Abstract
Anthropogenic climate change driven by increased carbon emissions is leading to more severe fire seasons and increasing the frequency of mega-fires in the Amazon. This has the potential to convert Amazon forests from net carbon sinks to net carbon sources. Although modern human influence over the Earth is substantial, debate remains over when humans began to dominate Earth’s natural systems. To date, little is known about the history of human land use in key regions like the Amazon. Here, we examine the history of human occupation from a ~8,500 year-old sediment core record from Lake Caranã (LC) in the eastern Amazon. The onset of pre-Columbian activity at LC (~4,500 cal yr B.P.) is associated with the beginning of fire management and crop cultivation, later followed by the formation of Amazonian Dark Earth soils (ADEs) ~2,000 cal yr B.P. Selective forest enrichment of edible plants and low-severity fire activity altered the composition and structure of forests growing on ADEs (ADE forests) making them more drought susceptible and fire-prone. Following European colonization (1661 A.D.), the Amazon rubber boom (mid-1800s to 1920 A.D.) is associated with record-low fire activity despite drier regional climate, indicating fire exclusion. The formation of FLONA Reserve in 1974 A.D. is accompanied by the relocation of traditional populations and a fire suppression policy. Despite suppression efforts, biomass burning and fire severity in the past decade is higher than any other period in the record. This is attributed to combined climate and human factors which create optimal conditions for mega-fires in ADE forests and threatens to transform the Amazon from a net carbon sink to a net carbon source. To help mitigate the occurrence of mega-fires, a fire management policy reducing fire-use and careful fire management for farming may help to reduce fuel loads and the occurrence of mega-fires in fire-prone ADE forests. As both natural and anthropogenic pressures are projected to increase in the Amazon, this study provides valuable insights into the legacy of past human land use on modern ADE forest composition, structure, and flammability that can inform ecological benchmarks and future management efforts in the eastern Amazon.
Highlights
Contextualizing Past Human Land Use and Fire ManagementIncreased temperatures driven by anthropogenic carbon emissions, earlier springs, decreased soil moisture, and drier fuels are leading to longer, more severe fire seasons in the Amazon (Malhi et al, 2002; Phillips et al, 2009; Castree, 2015; Zalasiewicz et al, 2015; Waters et al, 2016; Aragão et al, 2018)
A ∼8,500 year multiproxy paleoecological reconstruction spans the history of human occupation at Lake Caranã (LC) in the eastern Amazon, beginning with the arrival of pre-Columbian people ∼4,500 cal yr B.P., followed by the formation of Amazonian Dark Earth soils (ADEs) soils ∼2,000 cal yr B.P., and continues with European colonization and subsequent rubber boom ∼300 to 30 cal yr B.P., modern human occupation in the past century, and the establishment of FLONA reserve in 1974 A.D
Pollen data indicate the enrichment of edible plant species ∼2,500 cal yr B.P., which persists in modern ADE forest composition
Summary
Increased temperatures driven by anthropogenic carbon emissions, earlier springs, decreased soil moisture, and drier fuels are leading to longer, more severe fire seasons in the Amazon (Malhi et al, 2002; Phillips et al, 2009; Castree, 2015; Zalasiewicz et al, 2015; Waters et al, 2016; Aragão et al, 2018) These changes have the potential to alter the composition and structure of Amazon ecosystems and increase the frequency of large-scale wildfires (>100,000 acres; “mega-fires”) (Cochrane, 2009). The onset of agriculture or the formation of anthropogenic soils may not be the “golden spike” of the Anthropocene epoch (Certini and Scalenghe, 2011), a better understanding of the progressive intensification of past human land use and fire management practices will help to contextualize when humans began to dominate ecosystems in key regions such as the Amazon Basin
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