Estimating Terrestrial Wood Biomass from Observed Concentrations of Atmospheric Carbon Dioxide

We estimate terrestrial disequilibrium state and wood biomass from observed concentrations of atmospheric CO2 using the CarbonTracker system coupled to the SiBCASA biophysical model. Starting with a priori estimates of carbon flux from the land, ocean, and fossil fuels, CarbonTracker estimates net carbon sources and sinks from 2000 to 2007 that are optimally consistent with observed CO2 concentrations. The a priori terrestrial Net Primary Productivity (NPP) and heterotrophic respiration (Rh) from SiBCASA assume steady state conditions for initial biomass, implying mature ecosystems with no disturbances where growth balances decay and the long-term, net carbon flux is zero. In reality, harvest, fires, and other disturbances reduce available biomass for decay, thus reducing Rh and resulting in a long-term carbon sink. The disequilibrium state is the ratio of Rh estimated from CarbonTracker to the steady state Rh from SiBCASA. Wood is the largest carbon pool in forest ecosystems and the dominant source of dead organic matter to the soil and litter pools. With much faster turnover times, the soil and litter pools reach equilibrium relative to the wood pool long before the wood pool itself reaches equilibrium. We take advantage of this quasi-steady state to estimate the size of the wood pool that will produce an Rh that corresponds to the net carbon sink from CarbonTracker. We then compare this estimated wood biomass to regional maps of observed above ground wood biomass from the US Forest Inventory Analysis.