Scientists from the University of East Anglia in England say that until now estimates of tree mortality following forest fires have been based on only short-term studies of up to two years.

Current estimates are that Amazon forest fires contribute 5% of annual carbon emissions during severe El Niño years. However, if the study’s findings are correct, such events could be equivalent to 10-12.5% of annual global carbon emission from fossil fuels.

The researchers studied trees with a diameter greater than 10cm across 32 square kilometres of Brazilian Amazon rainforest, including areas that were subjected to a low intensity surface fire in the 1997-8 El Niño drought period. After three years, they found that there had been an average of an additional 74 trees per hectare that had died.

Three years after a fire, live stem density is just 52% of that in unburned areas, but biomass suffers even more, having only 49% that of unaffected areas.

The largest trees, those with a diameter equal to or greater than 50cm suffered the highest level of mortality, accounting for 67% of the loss of live biomass.

There are a number of possible reasons for the dieback of larger trees, such as reduced resistance to pathogens, increased water stress, or vulnerability to wind.

“These wildfires make substantial contributions to atmospheric carbon dioxide,” said Dr Jos Barlow, of UAE’s Centre for Ecology, Evolution and Conservation, and lead author of the report. The significance of these fires will take on further significance as El Niño events threaten to become more frequent and severe, he added.

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