In mountainous areas, cold air flows along the surface of the earth from mountain tops to valleys, and as it moves, it dramatically affects local temperatures. “Many ecosystem processes – including carbon uptake and storage – are affected by temperature,” says U.S. Forest Service research ecologist Chris Oishi.
Oishi recently contributed to a study on how the movement of cold air affects ecosystem productivity. The study was led by Kimberly Novick, an assistant professor at Indiana University, and published in Global Change Biology.
The study took place at the Forest Service Southern Research Station (SRS) Coweeta Hydrologic Laboratory, an outdoor living laboratory located in the mountains of western North Carolina. As in all mountainous areas, the climate of valleys and other small areas is affected by topography. Areas with distinct atmospheric characteristics are said to have microclimates.
Air behaves like an invisible fluid, flowing over mountains and tumbling through the sky in multiple, whirling eddies. The movement can be measured by eddy covariance flux towers. At Coweeta, one of these towers juts into the forest canopy, where it measures air movement as well as the exchange of carbon dioxide.
Carbon dioxide is released into the atmosphere by respiration, combustion, and other processes, and it is taken out of the atmosphere by trees. The balance between carbon release and carbon uptake is called net ecosystem carbon exchange, and is measured using flux towers. Oishi and his colleagues used carbon flux observations as well as temperature data to quantify the links between the movement of air, the carbon cycle, and climate – both the regional climate and the microclimate in the Coweeta study area.
The scientists found that at night and for a few hours before and after sunset, the cold air was enough to lower the temperature by as much as 7 degrees F in valleys, which was enough to lower plant and soil respiration by about 8 percent. Respiration is the process of breaking down sugars to release energy, and the reduced respiration allowed the studied forest to sequester more carbon.
Compared to estimates that ignored topographic effects on air movement, cold air drainage caused carbon intake to increase by more than 10 percent during the growing season, and when scientists measured carbon uptake across the whole year, they found it was increased by more than 15 percent.
“Forests are more efficient at sequestering carbon when respiration costs go down,” says SRS project leader and coauthor Chelcy Miniat. “The effect of cold-air flowing downslope in mountain forests hasn’t been adequately represented in climate or carbon sequestration models.”
The effect is strongest when the skies are clear and the air is dry – essentially when conditions are drought-like. Climate models are predicting increased frequency and duration of droughts, so the findings suggest that in mountainous areas, the flow of cold air can be a hidden protector of ecosystem productivity. Across the globe there are more than 11 million square miles of mountainous terrain.
Via the flow of cold air from their peaks to the valleys, mountains may buffer ecosystem processes from some effects of regional and global climate change. “Our results highlight that the relationship between topography, ecosystem processes, and climate change certainly deserves further study,” says Miniat.