The resilience of Amazon tree cover to past and present drying

Abstract

The Amazon forest is increasingly vulnerable to dieback and encroachment of grasslands and agricultural fields. Threats to these forested ecosystems include drying, deforestation, and fire, but feedbacks among these make it difficult to determine their relative importance. Here, we reconstruct the central and western Amazon tree cover response to aridity and fire in the mid-Holocene-a time of less intensive human land use and markedly drier conditions than today-to assess the resilience of tree cover to drying and the strength of vegetation-climate feedbacks. We use pollen, charcoal, and speleothem oxygen isotope proxy data to show that Amazon tree cover in the mid-Holocene was resilient to drying in excess of the driest bias-corrected future precipitation projections. Experiments with a dynamic global vegetation model (LPJ-GUESS) suggest tree cover resilience may be owed to weak feedbacks that act to amplify tree cover loss with drying. We also compare these results to observational data and find that, under limited human interference, modern tree cover is likely similarly resilient to mid-Holocene levels of aridification. Our results suggest human-driven fire and deforestation likely

The Zonal Patterns in Late Quaternary Tropical South American Precipitation

“…During the mid-Holocene, the reactive transport model simulates rainfall decreasing to ∼1,200 mm/yr (about half of modern; red distribution of Figure 4a). As discussed in Kukla et al (2021), the Δδ 18 O values in the mid-Holocene straddle zero-the theoretical maximum value for a single moisture trajectory. At this point, further drying has a negligible effect on Δδ 18 O.…”

“…While at the hydrostat, Δδ 18 O does not capture further drying that likely distinguishes the early mid Holocene conditions from the mid-late Holocene (P. A. Cheng et al, 2013;Fritz et al, 2004;Kukla et al, 2021). The western domain gradient shows a mostly opposing trend, with Δδ 18 O near ∼−2‰ at the LGM and increasing to zero, the hydrostat, by the mid-Holocene before decreasing to present.…”

“…We use a reactive transport model that simulates Δδ 18 O as a function of these fluxes to quantify past precipitation rates from Δδ 18 O data. To do so, we randomly sample from uniform distributions of reactive transport model input parameters to estimate past precipitation from the simulations that agree with Δδ 18 O data (Kukla et al, 2019(Kukla et al, , 2021.…”

“…Our application of the reactive transport model to the central-to-western isotope gradient follows that of Kukla et al (2021) with one key change. Kukla et al (2021) used modern reanalysis data to analyze both the late Holocene and mid-Holocene isotope gradients because PMIP3/CMIP5 results (Braconnot et al, 2012) show that reactive transport model inputs are similar for both time periods. However, modern reanalysis data cannot be reasonably applied to the LGM due to the ∼5°C of tropical cooling.…”

Climate and environmental changes in southwestern Amazon Basin over the last 3000 years based on speleothem isotope records, Brazil

“…Villages. We suggest these populations may have taken advantage of a more open vegetation, considering the evidence for earthworks (causeways, roads) connecting the sites over long distances, and that it is unlikely that widespread clearance was practiced Iriarte et al, 2020;Saunaluoma et al 2021;Kukla et al, 2021). The transition period between MCA and LIA, when the paleoclimate records from the Amazon basin present very heterogonous conditions, was also marked by significant demographic changes documented by archaeological data (Fig.…”