Cold season ecosystem respiration: The roles of snow and atmosphere in controlling carbon flux

Background/Question/Methods

Cold and ʻshoulder' season ecosystem respiration (R) is thought to comprise 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 advection to total trace gas transport through snow may be non-trivial, but most approaches to date assume that diffusion dominates trace gas flux through snow and consequently underestimate total cold season efflux. These underestimates obscure the important role of biological functioning during winter. We measured R in 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 using a combination of flux-gradient, chamber, and eddy covariance techniques. The role of wind speed, atmospheric pressure, and subnivean pressure are explicitly accounted for to quantify the magnitude and timing of advective flux through snow.

Results/Conclusions

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, and that wind speed is progressively less important as leaf area increases. A novel spectral Grainger Causality analysis demonstrates thresholds in the subnivean response to atmospheric pressure variability, which is a complex function of snowpack development. The ‘enhanced diffusivity’ of trace gasses through snow due to Venturi effects from wind can be effectively modeled using first-order closure models for within-canopy wind speed combined with a simple estimate of total snowpack conductance to trace gas transport. When integrated across the year, results suggest that cold season respiration often comprises nearly 20% of annual ecosystem respiration and cannot be ignored in the annual ecosystem carbon balance. 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.