Subarctic environments such as Denali National Park and Preserve’s tundra and boreal forest, have undergone drastic changes over the past few decades, likely as a result of a changing climate–increasing temperatures. Because Denali lays at the southern limits of permafrost, the average temperature of the park’s permafrost hovers just below freezing, making it particularly sensitive to thawing with just a small increase in mean annual temperature. Warmer temperatures are expected to thaw permafrost, which will greatly affect ecosystems locally. But one of the farthest–reaching consequences of climate change in northern ecosystems may be the effect of thawing permafrost on the global carbon cycle.
Net Balance of C
More than 50% of global terrestrial carbon (C) is stored in permafrost regions as soil organic matter. Carbon naturally enters any terrestrial ecosystem by photosynthesis, since plants take in carbon dioxide (CO2) from the atmosphere as they grow. Carbon returns to the atmosphere CO2 from the metabolic respiration of plants, animals, and microbes (bacteria). In water logged areas carbon can also be released in the form of methane (CH4). The ecosystem carbon balance is the difference between carbon uptake and emissions.
When carbon uptake by plant growth is greater than carbon emissions by metabolic respiration, the ecosystem is a carbon sink, meaning that atmospheric C is stored in biota and soils. When carbon emissions are greater than carbon uptake, the ecosystem is a carbon source, meaning that carbon from the ecosystem is released (from biota and soils) to the atmosphere.
Potential Effects of Thawing Permafrost
Permafrost thaw associated with climate warming can lead to two different impacts that change ecosystem carbon balance to a sink or a source. Warming increases plant growth and it promotes the invasion of shrubs and trees into tundra landscapes. These processes can increase the amount of C stored in plant biomass thus reducing the amount of C in the atmosphere.
At the same time, permafrost thaw and the associated environmental changes (e.g., ground surface collapse or subsidence) stimulate the microbial decomposition of soil organic matter. This decomposition can decrease the amount of stored C by releasing more CO2 into the atmosphere.
These metabolic by-products (CO2 and CH4) are the same “greenhouse gases” involved in climate change. Thus, when permafrost thaws it may affect the cycling of C to or from the atmosphere which can create additional global-scale impacts.
Studying the Effects of Permafrost Thaw on C
To learn more about how permafrost thaw impacts the ecosystem carbon balance, Dr. Ted Schuur of Northern Arizona University, conducts research at a site just outside the northeastern boundary of the park. This tundra site near Eight Mile Lake is underlain by permafrost that researchers first noticed was thawing in the 1980’s.
Schuur and his research group continuously measure soil temperature and moisture using sensors connected to a data-logger to track physical changes in the environment as the permafrost thaws. The site’s carbon balance is monitored using an eddy covariance tower that measures the wind speed and direction as well as the CO2 concentration of the atmosphere. The site is a well-drained wetland so the contribution of CH4 to the landscape carbon balance is minimal. Data from the tower sensors are then analyzed to determine the Net Ecosystem Exchange (NEE) of CO2 between the tundra and the atmosphere.
Every year, the NEE of the tundra landscape oscillates between being a carbon source in the winter and a carbon sink during the growing season (May to September). During the growing season, NEE is negative and indicates that the landscape is acting as a carbon sink of atmospheric CO2. This is because growing tundra plants incorporate carbon into new leaves, roots and stems through the process of photosynthesis, outweighing CO2 losses from metabolic processes. During the winter, however, it is too cold and dark for plants to photosynthesize so the tundra landscape has a positive NEE. A positive NEE rate shows that landscape is acting as a carbon source: it is releasing CO2 from plants and soils to the atmosphere. This CO2 is released as tundra plants and soil microbes break down carbon compounds and use the energy stored in those carbon compounds to fuel their metabolism. Metabolic respiration of CO2 by plants and microbes takes place year round but during the growing season this release of CO2 is overshadowed by the photosynthetic uptake of CO2 of the plant community.
Dr. Schuur and his team monitor NEE year round so that they can capture daily and seasonal changes in the landscape carbon balance as permafrost thaws. Year round measurements of NEE are also used to determine whether the strength of the growing season carbon sink is equal to the strength of the winter carbon source. When this is true, the ecosystem carbon balance is stable and the carbon storing capacity of the landscape does not change. The long–term carbon balance of a permafrost landscape is important because it allows scientists to understand how this ecosystem is responding to climate change.
Findings: Six years of carbon balance monitoring
Monitoring the ecosystem C balance of the tundra near Eight Mile Lake has revealed that that this landscape acted as a net carbon source from 2008–2013. Photosynthesis by plants in the growing season did not offset the year round metabolic respiration by plants and soils in five of the six years studied. As a result, the carbon storing capacity of this landscape was reduced from 2008 to 2013: 276.25 g of C was lost from each m2 of tundra. This C was previously stored in tundra plants and soils was released to the atmosphere in the form of CO2.
Carbon dioxide is a greenhouse gas that is expected to further accelerate global warming and could cause further thaw of permafrost in the future so it is critical that the scientific community understands whether the carbon balance at this site is typical of warming tundra landscapes. Dr. Schuur and his research team collaborate with permafrost researchers from around the world to compare results from sites and determine the impact permafrost thaw will have on our future climate.