Evaluating Amazonian Forest Response to Increased Disturbance: Lessons From a Gap Model for Future Model Improvement

Tuesday, May 13, 2014 - 07:00
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Uncertainties surrounding vegetation response to increased disturbance rates associated with climate change remains a major global change issue for Amazon forests. This project aims to evaluate the mortality and disturbance processes in the Community Land Model (CLM), a global land surface model that is part of the Community Earth System Model (CESM). This task will benefit the Integrated Earth System Model (iESM) project, which combines CLM/CESM with a fully integrated human system component. To assist in accurately developing mitigation strategy targets, and evaluate energy market shifts and carbon policy changes in iESM the land component CLM needs to more accurately simulate plant mortality, coarse litter inputs, carbon fluxes, and accelerated growth processes associated with disturbance-recovery events. To address these issues, we evaluated disturbance-recovery processes under two scenarios of increased disturbance rates in a complex Central Amazon forest using first ZELIG-TROP, a dynamic vegetation gap model, and second using CLM. Upon doubling the mortality rate in the Central Amazon to mirror the natural disturbance regime in the Western Amazon of ~2% mortality, AGB significantly decreased by 41.9% and there was no significant difference between the modeled AGB of 104 Mg C ha-1 and empirical AGB from the western Amazon datasets of 107 Mg C ha-1. We confirm that increases in natural disturbance rates will result in terrestrial carbon loss. However, different processes were responsible for the reductions in AGB between the models and empirical datasets. In addition, stem density, specific wood density, and basal area growth rates differed between the modeled and empirical Amazonian dataset. This suggests that: 1) the variability across Amazonia cannot be entirely explained by the variability in disturbance regime, but rather potentially sensitive to intrinsic environmental factors, which should be taken into account in CLM; or 2) the models are not accurately simulating all forest characteristics in response to increased disturbances. Last, to help quantify the impacts of increased disturbances on climate and the earth system, and eventually iESM, we evaluated the fidelity of elevated tree mortality in CLM. For a 100% increase in annual mortality rate, both ZELIG-TROP and CLM were in close agreement with each other and predicted a net carbon loss of 41.9% and 49.9%, respectively, with an insignificant effect on aboveground net primary productivity (ANPP). Likewise, a 20% increase in mortality every 50 years (i.e. periodic disturbance treatment) resulted in a reciprocal biomass loss of 18.3% and 18.7% in ZELIG-TROP and CLM respectively.

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