Forest response to increased disturbance in the central Amazon and comparison to western Amazonian forests

[摘要] Uncertainties surrounding vegetation response to increased disturbance ratesassociated with climate change remains a major global change issue for Amazonianforests. Additionally, turnover rates computed as the average of mortalityand recruitment rates in the western Amazon basin are doubled when comparedto the central Amazon, and notable gradients currently exist in specific wooddensity and aboveground biomass (AGB) between these two regions. This studyinvestigates the extent to which the variation in disturbance regimescontributes to these regional gradients. To address this issue, we evaluateddisturbance–recovery processes in a central Amazonian forest under two scenariosof increased disturbance rates using first ZELIG-TROP, a dynamic vegetationgap model which we calibrated using long-term inventory data, and secondusing the Community Land Model (CLM), a global land surface model that ispart of the Community Earth System Model (CESM). Upon doubling the mortalityrate in the central Amazon to mirror the natural disturbance regime in thewestern Amazon of ∼2% mortality, the two regions continued todiffer in multiple forest processes. With the inclusion of elevated naturaldisturbances, at steady state, AGB significantly decreased by 41.9% withno significant difference between modeled AGB and empirical AGB from thewestern Amazon data sets (104 vs. 107 Mg C ha⁻¹, respectively).However, different processes were responsible for the reductions in AGBbetween the models and empirical data set. The empirical data set suggests thata decrease in wood density is a driver leading to the reduction in AGB. Whiledecreased stand basal area was the driver of AGB loss in ZELIG-TROP, a forestattribute that does not significantly vary across the Amazon Basin. Furthercomparisons found that stem density, specific wood density, and basal areagrowth rates differed between the two Amazonian regions. Last, to helpquantify the impacts of increased disturbances on the climate and earthsystem, we evaluated the fidelity of tree mortality and disturbance in CLM.Similar to ZELIG-TROP, CLM predicted a net carbon loss of 49.9%, with aninsignificant effect on aboveground net primary productivity (ANPP).Decreased leaf area index (LAI) was the driver of AGB loss in CLM, anotherforest attribute that does not significantly vary across the Amazon Basin,and the temporal variability in carbon stock and fluxes was not replicated inCLM. Our results suggest that (1) the variability between regions cannot beentirely explained by the variability in disturbance regime, but ratherpotentially sensitive to intrinsic environmental factors; or (2) the modelsare not accurately simulating all tropical forest characteristics in responseto increased disturbances.

[学科分类] 地球化学与岩石