Due to a range of factors, mountain forests store more carbon than forests on flat land or low slopes, new research indicates.
Trees are one of Earth’s most important carbon reservoirs, absorbing carbon dioxide—a greenhouse gas—from the atmosphere as part of their process of respiration. Yet, in mountain landscapes, trees aren’t lone agents at storing carbon.
There is more going on; factors such as nutrient availability, soil depth, precipitation, and overland water flow, among other things, all impact the health of a forest and its ability to store carbon.
Ecologist Tyson Swetnam and colleagues study this bigger picture when thinking about how complex terrain influences the carbon cycle. The critical zone is defined as the zone from the tops of trees to the bottom of groundwater storage in crystalline bedrock that interact and are impacted by changes to climate and land use.
“Mountains first capture atmospheric moisture as it cools and condenses at altitude, and that rain and snow then provides catchments with moisture that eventually moves into valley bottoms,” says Swetnam, a science informatician at National Science Foundation-funded and University of Arizona-headquartered CyVerse, a national cyberinfrastructure project that provides computational support for life sciences. Snowmelt from mountains feeds the rivers of the Colorado plateau, where the study took place.
The team analyzed precipitation records for three study sites in the Boulder Creek CZO in the Rocky Mountains near Boulder, Colorado, as well as LIDAR (light detection and ranging) data on tree density and height collected by an aircraft flown over the study sites as part of a National Center for Airborne Laser Mapping initiative.
“Our study is the first to consider the variability of carbon compared with moisture distribution across elevation gradient,” Swetnam says.
Conventional thinking would indicate that trees growing at higher elevations should do better, since there is more precipitation at altitude, but “when you look across an entire watershed, forest productivity in valley bottoms far outweighs that of ridges,” Swetnam notes.
Trees located in valleys, where soils are deep and moisture is collected from precipitation that flows down from surrounding summits, are more productive and better at storing carbon.
“Concentrated areas of soil moisture lead to increased forest productivity, and greater biomass leads to more carbon sequestration,” Swetnam adds. It turns out that mountains provide the special mixture for optimum forest carbon storage.
Variable topography is even more important in arid or moisture-stressed ecosystems, Swetnam says.
“In the desert Southwest, if we didn’t have mountains we wouldn’t have forests. We need the complex terrain to create precipitation,” he says. “We see this every summer in Tucson, when rain clouds form over the Santa Catalina Mountains during monsoon season.”
It is unknown how climate change will influence mountain forests, Swetnam says.
“We anticipate that it’s going to get hotter and drier, and forests will be in competition with each other for water,” he says. “If trees die on upper slopes, maybe more water will be available to trees below, but conversely, maybe the trees which are acclimated to having constant water will die earlier when their water resources are reduced.”
Regardless, he says, it’s important that land-use decision makers—who are considering which timber stands to treat or preserve—consider that topography influences forests’ robustness, productivity, and ability to store carbon.
The study appears in the journal Ecosphere.
National Science Foundation grants to CyVerse, the Boulder Creek Critical Zone Observatory, and the National Center for Airborne Laser Mapping at the University of Houston, and the Department of Energy’s Terrestrial Ecosystem Science Program partially supported the study.
Source: University of Arizona