The resilience of Amazon tree cover to past and present drying

Abstract:

Amazon forests are widely regarded as increasingly vulnerable to dieback with the expansion of agricultural fields and grasslands. Possible threats to forested ecosystems include drying, fire, and deforestation, but feedbacks among these make it difficult to determine the relative importance of each. To further complicate future projections, models disagree on the magnitude or sign of change (wetter or drier) in Amazon rainfall and the fundamental relationship between vegetation cover and precipitation is debated. It is unclear whether grasslands gradually replace forests with drying, or whether the transition occurs abruptly at a “tipping” point. Here, we use proxy data and models to characterize Amazon aridity, fire, and tree-cover in the mid-Holocene—a time of human land use under markedly drier conditions than today. Combining stable oxygen isotope, charcoal, and pollen proxy data, we show that mid-Holocene precipitation was substantially lower than the driest bias-corrected future projections while increases in fire and tree-cover loss were minor and restricted to forest fringes. Dynamic vegetation modeling of mid-Holocene conditions supports these results, showing that tree cover remains high as annual precipitation decreases to half the modern rate. The resilience of Amazon tree cover to drying suggests human-driven fire and land use change likely pose a greater threat to the future of Amazon vegetation than aridification alone.