In mid-April 1921, a huge snowstorm hit Silver Lake, Colo. Within 24 hours, the town was buried beneath more than 1.8 meters of snow.
Although Silver Lake’s record-breaking snowfall has yet to be equaled, the Rocky Mountains remain famous for their snowy peaks. A good portion of the snow that falls there sublimates directly from solid snow to water vapor, leaving less snow on the ground to melt. Snow caught on tree branches sublimates fastest, often disappearing within a few days of a snowfall.
But in the mid-2000s, an endemic pest upset the balance. By 2010, a spruce beetle outbreak had killed 75%–85% of mature spruce trees in some areas, opening the canopy. Snow could no longer accumulate there and instead fell between the branches to the slow-sublimating snowpack on the ground.
Frank et al. investigated how canopy loss has affected sublimation rates—a critical factor in the ecosystem’s water dynamics. In a snow-dominated ecosystem, plant and animal life, water chemistry, and water resources are all influenced by sublimation rates. Decreased canopy would decrease the fast-sublimating clumps of snow that gather on branches, but opening up the forest could also increase the amount of wind and sunlight reaching the snowpack on the ground, speeding up its sublimation.
The team used data gathered over 17 years from the Glacier Lakes Ecosystem Experiments Sites in Wyoming’s Snowy Range, a research site managed by the U.S. Department of Agriculture’s Forest Service.
First, the researchers compared sublimation within the canopy with sublimation on the ground and confirmed that canopy snow sublimates more easily than ground snow. Even after the beetle outbreak, half to two thirds of all sublimation came from the diminished canopy. In fact, as more snow falls, a greater percentage of sublimation occurs in the canopy.
The beetle outbreak reduced the amount of snow intercepted by the spruce canopy to a third of its original size. The researchers observed 32% less sublimation from the canopy and only a 3% sublimation increase from snow on the increasingly exposed ground. Altogether, that means that nearly a quarter less sublimation is happening between November and April. The problem extends beyond winter: Another study found that the reduced canopies are releasing about a third less water into the air in the summer.
Ultimately, a significant amount of water remains within the forest rather than returning to Wyoming’s dry atmosphere. The reduction in sublimation adds as much water to the ecosystem as a 6% increase in snowfall would. In the western United States’ semiarid climate, this difference in water availability has both ecological and social implications. The hydrologic consequences are unknown, and studies so far have found conflicting or surprising results, but trends will also continue to change as the understory responds and the forest begins to recover from the beetle epidemic. (Water Resources Research, https://doi.org/10.1029/2018WR023054, 2019)
—Elizabeth Thompson, Freelance Writer