How Mountains Hold Carbon (And Do a Good Job of It)
UA ecologist Tyson Swetnam led an interdisciplinary team of U.S. researchers to discover that mountain forests are better at storing carbon than forests found on flat land.

By Shelley Littin, CyVerse

July 12, 2017

Gordon Gulch study site in the Rocky Mountains near Boulder, Colorado (Image courtesy of Boulder Creek CZO)

Mountain forests are better at storing carbon — actually, they're better at everything — according to a new study led by University of Arizona ecologist Tyson Swetnam and published in the open-access science journal Ecosphere.

Swetnam, a member of the BIO5 Institute, is a science informatician at National Science Foundation-funded and UA-headquartered CyVerse, a national cyberinfrastructure project that provides computational support for life sciences. CyVerse resources allowed Swetnam to network and share data with collaborators at other NSF-funded projects to complete the study.

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.

Swetnam and colleagues with the NSF Critical Zone Observatories 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," Swetnam said. Snowmelt from mountains feeds the rivers of the Colorado plateau, where the study was conducted.

Analysis From Three Sites

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 said.

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 noted. 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 added. 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 said.

"In the desert Southwest, if we didn't have mountains we wouldn't have forests. We need the complex terrain to create precipitation," he said. "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 said.

"We anticipate that it's going to get hotter and drier, and forests will be in competition with each other for water," he said. "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 said, 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.

CyVerse Speeds Computations

Swetnam used CyVerse computational resources to conduct GIS (geographic information system) and statistical analyses for the study. "The availability of CyVerse and Jetstream computational resources makes GIS computations faster and easier," Swetnam noted.

The team shared its data and calculations via the CyVerse Data Store and the NSF's Jetstream Cloud hosted by the Extreme Science and Engineering Discovery Environment, and it used an open science strategy supported by CyVerse to make data, analyses and code for its calculations all freely available for other researchers to reuse.

The data are available for download from the CyVerse Data Store and Critical Zone Observatories data repository. Code for the calculations is available in the paper's supplemental sections.

The study was partially supported by multiple NSF grants to CyVerse, the Boulder Creek Critical Zone Observatory and the National Center for Airborne Laser Mapping at the University of Houston, and by the Department of Energy's Terrestrial Ecosystem Science Program.