Ninth International Conference on Permafrost ... - IARC Research

Non-Summer CO 2Measurements Indicate Tundra Ecosystem Annual Net Source ofCarbon Double Net Summer SinkCheryl LaskowskiSan Diego State University, San Diego, CA, USAGeorge BurbaLiCor, Inc., Lincoln, NE, USAWalter OechelSan Diego State University, San Diego, CA, USAIntroductionThe Arctic tundra ecosystems are dominated by winterconditions, but data during the non-summer months is oftenlacking in Arctic annual carbon exchange estimates. Theextreme conditions (temperatures regularly below freezing,low light levels, and snow-covered frozen ground) are oftenless than ideal for biological processes, and it has been largelyassumed that little activity occurs under these conditions(McKane et al. 1997). While winter arctic conditionsmay not be ideal for high levels of photosynthesis andrespiration, biological activity still occurs under snow andin subzero temperatures (Romanovsky & Osterkamp 2000,Sturm et al. 2005). Although there is substantial evidenceshowing the importance of non-summer periods to Arcticcarbon metabolism, the difficulty of making ecosystemmeasurements under Arctic winter conditions means thatestimates of carbon flux rates across the tundra are typicallymade based solely on summer (June–August) carbon fluxdata (Kwon et al. 2006, Vourlitis & Oechel 1999).Annual estimates that ignore the non-summer periodare likely to be largely in error with respect to the annualcarbon balance (Oechel et al. 1997). Given that snowmeltis occurring earlier (Stone et al. 2002) and the growingseason has lengthened (Chapman & Walsh 1993, Keyseret al. 2000), the traditional 10- to 12-week field campaignno longer encompasses even the full snow-free period.Prior attempts to predict annual carbon budgets based onwinter field measurements have often lacked continuousmonitoring throughout the year, relying on a few data pointsto model seasonal carbon estimates (Zimov et al. 1996,Oechel et al. 1997). Attempts at continuous monitoring(i.e., through the eddy covariance method) have had limitedsuccess while highlighting some of the major limitations.The limitations that have posed the greatest hindrance tocontinuous monitoring in the past are now diminishing, andinclude instrument icing (minimized by heaters), instrumentoperating limits (that have been recently modified to operateat very low temperatures), and mismatch between CO 2fluxestimates and ecological/biological theory (for which thereis a new correction provided by Burba et al. 2006).MethodsDirect and continuous measures of mass (water vaporand CO 2), momentum, and energy exchange were measurednear the village of Atqasuk, Alaska, located 100 km south ofBarrow, Alaska, during 2006. Net ecosystem exchange wasmeasured using the open-path eddy covariance method andincluded an open-path infrared gas analyzer (IRGA, Li-7500,LiCor, Inc., Nebraska, USA) and ultrasonic anemometer(R3, Gill Instruments, Lymington, England).Eddy covariance data were calculated in half-hour dataseries using the EdiRe program (University of Edinburgh,Edinburgh, England), and applying standard corrections forsimultaneous latent and sensible heat measurements (Webbet al. 1980) and quality control techniques (AmeriFlux 2006).Data gaps were filled using techniques outlined in Falge etal. (2001) to represent seasonal/annual carbon exchangeestimates. In addition, the data are corrected for an apparentuptakesignal due to heating of the air mass in the absorptionpath of the IRGA. The correction is modified from Burba etal. 2006, to account for non-vertical sensor mounting angle.ResultsWeather conditions during 2006 were typical of historicclimate averages for Atqasuk, Alaska, with July as thewarmest month of the year and March, the coldest month(air temperatures of 8.2°C and -29.8°C, respectively). Eddycovariance data availability varied greatly, from a low of14% in February to a high of 80% in June. Average summer(June–August) data recovery was 71%, and data wereprimarily rejected due to precipitation events. Average nonsummer(September–May) data recovery was 36%, and dataloss was mainly due to icing events and conditions outsideof the instruments’ operating specifications.Summer carbon flux data showed a distinct diurnal patterncharacterized by a midday CO 2drawdown and slight midnightrelease, with the pattern being strongly temperatureandlight-dependent. Non-summer diurnal patterns werenot evident, due to the lack of intense solar radiation duringmuch of the season.Cumulative carbon exchange during the summer monthsresulted in a net uptake of 21.5 g C m -2 , whereas thecumulative non-summer carbon exchange showed a net lossof 53.5 g C m -2 . This resulted in an annual carbon release of31.8 g C m -2 to the atmosphere (Fig. 1).DiscussionThe results show that non-summer season carbon exchangeis not only significant, it can be of greater cumulativemagnitude than summer carbon exchange—in this case,165