The Amazon rainforest may be able to survive long-term drought caused by climate change, but adjusting to a drier, warmer world would exact a heavy toll, a study suggests.
The findings show that adjusting to cope with the effects of climate change could see some parts of the Amazon rainforest lose many of its largest trees.
This would release the large amount of carbon stored in these trees to the air, and reduce the rainforest's immediate capacity to act as an important carbon sink, researchers say.
Parts of the Amazon are expected to become drier and warmer as the climate changes, but the long-term effects on the region's rainforests – which span more than 2 million square miles – are poorly understood.
Previously, research has raised concerns that a combination of severe warming and drying, together with deforestation, could lead to lush rainforest degrading to a sparser forest or even savanna.
Now, findings from the world's longest-running drought study in tropical rainforest have revealed some of the profound changes the Amazon could undergo in a drier world.
Over a 22-year period, a one-hectare area of rainforest in north-eastern Amazonian Brazil – roughly the size of Trafalgar Square – has been subjected to long-term drought conditions.
The experiment began in 2002, with thousands of transparent panels installed above the ground to redirect roughly half of the rainfall to a system of gutters, taking it away from the trees.
Analysis by a team co-led by scientists from the University of Edinburgh and the Federal University of Para, Brazil, shows that most of the study area's largest trees died during the first 15 years of the experiment, after which the forest stabilised.
The team's findings show that for the seven years after the large initial biomass losses the availability of water increased for the surviving trees. Tests on these remaining trees showed they were now no more drought-stressed than those in nearby rainforest not subjected to drought.
Overall, the area lost more than one-third of its total biomass – the trunks, branches, stems and roots where carbon is stored in living vegetation. Such widespread losses across the Amazon would see the rainforest release vast amounts of carbon, and greatly reduce its immediate capacity to act as a sink for emissions from human activities, the team says.
Having lost carbon through excess tree deaths during the first 15 years of the study, surviving trees in the area are now making slight carbon gains, the team says.
While the study area has less woody biomass than normal rainforests in the Amazon, it still has more than many dry forests and savannas. This indicates that the rainforest has some long-term resilience to the drier conditions it could experience due to climate change, but that this comes at a high cost.
The amount of biomass the Amazon could lose, and the time required for it to stabilise, may be underestimated, as the study only assessed the effects of soil drought, the team says.
Further research is needed to assess other likely impacts, such as changes to moisture in the air, temperature and the compounding effects of other climate-related factors such as storms or fires, they add.
The study, published in the journal Nature Ecology and Evolution, was carried out by a team led by Professors Patrick Meir of the University of Edinburgh and Antonio Carlos Lôla Da Costa of the Universidade Federal do Pará and the Museu Paraense Emílio Goeldi, Brazil. It also involved researchers from the Universities of Exeter and Cardiff, and CREAF in Spain. The research was supported by the Natural Environment Research Council (NERC), the Royal Society and the UK Met Office Newton Fund.
Lead author Dr Pablo Sanchez Martinez, of the University of Edinburgh's School of GeoSciences, said: "Our findings suggest that while some rainforests may be able to survive prolonged droughts brought on by climate change, their capacity to act as both a vital carbon store and carbon sink could be greatly diminished."
Professor Patrick Meir, of the University of Edinburgh's School of GeoSciences, said: "Ecological responses to climate can have very large impacts on our environment, locally and globally; we cannot understand and predict them without long-term collaborative research of this sort."
