Ninth International Conference on Permafrost ... - IARC Research

Spatial Variation in CO 2Release from Arctic Tundra as a Result of PermafrostThawing and Thermokarst DevelopmentHanna LeeDepartment of Botany, University of Florida, Gainesville, FL 32601-8526, USAEdward A.G. SchuurDepartment of Botany, University of Florida, Gainesville, FL 32601-8526, USAJason G. VogelDepartment of Botany, University of Florida, Gainesville, FL 32601-8526, USAIntroductionOne of the biggest potential feedbacks to global climatechange from high latitude ecosystems may come from thawingof permafrost, which stores 30% of the total global terrestrialsoil organic carbon (SOC) (Gorham 1991). Permafrostthawing may accelerate decomposition of soil organicmatter (SOM) and increase carbon dioxide (CO 2) emissions,which could lead to further climatic warming (Oechel et al.2000). In particular, thermokarst formation in response topermafrost thawing could change C cycling in high latitudeecosystems beyond simple increases in temperature becauseit has unique effects on soil conditions.When permafrost thaws and drains in ice-rich areas, itcreates localized surface subsidence called thermokarst(Jorgenson et al. 2001). The changes in the ground surfacetopography can induce variations in soil properties such assoil temperature, moisture content, and nutrient availability(Chapin et al. 2000). Our objective was to determine howpermafrost thawing and thermokarst development affect SOMdecomposition and ecosystem C exchange. We hypothesizedthat there will be a positive relationship between the degreeof ground subsidence and CO 2emissions from SOM.Materials and MethodsField siteThis study was established at the Eight Mile Lake (EML)tundra site located 5 miles outside of Denali National Park. Previouswork by Osterkamp and Romanovsky (1999) monitoreddeep soil temperatures at this site to 27 m belowground for theprevious two decades and observed increased permafrost temperaturesand thermokarst development (Osterkamp 2007).Three sites were established at EML as an observationalnatural gradient study based on the degree of thermokarstdevelopment. The observed gradient was divided into threecategories: Minimal Thaw, where typical tussock tundra appearsleast disturbed; Moderate Thaw, where thermokarstdevelopment started about 20 years ago; and Severe Thaw,where there were significant surface depressions. Based on1951 aerial photographs, thermokarst development at SevereThaw was estimated to be present for at least 50 years(Schuur et al. 2007).Defining microtopographyWe defined the degrees and patterns of depression createdby thermokarst using a topographic survey. Twelve transectswere established within a 50 m × 50 m plot at each of thethree sites, and 600 (±50) points were surveyed. The majorindependent variable was elevation representing variationin microtopography created by thermokarst; lower surfacesrepresent subsidence by thermokarst development.Establishing the plotsA 50 × 25 m subplot was established within the surveyedarea at each site. In each plot, 50 equally spaced points wereselected and surveyed again with a fine-scale GPS (Trimble5700) unit within the plot to relate microtopography to soiltemperature, moisture, active layer thickness, ecosystemrespiration, and photosynthesis.Soil propertiesSoil temperature, volumetric water content (VWC), andactive layer thickness were measured across the sites. Ahandheld soil temperature probe was used to measure soiltemperature at 10, 20, and 30 cm during the growing season.VWC was measured at 10 and 20 cm belowground usinga soil moisture reflectometer (Campbell Scientific CS616),and a 1/8 inch rod was used as a depth probe to measureactive layer thickness.Carbon emissionsCO 2flux was quantified 4 times during the summer of 2006and 2007 at the peak of the growing season to measure netecosystem exchange of carbon using an IRGA (infrared gasanalyzer, LiCOR820) attached to a 40 × 40 × 40 cm plasticchamber. Both dark CO 2chamber measurement and lightmeasurement were taken covered with a reflecting cloth, , com-pletely intercepting light and uncovering the chamber. Darkmeasurements estimate the rate of carbon emissions fromecosystem respiration, whereas light measurements estimatecarbon emissions from ecosystem respiration as well as carbonuptake by photosynthesis. Differences between light and darkmeasurement served as an estimate of photosynthesis.AnalysesTo compare the effects of microtopography among sites,the plot level elevations were normalized to relative valueswithin each site and semivariograms and correlograms werecalculated to mathematically define the surface structure.A multiple regression with stepwise selection was used toobtain the best relationship between microtopography andmeasured soil and ecosystem properties.173