GLOBAL BIOGEOCHEMICAL CYCLES, VOL. 15, No. 2, PAGES 321-335, JUNE 2001 Carbon gas exchange at a southern RockyMountain wetland, 1996-1998 Kimberly P. Wickland U.S. Geological Survey, Boulder, Colorado Robert G. Striegl, M. Alisa Mast,and David W. Clow U.S. Geological Survey, Denver, Colorado Abstract. Carbon dioxide (CO2)and methane (CH4)exchange between theatmosphere anda subalpine wetland located in Rocky Mountain National Park, Colorado, at 3200 rnelevation weremeasured during 1996-1998. Respiration, net CO2flux, andCH4 flux weremeasured using the closed chamber method during snow-free periods and using gas diffusion calculations m-2d-• during snow-covered periods. The ranges of measured flux were 1.2-526 mmol CO2 m -2 d -1 (respiration), -1056-100 mmol CO2 m-2-d '• (net CO2 exchange), and 0.1-36.8 mmol CH4 (a positive value represents effiux to theatmosphere). Respiration and CH4emission were significantly correlated with 5 cm soil temperature. Annual respiration and CH4 emission were modeled by applying the flux-temperature relationships toa continuous soil temperature record during 1996-1998. Gross photosynthesis wasmodeled using a hyperbolic equation relating gross photosynthesis, photon flux density, and soil temperature. Modeled annual flux estimates indicate that the wetland wasa net source of carbon gasto the atmosphere each of the three years' 8.9 mol Crn '2 yr -•in1996, 9.5 mol Cm -2 yr -•in1997, and 9.6 mol Crff 2 yr -• in1998. This contrasts with the long-term carbon accumulation of-0.7mol m -2 yr -• determined from •4C analyses ofapeat core collected from the wetland. 1. Introduction Incomplete decomposition of organic material results in the accumulation of carbon and nutrients in most wetlands. Wetlands gaina large amount of their carbon autotrophically fromatmospheric CO:, andthey lose much of it back to the atmosphere as CO: and CH4 emissions derived from decomposition andrespiration. There is concern that changes in climate, such as warmer temperatures or decreased precipitation, may accelerate decomposition rates in some wetlands, causing them to become net sources of carbon tothe atmosphere [Gorham, 1991; Oechel et al., 1995].Carbon cycling in northern, high-latitude wetlands has been a focus of recent researchbecausethese wetlands store about one third of thetotal global soilcarbon pool [Gorham, 1991]and because the carbon balance of these wetlands appears to be sensitive to small changes in climate [Oechel et al., 1995; Carrolland Crill, 1997; Oechelet al., 1998;Aim et al., 1999;Soegaard and Nordstroem, 1999]. Carbon cycling in high-altitude wetlands may alsobe sensitive to minorclimate variations. Baron et al. [2000] modeledthe response of an alpine- subalpine watershed to different climate change scenarios and found thatchanges in the winter andspring climate, such as snow accumulation andthetimingof snowmelt, could have a This paper is not subject to U.S. copyright. Published in 2001 by the American Geophysical Union. Papernumber 2000GB001325. significant impact on hydrology and vegetation dynamics. Short- or long-term changes in temperature and/or precipitation could invoke changes in the carbon balance of high-altitude wetlandssimilar to that predicted in high- latitude wetlands. During the past century, theEuropean Alps experienced an increase of 2øC in minimum temperatures and a somewhat smaller increase in maximum temperatures [Haeberli and Beniston, 1998], and recentmelting of Rocky Mountain glaciers is well documented [Hall et al., 1994; Fagre, 1998]. Climate records at Niwot Ridge, Colorado (3750 m altitude, 30 km southof the studywetland), indicate trends of increased precipitation and decreased solar radiation in the Colorado Front Range, but no significant trend in temperature from 1951 to 1994 [Williamset al., 1996]. The precipitationand solar radiation trends are consistent with climate changepredictions simulated by the nested regional MM4 model under 2 x CO2 scenarios [Giorgi et al., 1994; Williams et al., 1996]. However, the model also predicts an increase in annualtemperatures. General circulation models predict an increase in summerand winter temperatures and slight changes in precipitation in boththeAlps [Haeberliand Beniston, 1998] and in the southern Rocky Mountains [Baron et al., 2000] if greenhouse gas concentrations continueto increase. An accurate estimate of the global extent of mountain wetlands does not exist, but there are certain geomorphic characteristics that favor the formation of wetlands in mountainous regions, primarily in high mountain valleysand in intermountain basins [Windell et al., 1986]. In mountain 321