The combination of fire and extreme weather could accelerate tree mortality in the Amazon, a study has suggested.
Researchers said field experiments showed that severe droughts could trigger dieback of forests in the near-term.
Efforts to curb deforestation needed to be accompanied by initiatives to stop land management fires spreading into adjoining forest reserves, they added.
Co-author Paulo Brando, a researcher from Brazil's Amazon Environmental Research Institute and the Woods Hole Research Center, US, explained that the findings were based on data gathered from a long-running field experiment, described as the first of its kind.
In 2004, the team established three plots in the south-east Amazon – one of which was burned annually, another was burned every three years, while the remaining one was left untouched as a control plot.
"If we burned every year, we did not have enough fuel (leaves and trees) to drive a high intensity fire – fires that will kill a lot of trees – so we published a few papers saying that this kind of forest was quite fire resistant," explained Dr Brando.
But he told BBC News that the team was surprised in 2007: "We burned both plots (fourth burn for the annual plot and second burn for three-year plot) and both plots responded really strongly to the fires.
"There was a shift in the system from a forest to a savannah-like environment… grasses were invading the forest and there were fundamental changes within the ecosystem.
He added that drought conditions during that year triggered the change: "There were more leaves and twigs on the ground, they were drier so the fires were quite intense.
"To our surprise, the major effects were not observed in the plot we burned every year but rather in the one we burned every three years.
"This was because there was more time for fuel accumulation, and also it was because the productivity of that plot was still quite high, whereas in the [annual burn] plot, the fuel was being removed every year and the productivity of the system was reduced."
Dr Brando said the abrupt high tree-mortality rates had a lasting impact on the landscape as grasses invaded the space once occupied by trees.
"You do not need drought anymore to observe high density fires because grasses can accumulate way more fuel than the native, wood vegetation," he explains.
Written By: Mark Kinver
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