Radiative forcings from albedo and carbon dynamics after clear cut in Massachusetts forests

Background/Question/Methods

Recent efforts have sought to compare and contrast the radiative forcings excited by forest disturbances due to both biogeochemical and biogeophysical mechanisms (Bonan et al., 2008; e.g. Randerson et al., 2005; Brovkin et al., 2004; Randerson et al., 2006).  Study in boreal forests led to the important finding that the albedo increase from snow exposure after canopy destroying fire offsets the warming from carbon emissions (Randerson et al., 2005).  Similar study is lacking for temperate forests, leading to uncertainty about the net effect of albedo and carbon forcings following their disturbance.  This work begins to address that uncertainty by quantifying the gross and net radiative forcings from albedo and carbon mechanisms after clear cutting in western Massachusetts.  High temporal resolution albedo and carbon flux measurements at a clear cut and mature hardwood forest site at Harvard Forest are compared to find the net radiative forcing of clear cut over four years.  Carbon fluxes are estimated from detailed biomass inventories at both sites, as well as additional measurement with eddy covariance at the clearing.  Associated radiative forcing is estimated with conventional methods quantifying the perturbation to CO₂ in the atmosphere and its lifetime considering ocean uptake (and vegetation regrowth.  

Results/Conclusions

For the first four years after clear cut, albedo change led to a cooling radiative forcing throughout the year (seasonal midday means as high as JFM: -130; AMJ: -50; JAS: -60; OND: -50 W m^-2).  Annual albedo related cooling was large enough to offset the carbon related warming (leading to net cooling of -25-40 W m^-2 over the span of observation).  Summer albedo related radiative forcing was large enough during observation to lead to a net cooling radiative forcing, even if snow was not present during the winter.  To expand the temporal window and spatial variability, annual estimates of albedo were derived from a Landsat based albedo product (Shell et al., 2008).  The albedo related cooling diminishes to zero within two decades of a clear cut.