Tuesday, August 7, 2007 - 1:30 PM

COS 39-1: Climate sensitivity in the carbon cycle of a Colorado subalpine forest

Russell K. Monson, University of Colorado, William J. Sacks, University of Wisconsin, David J.P. Moore, CIRES and National Center for Atmospheric Research, Sean P. Burns, University of Colorado and NCAR, and David S. Schimel, National Ecological Observatory Network.

After eight years of continuous observations of forest-atmosphere CO2 fluxes at the Niwot Ridge AmeriFlux site, we are beginning to understand how seasonal and interannual variations in climate affect the forest carbon budget and its component processes.  We have used a combination of direct observations using our tower flux systems and data assimilation into an ecosystem process model to evaluate component processes of the observed net CO2 fluxes.  Net ecosystem CO2 exchange (NEE) exhibits a biphasic seasonal pattern, with relatively high rates of CO2 uptake during the spring and autumn, and low rates during the summer.  The biphasic nature of this response is controlled by a slow springtime increase in soil respiration as the snow melts and exposes the soil, and a quick autumnal decrease in soil respiration as the soil dries out just prior to the onset of winter.  Sensitivity of the annual rate of NEE to interannual climate variation is due to the differential effects of early versus late spring snow melt on the magnitude of gross ecosystem CO2 uptake (GEE), and the timing of the summertime rise in soil respiration rate.  Unlike many temperate- and northern-latitude forests that respond to earlier spring warming through increased annual CO2 uptake, the Niwot Ridge forest responds to earlier spring warming through decreased annual CO2 uptake;  this is due to high sensitivity of GEE to low air temperatures and low atmospheric humidity when the initiation of snow melt is forced earlier in the spring.

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