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Ancient Amazon fires linked to human populations

Ancient Amazon fires linked to human populations

Ancient Amazon fires linked to human populations
Rhett A. Butler,
February 20, 2008

European contact resulted in both a steep decline in indigenous populations and burning in the Amazon rainforest — at least until now.

Analysis of soil charcoal in South America confirms that from a historical perspective, fire is rare in the Amazon rainforest, but when it does occur, it appears linked to human activities.

The research, published in Philosophical Transactions of the Royal Society B, is based on dating of soil carbon, which provides a good indication of when fires occurred in Amazonia, according to lead author Mark Bush, head of the Department of Biology at Florida Institute of Technology.

“Charcoal particles provide a good indication of the age of the fire in Amazonia,” he told via email. “Trees rot quickly in tropical forests and do not lie around on the forest floor for centuries until burned. Further, fires generally only char the outer few centimeters of the trunk. Consequently, once calibrated, the carbon-14 ages of the charcoal provide good ages for tropical fire events.”

228 14C ages for dated charcoal horizons in Amazonian soils. Pink represents the number of charcal records (i.e. fire)while purple indicates atmospheric D14C as a proxy for solar output. Turquoise indicates number of El Niño events per century. ? indicates uncertainty as to the timing of a shift in Amazonian cultural adaptations.

Conducting a review of all the ages of soil charcoal published for Amazonia, Bush and co-authors Miles R. Silman, Crystal McMichael and Sassan Saatchi found three peaks of fire activity that corresponded to solar activity minima — lows of sunspot activity and reduced solar output — at 750, 1150, and 1400 AD.

“Solar minima are linked to thermal oscillations in the tropical Atlantic that diminish the flow of moisture onto the continent and thereby induce droughts within Amazonia,” said Bush. “We hypothesize that these fire events took place within times of drought and that during these events human-induced fires went out of control and became wildfires. The spatial extent of these fires is not known, but they do appear to be anthropogenic in origin.”

Bush says a tell-tale sign that humans caused ancient fires comes from the lack of fire activity during one of the strongest periods of solar activity minima, the Maunder minimum, a period from 1645-1715 when the Amazon had been greatly depopulated — 50-95 percent by some estimates — following the introduction of European diseases to the New World.

“After the time of European contact, fires became much scarcer within Amazonia,” write the authors.

Seasonal probabilities of fire occurrence in Amazonia. (a) September to November, (b) June to August, (c) December to February and (d ) March to May. Colors represent the cumulative probability of fire occurrence for any season over 17 years derived from AVHRR satellite observation. Percentages represent the probability of an area burning in a given year. The seasonality of fires closely follows seasonality in rainfall.

Bush says the work provides further evidence that fire is an unnatural occurrence in the heart of the Amazon.

“From the paleoecological data fire appears to be a truly rare natural event in Amazonia,” Bush explained. “Some of the more flammable zones, where seasonality is high and precipitation relatively low, may have burned naturally, but across much of the wetter portions of the Amazon basin, at least within the last 5000 years, fire is almost uniquely associated with human activity.”

Bush says that a new grant from the National Science Foundation will fund the next round of research to investigate the area and habitat types of Amazonia influenced by pre-Colombian fires.

M.B. Bush at al (2008). Fire, climate change and biodiversity in Amazonia: a Late-Holocene perspective [FREE OPEN ACCESS]. Phil. Trans. R. Soc. B, DOI: 10.1098/rstb.2007.0026