Jan Bogaert and Jean-Louis Doucet take part in an international study

Tropical forests can withstand heat to a certain extent...



Tropical forests face an uncertain future under the pressure of climate change, but new scientific research published in Science suggests that they can continue to store large amounts of carbon in a warmer world if countries limit greenhouse gas emissions.

The world's tropical forests store a quarter of a century of fossil fuel emissions in their trees. There is concern that global warming could reduce this storage if tree growth declines or tree mortality increases, accelerating climate change.

An international research team has measured more than half a million trees in 813 tropical forests to assess the amount of carbon stored by forests growing under different climatic conditions today.

The team reveals that tropical forests continue to store high levels of carbon under high temperatures, showing that in the long term, these forests can withstand heat up to an estimated threshold of 32 degrees Celsius of daytime temperature.

However, this positive conclusion is only possible if forests have time to adapt, if they remain intact and if global warming is strictly limited to prevent global temperatures from exceeding the critical threshold.

Lead author Dr. Martin Sullivan, from the University of Leeds and Manchester Metropolitan University, said: "Our analysis reveals that, up to a certain point of warming, tropical forests are surprisingly resilient to small temperature differences. If we limit climate change, they can continue to store large amounts of carbon in a warmer world. The 32-degree threshold underlines the critical importance of urgently reducing our emissions to avoid pushing too many forests beyond the safe zone. For example, if we limit global average temperatures to an increase of 2°C above pre-industrial levels, this would push nearly three-quarters of tropical forests above the heat threshold we have identified. Any further increase in temperature will lead to rapid losses of forest carbon.

Forests release carbon dioxide into the atmosphere when the amount of carbon gained through tree growth is less than that lost through tree mortality and degradation.

The study is the first to analyse long-term climate sensitivity through direct observation of entire forests in all tropical regions. This research suggests that in the long term, temperature, by reducing tree growth, has the greatest effect on forest carbon stocks, with drought being the second key factor in killing trees.

Researchers conclude that tropical forests have a long-term capacity to adapt to some climate changes, partly because of their high biodiversity, because in the long term tree species that can tolerate new climatic conditions grow well and replace less well-adapted species.

But to maximize this potential climate resilience, forests must be kept intact.

Professor Beatriz Marimon of the State University of Mato Grosso in Brazil, co-author of the paper, studies some of the world's warmest tropical forests in central Brazil. She said: "Our results suggest that intact tropical forests are able to withstand some climate change. However, these heat-tolerant trees also face immediate threats from bush fires and fragmentation. Achieving climate adaptation means first of all protecting and connecting the remaining forests. Professor Marimon warns of clear limits to adaptation. "The study indicates a heat threshold of 32 degrees Celsius daytime temperature. Above this point, tropical forest carbon decreases more rapidly with temperature increments, regardless of the species present. Each increment above the 32-degree Celsius limit releases four times more carbon dioxide than would be released below the limit".

Information on how the world's tropical forests respond to climate has only been obtained through decades of painstaking fieldwork, often in remote locations. The global team of 225 researchers combined observations of forests in South America (RAINFOR), Africa (AfriTRON) and Asia (T-FORCES). In each monitoring plot, the diameter of each tree and its height were used to calculate the quantity of trees in the forest.

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