Ninth International Conference on Permafrost ... - IARC Research

Vegetation Change and Thermokarst Development: Effects on Ecosystem CarbonExchange in Upland Tussock TundraJason G. Vogel, Hanna Lee, Christian Trucco, Edward A.G. SchuurDepartment of Botany, University of Florida, Gainesville, FL 32601-8526, USAJames SickmanDepartment of Environmental Sciences, University of California Riverside, Riverside, CA 92521, USAIntroductionThermokarst development generally alters the vegetationcomposition of an area because of changes in soil temperatureand moisture (Camill et al. 2001). In upland tussock tundra,thermokarst depressions are wetter and warmer thanundisturbed areas, while areas along the sides of thermokarstare drier due to drainage (Schuur et al. 2007). The changesin soil climate and vegetation composition that occur withthermokarst could affect ecosystem C cycling. Plant growth,or net primary productivity (NPP) and microbial respiration(Rm) of organic matter are both affected by soil climate, andit is the balance between these two processes that determinesnet ecosystem exchange (NEE). Vegetation composition candirectly affect NPP because plant species differ in growthpotential, and can indirectly affect Rm because tissuechemistry strongly influences decomposition rates (Chapin& Shaver 1996). Potential C loss from permafrost soils isimportant to the global C budget because these soils store~1.6 times more C than is currently in the atmosphere(Schuur et al., in press).The objective of this study was to examine how vegetationcomposition and NPP, and thermokarst development maybe affecting ecosystem C cycling. In a previous study, wereported that thermokarst depressions in upland tussocktundra apparently caused the loss of tussock-formingspecies (Eriophorum vaginatum, Carex bigelowii) and again in deciduous shrubs and mesophilic Sphagnum species(Schuur et al. 2007). In this study, we relate these changes invegetation composition, mortality, and NPP to measurementsof seasonal change in ecosystem respiration (Reco), grossprimary productivity (GPP), and net ecosystem exchange(NEE).Materials and MethodsField siteThe study area was in the Eight Mile Lake (EML)watershed in central Alaska. The EML watershed is located7 miles west of the town of Healy, and is near the northend of Denali National Park and Preserve. Osterkampand Romanovsky (1999) have monitored permafrosttemperatures to 27 m since 1985. Between 1990 and 1998the permafrost profile warmed by ~0.7–1.2°C, warmingthat coincided with thermokarst development (Osterkamp2007). Since 1999, permafrost temperatures have stabilizedor slightly decreased (~0.2°C).A natural gradient study was established within 400 m ofthe permafrost monitoring borehole. Three sites were located:“Minimal Thaw,” where surface topography and tussocktundra vegetation appeared little changed by thermokarst;“Moderate Thaw,” where thermokarst development beganabout 15 years ago; and “Extensive Thaw,” where surfacedepressions are wider and deeper than Moderate Thaw dueto a prolonged period (minimum of 50 years) of thermokarstdevelopment (Schuur et al. 2007).Vegetation samplingVegetation composition and NPP were sampled in twelve0.7 x 0.7 m quadrats (chamber base) per site that weredistributed in pairs across a 40 m transect The “point frame”method was used to estimate vegetation characteristics,where a thin metal rod is passed vertically through the canopyand the number of interception points with vegetation usedto estimate biomass. Site-specific relationships betweenthe number of point intercepts and vegetation biomasswere developed in 2004 (Schuur et al. 2007) and applied tosurveys in 2004 and 2006. We estimated ground coverage oflive and dead mosses and Eriophorum vaginatum using theline-intercept method. The five dominant moss groups withinthe quadrats were identified, including the area coverage ofdead Sphagnum spp.Ecosystem carbon exchangeNEE and Reco were estimated with both an automatic andmanually operated closed chamber system. The chamberswere 0.4 m high and were placed on the same 0.7 x 0.7 areasor chamber bases where NPP and plant species compositionwere measured. The air inside a chamber was circulatedto an infrared gas analyzer (LI-820) and the rate change inCO 2concentration recorded on either a Campbell CR10x(automatic chamber) or Palm Tungsten C palm pilot (manualchambers). Measurements began in the first few weeks ofMay and continued until the end of September. Responsecurves of NEE to light and Reco to temperature weredeveloped and growing season estimates constructed withthese response equations. Gross primary productivity wasestimated as the difference between growing season NEEand Reco.AnalysesMultiple regression analysis was used to determine ifvegetation characteristics of a chamber base (proportionof dead moss and NPP of functional groups) covaried withchamber level variation in growing season NEE, GPP, andReco. The best parameters for the regression model wereselected based on the maximum coefficient of variation335