Long‐term fire and vegetation change in northwestern Amazonia

Abstract

Amazonian forest plots are used to quantify biodiversity and carbon sequestration, and provide the foundation for much of what is known about tropical ecology. Many plots are assumed to be undisturbed, but recent work suggests that past fire, forest openings, and cultivation created vegetation changes that have persisted for decades to centuries (ecological legacies). The Yasuní Forest Dynamics plot is one of the most biodiverse places on earth, yet its human history remains unknown. Here, we use charcoal and phytolith analysis to investigate the fire and vegetation history of the Yasuní forest plot, and compare results with nearby forest plots in Colombia (Amacayacu) and Peru (Medio Putumayo‐Algodón [MPA]) to explore the spatial variability of past disturbances and ecological legacies in northwestern Amazonia. Three 14C dated charcoal fragments provided evidence for a modern (1956 CE) and a past fire event ca. 750 years ago at Yasuní, compared with fire ages of 1000–1600 years ago documented at Amacayacu and MPA. Small‐scale disturbances and localized canopy openings also occurred in the Yasuní plot. Phytolith assemblages from Yasuní and Amacayacu showed more variability in past vegetation change than MPA. Low‐intensity, non‐continuous disturbances occurred at all three plots in the past, and our results highlight the variability of past human activities both in space and time in northwestern Amazonia. Our data also suggest that post‐Columbian human disturbances from the Rubber Boom (AD 1850–1920) and subsequent oil exploration have likely left stronger ecological legacies than those left by pre‐Columbian peoples in our studied regions.

Assessing charcoal and phytolith signals for pre-Columbian land-use based on modern indigenous activity areas in the Upper Xingu, Amazonia

“…Sampling in the field was made from excavated soil profiles, a strategy that allowed us to record the soil stratigraphy at each sampling location, check for the presence of charcoal lenses that could indicate in-situ burning events, and avoid areas of obvious bioturbation while sampling. A sampling resolution of every 5 cm was used to test whether there were any finer temporal patterns detectable in the off-site phytolith and charcoal signatures that could otherwise be obscured by sampling every 10 cm (sensu Heijink et al, 2020Heijink et al, , 2023Iriarte et al, 2020;Piperno et al, 2021) or 20 cm (sensu McMichael et al, 2012b;Piperno et al, 2015). We do this by averaging the proxy data over these different depth intervals and presenting the results alongside our data.…”

Past human‐induced ecological legacies as a driver of modern Amazonian resilience

“…We predict that forests that had mid-intensity disturbances in the last 200-600 years are those most likely to have increased resilience as they would probably show the strongest resistance and speediest recovery to a highly diverse state following modern drought and fire regimes (Figure 3a). We also suggest that determining the successional state of Amazonian forests, such as recent efforts to determine the time of last fire or disturbance (Heijink et al, 2020(Heijink et al, , 2022, is crucial to understanding modern forest resilience.…”

“…Recent studies have begun assessing the time since the last fire or deforestation event via palaeoecological surveys in forest inventory plots, where biodiversity and carbon dynamics in Amazonian forests have been measured (Feldpausch et al, 2022;Heijink et al, 2020Heijink et al, , 2022Piperno & Becker, 1996; Figure 4). Archaeological surveys within the forest plots would also provide information into the size of past occupation sites and the manner in which people were using the land (Iriarte et al, 2020;Mayle & Iriarte, 2014).…”

“…The integration of palaeoecological, archaeological and modern ecological data are becoming more common (Iriarte et al, 2020;Mayle & Iriarte, 2014). Palaeoecological and archaeological analyses performed on soil samples can successfully assess (i) the timing of the last fire(s), (ii) recurrence of fire events, (iii) signals of palm enrichment or depletion, (iv) the duration of past occupations, (v) the cultural practices of past Indigenous people and (vi) general patterns of vegetation change (De Souza et al, 2019;Heijink et al, 2020Heijink et al, , 2022Maezumi, Whitney, et al, 2018;McMichael et al, 2015;Piperno & Becker, 1996;Watling et al, 2017;Figure 4). Relatively few of these surveys, however, have been paired with field-based data on modern vegetation and estimated resilience of forests to future climate and anthropogenic change.…”

“…The loss of biomass, and thus the potential to store carbon, can occur when a forest transitions from a mid‐successional to a mature state due to the loss of long‐lived pioneer and mid‐successional species (Poorter et al, 2016). The extent to which the observed slowing down of the carbon sink in Amazonia is related to this transition remains relatively unexplored (Feldpausch et al, 2022; Heijink et al, 2022; McMichael, 2021).…”