Cooperative Institute for Research in Environmental Sciences

Science Rendezvous > 2009 Posters
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Snow, Soil Freeze/Thaw, and the Carbon Cycle

Kevin Schaefer, Tingjun Zhang, Lixin Lu, and Ian Baker

We evaluate how snow cover, soil temperature, and soil freeze-thaw processes influence terrestrial net carbon flux. The net carbon flux or Net Ecosystem Exchange (NEE) is the small difference between two large gross fluxes: R - GPP, where R is respiration and GPP is Gross Primary Productivity or photosynthesis. GPP for plant growth removes CO2 from the atmosphere while R due to the decay of dead plants returns CO2 to the atmosphere. Negative NEE indicates net biological uptake of CO2. We used time-lagged correlation analysis of model output to isolate the complex interactions between snow, soil temperature, and the carbon cycle. We ran the Simple Biosphere Carnegie-Ames-Stanford Approach (SiBCASA) model driven globally by the NCEP reanalysis at 2x2 degrees and driven locally at eddy covariance flux towers by the NARR reanalysis at 32 km resolution.

Variations in winter snow depth are preserved as variations in soil temperature, producing time-delayed effects on R and GPP and modulating the NEE for the entire year. Deeper snow in winter insulates the soil, resulting in warmer soil temperatures the following spring and summer. This increases both R and GPP, but at different magnitudes throughout the year. In spring and fall, warmer soils increase R, but GPP is low, resulting in increased NEE. The magnitude of the effect of winter snow depth on summer NEE depends on what environmental factors limit GPP. In permafrost, the active layer depth limits summer GPP, so warmer winter soil temperatures increase active layer depth in summer, increasing GPP and decreasing NEE. In boreal forests, summer GPP is limited by water availability, so winter snow depth influences spring and fall NEE by modulating R, but exerts a much weaker influence on summer NEE.