Abstract

The impact of increases in drought frequency on the Amazon forest's composition, structure and functioning remain uncertain. We used a process- and individual-based ecosystem model (ED2) to quantify the forest's vulnerability to increased drought recurrence. We generated meteorologically realistic, drier-than-observed rainfall scenarios for two Amazon forest sites, Paracou (wetter) and Tapajós (drier), to evaluate the impacts of more frequent droughts on forest biomass, structure and composition. The wet site was insensitive to the tested scenarios, whereas at the dry site biomass declined when average rainfall reduction exceeded 15%, due to high mortality of large-sized evergreen trees. Biomass losses persisted when year-long drought recurrence was shorter than 2-7yr, depending upon soil texture and leaf phenology. From the site-level scenario results, we developed regionally applicable metrics to quantify the Amazon forest's climatological proximity to rainfall regimes likely to cause biomass loss >20% in 50yr according to ED2 predictions. Nearly 25% (1.8 million km2 ) of the Amazon forests could experience frequent droughts and biomass loss if mean annual rainfall or interannual variability changed by 2σ. At least 10% of the high-emission climate projections (CMIP5/RCP8.5 models) predict critically dry regimes over 25% of the Amazon forest area by 2100.

Highlights

  • The Amazon forest is the largest tropical rain forest in the world, storing c. 40% of the global biomass of tropical forests (Malhi et al, 2006; Saatchi et al, 2007, 2011)

  • The point at which average aboveground biomass (AGB) became significantly lower than the control scenarios varied depending on soil texture: for clayey soils, mean AGB decreased by > 15% at S = À0.8 (a 20% reduction in mean rainfall), whereas for sandy soils, mean AGB decreased by c. 5%, indicating that simulations with sandy soils showed greater resistance to changes in AGB compared to those with clayey soils (Fig. 4c,d)

  • Biomass losses were influenced by canopy phenology: under similar rainfall regimes, losses were more significant in evergreen communities (Fig. 4c) than in droughtdeciduous counterparts (Fig. 4d)

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Summary

Introduction

The Amazon forest is the largest tropical rain forest in the world, storing c. 40% of the global biomass of tropical forests (Malhi et al, 2006; Saatchi et al, 2007, 2011). Research suggests that parts of the Amazon may be susceptible to biome shifts, biodiversity loss and depletion of carbon stores because of changes in climate and climate variability Rainfall predictions for the region’s 21st century climate are uncertain, but results from multiple model predictions indicate stronger and longer dry seasons, in Southern and Eastern Amazonia (Malhi et al, 2008; Fu et al, 2013; IPCC, 2014; Boisier et al, 2015), and increased recurrence of droughts (Duffy et al, 2015), such as the widespread and intense Amazon droughts observed in 2005, 2010 and 2015–2016 (Marengo et al, 2008; Phillips et al, 2009; Lewis et al, 2011; Jimenez-Mun~oz et al, 2016; Erfanian et al, 2017). Future climate projections for the Amazon predicted by models participating in Phase 5 of the Climate Model Intercomparison Project (CMIP5) (Taylor et al, 2012)

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