Ancient Amazonian populations left lasting impacts on forest structure

M W Palace,S Frolking,B H Braswell,E Neves,C N H Mcmichael,S C Hagen,M B Bush,C Herrick,E Tamanaha

doi:10.1002/ecs2.2035

Abstract

AbstractAmazonia contains a vast expanse of contiguous tropical forest and is influential in global carbon and hydrological cycles. Whether ancient Amazonia was highly disturbed or modestly impacted, and how ancient disturbances have shaped current forest ecosystem processes, is still under debate. Amazonian Dark Earths (ADEs), which are anthropic soil types with enriched nutrient levels, are one of the primary lines of evidence for ancient human presence and landscape modifications in settings that mostly lack stone structures and which are today covered by vegetation. We assessed the potential of using moderate spatial resolution optical satellite imagery to predict ADEs across the Amazon Basin. Maximum entropy modeling was used to develop a predictive model using locations of ADEs across the basin and satellite‐derived remotely sensed indices. Amazonian Dark Earth sites were predicted to be primarily along the main rivers and in eastern Amazonia. Amazonian Dark Earth sites, when compared with randomly selected forested sites located within 50 km of ADE sites, were less green canopies (lower normalized difference vegetation index) and had lower canopy water content. This difference was accentuated in two drought years, 2005 and 2010. This is contrary to our expectation that ADE sites would have nutrient‐rich soils that support trees with greener canopies and forests on ADE soils being more resilient to drought. Biomass and tree height were lower on ADE sites in comparison with randomly selected adjacent sites. Our results suggested that ADE‐related ancient human impact on the forest is measurable across the entirety of the 6 million km2 of Amazon Basin using remotely sensed data.

Highlights

The Amazon Basin contains the largest continuous rainforest in the world (~6 million km2) and constitutes 40% of this ecotype globally (Olson et al 2001, Keller et al 2004)
Normalized difference vegetation index values were lower for Amazonian Dark Earths (ADEs) sites later in the dry season (August and September) for both drought years (2005 and 2009) and normal years (2004, 2010) compared with random sites; 2009 and 2010 showed greater variation (Fig. 2)
Amazonian Dark Earth sites had lower visible atmospherically resistance index (VARI) values than random sites for all months and for both 2004 and 2005, though VARI values were lower for ADE sites in 2005 and 2009 compared with 2004 and 2010, respectively (Fig. 2)

Summary

Introduction

The Amazon Basin contains the largest continuous rainforest in the world (~6 million km2) and constitutes 40% of this ecotype globally (Olson et al 2001, Keller et al 2004). Estimates of the pre-Columbian indigenous population of the Amazon Basin lowlands are highly uncertain, widely varying, and have been the subject to considerable controversy (Brondizio 2003, Meggers 2003, Bush and Silman 2007, Clement et al 2015). Estimates of pre-Columbian populations for the Amazon Basin have ranged from 500,000 to 10 million (Comas 1951, Moran 1974). Multiple mid-range estimates from 6.8 to 8 million people have been proposed (Denevan 1970, 1992, Clement et al 2015). One school of thought suggested that Amazonia was a counterfeit paradise whose lush vegetation hid nutrient-poor soils incapable of supporting large populations and complex societies (Meggers 1971, 2003). One point of agreement among the proponents of the extreme points of view, as well as those in between, is the scarcity of data regarding the preEuropean societies of Amazonia (Denevan 2014)

Methods

Results

Discussion

Conclusion