Amazon forest fires 'belated killers'

Years after the fire, the damage becomes clear

By Alex Kirby BBC News Online environment correspondent

Fires in the Amazon forest are killing more trees than anyone had realised, according to UK scientists.

They say the fires have a delayed effect, with trees often dying two or three years after they were damaged.

This, they say, could double present estimates of the amount of vegetation lost and carbon dioxide released.

This could become more important in the future if climatic patterns change significantly.

Carbon emissions during severe El Nino events could be equivalent to 10-12.5% of annual global carbon emissions from fossil fuels

The UEA scientists

The scientists, from the School of Environmental Sciences at the University of East Anglia, UK, report their findings in Ecology Letters.

They inspected all standing woody stems of 10 centimetres diameter or more in seven plots measuring 10 by 250 metres spread across 32 square kilometres of forest in the central Brazilian Amazon.

These plots had been burnt in a low-intensity surface fire during the 1997-8 drought linked with El Nino, the seasonal climatic phenomenon in the Pacific Ocean which causes widespread weather disruption.

Discounting the drought

The team examined the plots one year and three years after the fire, and compared their estimates of live biomass there with the amounts found in six plots of the same size in unburned forest.

Lead researcher Jos Barlow

They say these unburned plots were "statistically indistinguishable" and "similar floristically" to the damaged ones.

This persuaded them that the explanation for the deaths of trees in the burned plots was fire rather than drought.

The report says: "Patterns of stem mortality detected after one year were very similar to those reported up to two years after low-intensity surface fires in previous studies, with smaller stems suffering the highest mortality and larger, thicker-barked trees tending to survive.

"However, there was significant additional tree mortality between one and three years... with large trees showing the greatest decrease in abundance relative to their one-year post-burn levels.

"Whilst temporal increases in tree mortality up to two years have been previously reported, the increase in mortality of larger stems that we detected three years post-burn has not.

Steep upwards revision

"After three years post-burn the live stem density was just 52% of that found in the unburned forest, and live biomass had declined... to just 49% of that found in the unburned forest.

How it should be: Intact forest

During a severe El Nino, the authors say, it has been established that low-intensity Amazon fires could contribute as much as 5% of annual carbon emissions from all sources of human origin.

But they believe their research has doubled that estimate, with profound implications for the rest of the Amazon and beyond.

They write: "If our findings are characteristic of other burned areas in the Brazilian Amazon, carbon emissions... during severe El Nino events could be equivalent to 10-12.5% of annual global carbon emissions from fossil fuels..."

New predictions

But they acknowledge that uncertainties remain high, and say their results underline the need for more long-term monitoring, especially if severe El Ninos become more frequent.

The team leader, Dr Jos Barlow, told BBC News Online: "I think our results would certainly apply to forests in Africa and Indonesia too.

"There's a lot of carbon locked up there. We'll be passing on our findings to the Intergovernmental Panel on Climate Change (IPCC).

"They will want to prove what we've found for themselves, and then factor in to their predictions all this extra carbon that the forests may be releasing."

The team's work was funded by the Natural Environment Research Council (Nerc) and Conservation International.

Images courtesy of Dr Jos Barlow