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Extreme El Ni\u00f1o weather saw South America\u2019s forest carbon sink switch off<\/p>\n
Tropical forests in South America lose their ability to absorb carbon from the atmosphere when conditions become exceptionally hot and dry, according to new research. <\/p>\n
For a long time, tropical forests have acted as a carbon sink, taking more carbon out of the air than they release into it, a process that has moderated the impact of climate change. <\/p>\n
But research led by Dr. Amy Bennett, a Research Fellow at the University of Leeds, found that in 2015 \u2013 2016, when an El Ni\u00f1o climate event resulted in drought and the hottest temperatures ever recorded, South American forests were unable to function as a carbon sink. <\/p>\n
El Ni\u00f1o occurs when sea-surface temperatures in the Pacific Ocean increase sharply, triggering a major shift in the world\u2019s climate system. In 2015-2016, the result was exceptionally hot weather for South America. A similar event is underway now. <\/p>\n
Dr. Bennett, from the School of Geography at Leeds, said: \u201cTropical forests in the Amazon have played a key role in slowing the build-up of carbon dioxide in the atmosphere. <\/p>\n
\u201cScientists have known that the trees in the Amazon are sensitive to changes in temperature and water availability, but we do not know how individual forests could be changed by future climate change. <\/p>\n
\u201cInvestigating what happened in the Amazon during this huge El Ni\u00f1o event gave us a window into the future by showing how unprecedented hot and dry weather impacts forests.\u201d <\/p>\n
The researchers today report their findings in the journal Nature Climate Change. The study united the RAINFOR<\/a> and PPBio<\/a> research networks, with dozens of short-term grants enabling more than 100 scientists to measure forests for decades across 123 experimental plots. <\/p>\n The plots span Amazon and Atlantic forests as well as drier forests in tropical South America. <\/p>\n These direct, tree-by-tree records showed that most forests had acted as a carbon sink for most of the last 30 years, with tree growth exceeding mortality. When the 2015\u20132016 El Ni\u00f1o hit, the sink shut down. This was because tree death increased with the heat and drought. <\/p>\n Professor Beatriz Marimon, of Brazil\u2019s Mato Grosso State University, added \u201cHere in the southeastern Amazon on the edge of the rainforest, the trees may have now switched from storing carbon to emitting it. While tree growth rates resisted the higher temperatures, tree mortality jumped when this climate extreme hit.\u201d <\/p>\n Study\u2019s findings <\/strong><\/p>\n Of the 123 plots studied, 119 of them experienced an average monthly temperature increase of 0.5 degrees Celsius. 99 of the plots also suffered water deficits. Where it was hotter, it was also drier. <\/p>\n Prior to El Ni\u00f1o, the researchers calculated that the plots were storing and sequestering around one third of a tonne of carbon per hectare per year. This declined to zero with the hotter and drier El Ni\u00f1o conditions. <\/p>\n The change was due to biomass being lost through the death of trees. <\/p>\n Writing in the paper, the researchers noted that the greatest relative impact of the El Ni\u00f1o event were in forests where the long-term climate was already relatively dry. <\/p>\n The expectation was that wetter forests would be most vulnerable to the extreme drier weather, as they would be least adapted to such conditions. However, the opposite was the case. Instead, those forests more used to a drier climate at the dry periphery of the tropical forest biome turned out to be most vulnerable to drought. <\/p>\n This suggested some trees were already operating at the limits of tolerable conditions. <\/p>\n For Professor Oliver Phillips, an ecologist at the University of Leeds who supervised the research and leads the global ForestPlots<\/a> initiative, the findings offered hope about the resilience of the South American tropical nature. <\/p>\n He added: \u201cThe full 30-year perspective that our diverse team provides shows that this El Ni\u00f1o had no worse effect on intact forests than earlier droughts. Yet this was the hottest drought ever. <\/p>\n \u201cWhere tree mortality increased was in the drier areas on the Amazon periphery where forests were already fragmented. Knowing these risks, conservationists and resource managers can take steps to protect them. <\/p>\n \u201cThrough the complex dynamics that happen in forest environments, land clearance makes the environment drier and hotter, further stressing the remaining trees. <\/p>\n \u201cSo, the big challenge is to keep forests standing in the first place. If we can do that, then our on-the-ground evidence shows they can continue to help lock up carbon and slow climate change.\u201d <\/p>\n