Understanding the mechanisms behind observed biomass dynamics at 10 Amazonian field sites: a model-data intercomparison

The Amazonian rainforests play a vital role in global water, energy and carbon cycling. The sensitivity of this system to natural and anthropogenic disturbances therefore has important implications for the global climate. Field observations suggest that there has been a net increase in biomass in old growth Amazonian forests over the past two decades (Baker et al 2004), however the mechanisms behind this change remain unclear. A model-data intercomparison is conducted with four state-of-the-art terrestrial ecosystem models to disentangle the processes responsible for the observed trends and inter-annual variability in above-ground biomass dynamics. Each modeling group conducted a series of simulations for 10 priority sites chosen to represent a range of Amazonian ecosystem types including forest sites, savannah sites, and agricultural/pasture sites. All models were run using standard physical parameters and the same initialization procedure. In general, the model results compare reasonably well against the field observations, reproducing the diurnal cycles in energy and carbon fluxes and correctly simulating annual mean fluxes. However, most of the models used in this study struggle to reproduce the observed seasonality in carbon and energy fluxes. The models do predict the observed long-term increase in biomass. Preliminary results suggest that this increase may be primarily due to increasing atmospheric CO2 concentrations coupled with disturbance history. This study provides insight into tropical forest function and sensitivity to environmental conditions that will aid in predictions of the response of the Amazon rainforest to future anthropogenically induced changes.