Abstract
Abstract. Uncertainties surrounding vegetation response to increased disturbance rates associated with climate change remains a major global change issue for Amazonian forests. Additionally, turnover rates computed as the average of mortality and recruitment rates in the western Amazon basin are doubled when compared to the central Amazon, and notable gradients currently exist in specific wood density and aboveground biomass (AGB) between these two regions. This study investigates the extent to which the variation in disturbance regimes contributes to these regional gradients. To address this issue, we evaluated disturbance–recovery processes in a central Amazonian forest under two scenarios of increased disturbance rates using first ZELIG-TROP, a dynamic vegetation gap model which we calibrated using long-term inventory data, and second using the Community Land Model (CLM), a global land surface model that is part of the Community Earth System Model (CESM). Upon doubling the mortality rate in the central Amazon to mirror the natural disturbance regime in the western Amazon of ∼2% mortality, the two regions continued to differ in multiple forest processes. With the inclusion of elevated natural disturbances, at steady state, AGB significantly decreased by 41.9% with no significant difference between modeled AGB and empirical AGB from the western Amazon data sets (104 vs. 107 Mg C ha−1, respectively). However, different processes were responsible for the reductions in AGB between the models and empirical data set. The empirical data set suggests that a decrease in wood density is a driver leading to the reduction in AGB. While decreased stand basal area was the driver of AGB loss in ZELIG-TROP, a forest attribute that does not significantly vary across the Amazon Basin. Further comparisons found that stem density, specific wood density, and basal area growth rates differed between the two Amazonian regions. Last, to help quantify the impacts of increased disturbances on the climate and earth system, 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 an insignificant effect on aboveground net primary productivity (ANPP). Decreased leaf area index (LAI) was the driver of AGB loss in CLM, another forest attribute that does not significantly vary across the Amazon Basin, and the temporal variability in carbon stock and fluxes was not replicated in CLM. Our results suggest that (1) the variability between regions cannot be entirely explained by the variability in disturbance regime, but rather potentially sensitive to intrinsic environmental factors; or (2) the models are not accurately simulating all tropical forest characteristics in response to increased disturbances.
Highlights
One of the largest uncertainties in future terrestrial sources of atmospheric carbon dioxide results from changes to forest disturbance and tree mortality rates, in tropical forests (Cox et al, 2000, 2004; DeFries et al, 2002; Clark, 2007; Pan et al, 2011)
Results simulated by ZELIG-TROP for the mature central Amazon tropical forest were in close range to empirical data (Table 3), making ZELIG-TROP successful at predicting stand dynamics of a complex tropical forest
Since the mid-1970s observed tree mortality and recruitment rates have been increasing in the Amazon (Phillips et al, 2004), and higher than usual mortality rates have been associated with droughts and strong windstorm events (Nepstad et al, 2007; Chambers et al, 2009; Phillips et al, 2009; NegrónJuárez et al, 2010; Lewis et al, 2011), each of which could increase with human-induced climate change
Summary
One of the largest uncertainties in future terrestrial sources of atmospheric carbon dioxide results from changes to forest disturbance and tree mortality rates, in tropical forests (Cox et al, 2000, 2004; DeFries et al, 2002; Clark, 2007; Pan et al, 2011). There has been evidence that climate change and forest disturbance are linked such that a changing. Climate change related impacts such as water and heat stress, and increased vulnerability to fires could lead to increased forest dieback (i.e., tree mortality notably higher than usual mortality) and increased disturbance rates (Cox et al, 2004; Malhi et al, 2008, 2009; US DOE, 2012). Increased forest dieback in tropical locations could produce large economic costs, ecological impacts, and lead to climate related positive feedback cycles (Canham and Marks, 1985; Dale et al, 2001; Laurance and Williamson, 2001; Bonan, 2008)
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