20primary%20causes%20of%20SAD.

USDA Forest Service. Aspen Decline. https://www.fs.usda.gov/wildflowers/beauty/aspen/decline.shtml

Weber, Liz. Scars of Aspen Decline Persist Nearly 20 years after drought. Feb. 27, 2020. https://www.durangoherald.com/articles/scars-of-aspen-decline-persist-nearly-20-years-after-drought/