Forest Fire History in Amazonia Inferred From Intensive Soil Charcoal Sampling and Radiocarbon Dating

Abstract

Fire has a historical role in tropical forests related to past climate and ancient land use spanning the Holocene; however, it is unclear from charcoal records how fire varied at different spatiotemporal scales and what sampling strategies are required to determine fire history and their effects. We evaluated fire variation in structurally intact, terra-firme Amazon forests, by intensive soil charcoal sampling from three replicate soil pits in sites in Guyana and northern and southern Peru. We used radiocarbon (14C) measurement to assess (1) locally, how the timing of fires represented in our sample varied across the surface of forest plots and with soil depth, (2) basin-wide, how the age of fires varies across climate and environmental gradients, and (3) how many samples are appropriate when applying the 14C approach to assess the date of last fire. Considering all 14C dates (n = 33), the most recent fires occurred at a similar time at each of the three sites (median ages: 728–851 cal years BP), indicating that in terms of fire disturbance at least, these forests could be considered old-growth. The number of unique fire events ranged from 1 to 4 per pit and from 4 to 6 per site. Based upon our sampling strategy, the N-Peru site—with the highest annual precipitation—had the most fire events. Median fire return intervals varied from 455 to 2,950 cal years BP among sites. Based on available dates, at least three samples (1 from the top of each of 3 pits) are required for the sampling to have a reasonable likelihood of capturing the most recent fire for forests with no history of a recent fire. The maximum fire return interval for two sites was shorter than the time since the last fire, suggesting that over the past ∼800 years these forests have undergone a longer fire-free period than the past 2,000–3,500 years. Our analysis from terra-firme forest soils helps to improve understanding of changes in fire regime, information necessary to evaluate post-fire legacies on modern vegetation and soil and to calibrate models to predict forest response to fire under climate change.

Soil pyrogenic carbon in southern Amazonia: Interaction between soil, climate, and above-ground biomass

“…Studies over the past decade have begun to reveal the effects of historical fire on structurally intact forests with no history of recent fire throughout the Amazon Basin (Feldpausch et al, 2022), including higher than predicted PyC stocks (Bird et al, 2015;Koele et al, 2017). However, there remains large uncertainty about how soil PyC varies spatially (Carvalho et al, 2018), and the drivers of spatial variability, especially throughout the ZOT where fire risk is greater and fire has potentially affected vegetation and soils for millennia.…”

“…While ADEs are found throughout Amazonia (Palace et al, 2017), estimates suggest they only represent a small fraction (∼3%) of Amazonian soils (McMichael et al, 2014). Pre-Columbian land-use and fire management appears to have declined sharply across Amazonia following the Great Dying after 1492 (Goulart et al, 2017;McMichael and Bush, 2019;Feldpausch et al, 2022) due to the introduction of Old-World diseases during post-Columbian European colonization (Dull et al, 2010).…”

“…In the ever wet forests of the Amazon, PyC in the soil usually results from ancient fires, both natural and those caused by people from pre-Columbian civilizations (Masiello, 2004;Lehmann et al, 2008;Goulart et al, 2017;McMichael and Bush, 2019;Feldpausch et al, 2022), as inputs from aerosols are relatively small (Coppola et al, 2019). There is evidence for ancient and frequent fires in closed canopy forest in Amazonia (Hammond et al, 2007), especially during the Middle Holocene (Mayle and Power, 2008), with historical fire return intervals of ∼450 years (Feldpausch et al, 2022).…”

“…In the ever wet forests of the Amazon, PyC in the soil usually results from ancient fires, both natural and those caused by people from pre-Columbian civilizations (Masiello, 2004;Lehmann et al, 2008;Goulart et al, 2017;McMichael and Bush, 2019;Feldpausch et al, 2022), as inputs from aerosols are relatively small (Coppola et al, 2019). There is evidence for ancient and frequent fires in closed canopy forest in Amazonia (Hammond et al, 2007), especially during the Middle Holocene (Mayle and Power, 2008), with historical fire return intervals of ∼450 years (Feldpausch et al, 2022). However, as there was an advance of the Amazon transitional forests over savanna formations in more recent times (Mayle et al, 2000(Mayle et al, , 2004Marimon et al, 2006;Mayle and Power, 2008;Morandi et al, 2015), our plots that are currently forested could have previously supported savanna vegetation with lower biomass (Marimon et al, 2006).…”

“…Therefore, the majority of soil PyC found in forests with no history of recent fire is likely due to in situ historical fires and/or local edaphic or topographic conditions favoring the long-term accumulation of soil PyC. Radiocarbon dating of macro-charcoal fragments from the soil indicates Amazon forests have periodically burned throughout the Holocene (Sanford et al, 1985;Goulart et al, 2017;McMichael and Bush, 2019;Feldpausch et al, 2022), with some old-growth forests having fire return intervals of hundreds of years and having experienced no fire for the last ∼800 years (Feldpausch et al, 2022).…”