Controls on carbon dioxide flux through snow in forested and clearcut ecosystems

Cold and ‘shoulder' season ecosystem respiration (R) often comprises some 10-20% of the annual carbon balance in ecosystems with a seasonal snowpack, but biological and physical controls on these fluxes remain poorly understood. Specifically, the contribution of advective transport of trace gas through snow may be non-trivial, but most approaches to date assume that diffusion dominates flux through snow. We measured R subarctic birch and tundra ecosystems near Abisko, Sweden and in clearcut and lodgepole pine ecosystems in the Tenderfoot Creek Experimental Forest, MT, for multiple years. Flux-gradient, chamber, and eddy covariance techniques are compared, and the role of wind speed and atmospheric and subnivean pressure variations for eliciting flux responses are explored.

A wavelet coherence analysis demonstrates that hourly variability in wind speed and multi-day (mesoscale) variability in atmospheric pressure elicit a response in subnivean carbon dioxide concentration. A novel spectral Grainger Causality analysis is used to demonstrate thresholds in the subnivean response to atmospheric pressure variability, and changes to this response during snowpack development. Together, results suggest that the forest canopy plays an important role in regulating flux response to snow surface wind speed, but not atmospheric pressure, highlighting the important interaction between biological and physical processes in controlling the variability of cold-season surface-atmosphere exchange.