Ninth International Conference on Permafrost ... - IARC Research

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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
Ni n t h In t e r n at i o n a l Co n f e r e n c e o n Pe r m a f r o s tTable 1. Probability of each significant variable in a stepwiseregression at each site.Site Variables Step Sig. Prob.Minimal ThawModerate ThawSevere ThawVWC20cmVWC10cmActive LayerVWC20cmActive LayerVWC10cm1212120.00000.06830.00000.00000.00000.0008Figure 1. Semivariogram and correlogram of microtopography ofthe three sites in EML. Semivariograms show autocorrelation ofmicrotopography as a function of distance. Minimal Thaw site hashomogeneous surface; Moderate Thaw site has patchy depressionwithin the thermokarst; and Severe Thaw site has large scaledepression and is heterogeneous.ResultsMicrotopographic patternsOur semivariogram and correlogram results show thatMinimal Thaw site has the most uniform surface pattern ofall 3 sites. There are patchy depressions within ModerateThaw site, while Severe Thaw site shows large-scaleheterogeneous depressions that are not autocorrelated over15 m (Fig. 1).Soil propertiesAll of the measured soil properties were significantlycorrelated to topographic patterns, except for soil temperaturedue to a measurement error. Among the soil propertymeasurements, VWC showed the strongest relationshipwith topographic patterns (p < 0.008) for all three sites,implying that soil moisture conditions changed the most dueto thermokarst development (Table 1).Carbon emissionsMultiple regression analyses (Fig. 2) showed a negativecorrelation between ecosystem respiration and microtopographyat Severe and Minimal Thaw sites. Amongthe environmental variables, the best predictor variablefor ecosystem C exchange was VWC from changes in soilproperties.ConclusionsChanges in microtopography created by thermokarstdevelopment alter soil properties, especially soil moisturecontent by redistributing water when the ground surfacesubsides. Subsided areas showed higher CO 2emissions;therefore, we suggest thermokarst development maystimulate CO 2emissions in high latitude ecosystems.Figure 2. Relationship between microtopography and ecosystemrespiration measured using dark CO 2chamber.AcknowledgmentsWe thank C. Staudhammer and M. Lavoie for advice onspatial analysis; UNAVCO for Trimble rental and training.This research was funded by NSF grant DEB-0516326awarded to EAGS.ReferencesChapin III, F.S. et al. 2000. Arctic and boreal ecosystems ofwestern North America as components of the climatesystem. Global Change Biology 6: 211-223.Gorham, E. 1991. Northern peatlands: Role in the carboncycle and probable responses to climatic warming.Ecological Applications 1: 182-195.Jorgenson, M.T., Racine, C.H., Walters, J.C. & Osterkamp,T.E. 2001. Permafrost degradation and ecologicalchanges associated with a warming climate in centralAlaska. Climate Change 48: 551-579.Oechel, W.C. et al. 2000. Acclimation of ecosystem CO 2exchange in the Alaskan Arctic in response to decadalclimate warming. Nature 406: 978-981.Osterkamp, T.E. & Romanovsky, V.E. 1999. Evidence forwarming and thawing of discontinuous permafrost inAlaska. Permafrost and Periglac. Process 10: 17-37.Osterkamp, T.E. 2007. Characteristics of the recent warmingof permafrost in Alaska. Journal of GeophysicalResearch 112: F02S02, DOI: 10.1029.Schuur, E.A.G., Crummer, K.G., Vogel, J.G. & Mack, M.C.2007. Plant species composition and productivityfollowing permafrost thaw and thermokarst inAlaskan tundra. Ecosystems 10: 280-292.174
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ContentsPreface ...................
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Mapping and Modeling the Distributi
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Satellite Observations of Frozen Gr
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xiiIce Wedge Thermal Regime in Nort
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xivPermafrost Response to Dynamics
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NICOP SponsorsUniversitiesUniversit
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Deep Permafrost Studies at the Lupi
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Effect of Fire on Pond Dynamics in
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Cryological Status of Russian Soils
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Acoustical Surveys of Methane Plume
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Permafrost Delineation Near Fairban
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Preparatory Work for a Permanent Ge
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A Provisional Soil Map of the Trans
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Martian Permafrost Depths from Orbi
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Time Series Analyses of Active Micr
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Impact of Permafrost Degradation on
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DC Resistivity Soundings Across a P
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Modeling Thermal and Moisture Regim
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A Provisional Permafrost Map of the
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Alpine Permafrost Distribution at M
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Cryogenic Formations of the Caucasu
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Modeling Potential Climatic Change
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A Hypothesis: A Condition of Growth
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Modeled Continual Surface Water Sto
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Freeze/Thaw Properties of Tundra So
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Discontinuous Permafrost Distributi
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Thermal Regime Within an Arctic Was
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Seasonal and Interannual Variabilit
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Twelve-Year Thaw Progression Data f
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Continued Permafrost Warming in Nor
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Landsliding Following Forest Fire o
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A Permafrost Model Incorporating Dy
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Seasonal Sources of Soil Respiratio
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Greenland Permafrost Temperature Si
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The Importance of Snow Cover Evolut
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The Account of Long-Term Air Temper
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The Combined Isotopic Analysis of L
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Adaptating and Managing Nunavik’s
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Human Experience of Cryospheric Cha
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HiRISE Observations of Fractured Mo
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A Soil Freeze-Thaw Model Through th
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Mapping and Modeling the Distributi
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First Results of Ground Surface Tem
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Historical Changes in the Seasonall
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Rock Glaciers in the Kåfjord Area,
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Snowpack Evolution on Permafrost, N
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Climate Change in Permafrost Region
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Maximizing Construction Season in a
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Pleistocene Sand-Wedge, Composite-W
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