- Rising temperatures as a result of climate change are making tropical forests hotter, which translates into shorter life spans for tropical tree species, a new study shows.
- Tropical forests host about 50% of Earth’s biodiversity and 50% of its forest carbon stocks; their capacity to capture and store carbon depends on their health and longevity.
- The authors of the study warn that the shorter life span raises concerns about the future potential of forests to offset CO2 emissions from fossil fuel burning.
- They also warn that temperatures will keep rising in the near future — “even if we were to take drastic emissions reductions measures.”
Tropical trees have shorter life spans than trees in other parts of the world, living, for example, just over half as long as temperate trees. A new analysis suggests that, as the world warms up, tropical trees will live even shorter lives, spelling trouble for global biodiversity and carbon stocks.
A new study published in the journal Proceedings of the National Academy of Sciences (PNAS) suggests that in warm tropical lowlands, tree longevity decreases when forests become drier and when the mean annual temperatures is greater than 25.4° Celsius (77.7° Fahrenheit).
“Our findings — which are the first to demonstrate that there is a temperature threshold — suggests that for trees in these regions, their longevity is likely to be negatively affected,” study co-author Manuel Gloor from the University of Leeds, U.K., said in a statement.
The researchers examined tree-ring data from more than 100,000 trees belonging to 438 species from more than 3,300 sites around the world. Growth rings, found within tree trunks, represent one year of growth, allowing researchers to estimate the age of trees and speed of growth.
The authors used statistical modeling to assess how tree growth and longevity were related to climate in the tropics, taking into account factors that may influence the results, such as seasonal temperature, soil moisture, soil type, cloud cover, and human influence.
“Tropical forests may be more vulnerable to increasing heat than has been previously thought,” co-author Roel Brienen, also from the University of Leeds, said in a statement. “These results are a warning sign that, along with deforestation, global warming adds extra stress on the Earth’s tropical forests.”
In another recently published study, conducted in the Biosphere 2 dome in the Arizona desert, researchers determined dryness, not heat, was the biggest threat to tropical trees. The research team simulated extreme warming conditions for tropical forests, with temperatures reaching to 40°C (104°F).
The team found that trees could still photosynthesize at temperatures up to 38°C (100°F), which is 10°C (18°F) warmer than the current average temperature in tropical forests. However, they acknowledge the dome kept moisture levels high, whereas the warming of forests under natural conditions would also lead to drier air and increased water loss from plants, accelerating tree death.
“I find particularly interesting their finding showing that trees longevity is limited by both aridity and temperature in the tropics,” Adriane Esquivel-Muelbert from the Birmingham Institute of Forest Research, U.K., who was not involved in either study, told Mongabay. “This has a big implication to our understanding of how forest will respond to global change. [N]ext we need to know whether tree communities, as opposed to individual trees, respond similarly.”
Esquivel-Muelbert, who recently co-authored a study on why trees die in the Amazon says this new research, “brings new insights to our understanding on the longevity of tree species – particularly in the tropics [and] provide[s] a great synthesis on the life-history strategies of trees and how they vary across the globe.”
Tropical forests host about 50% of Earth’s biodiversity and 50% of its forest carbon stocks. The capacity of forests to capture and store carbon depends on their health and longevity. Thus, predictions of increasingly drier and hotter conditions in the tropics means impacts for global carbon stocks.
“If tropical trees die earlier, this will affect how much carbon these forests can hold, raising concerns about the future potential of forests to offset CO2 emissions from fossil fuel burning. It could also cause changes in biodiversity and a decrease in the number of species on the planet.” Giuliano Locosselli, lead author of the new study from the Institute of Biosciences, University of São Paulo (USP), Brazil, said in a statement.
Average temperatures in the tropics are between 21 and 30°C (70 and 86°F) but are expected to rise over the next 10 to 20 years, reaching 2.5°C (4.5°F) higher on average than pre-industrial levels. Data from the World Meteorological Organization show that the average global temperatures between 2016 and 2020 are among the hottest on record.
“Many regions in the tropics are heating up particularly rapidly and substantial areas will become warmer, on average, than approximately 25°C [77°F],” Gloor said.
Forests in the Amazon are already close to this critical temperature threshold. This November, Brazil recorded its highest temperature in more than 100 years: 44.8°C (112.6°F) in Nova Maringá in Mato Grosso state.
As the tropics get hotter, tree death is likely to accelerate in the Amazon, Pantanal and Atlantic forests of South America, the authors say. In the Congo rainforest, temperatures are lower, but as they increase, “we might begin to see signs of increased tree mortality. From this point of view, the scenario is quite bleak.” Locosselli said.
“Temperatures will keep rising in the near future even if we were to take drastic emissions reductions measures,” Marcos Buckeridge, director of USP’s Institute of Biosciences, who is also a co-author of the study, said in a statement.
That this critical temperature threshold will be passed is “unavoidable,” Buckeridge said, unless more is done to protect forests and curb greenhouse emissions.
Locosselli, G. M., Brienen, R. J., de Souza Leite, M., Gloor, M., Krottenthaler, S., de Oliveira, A. A., … Buckeridge, M. (2020). Global tree-ring analysis reveals rapid decrease in tropical tree longevity with temperature. Proceedings of the National Academy of Sciences. doi:10.1073/pnas.2003873117
Banner image of a rainforest tree by Rhett A Butler/Mongabay.
Liz Kimbrough is a staff writer for Mongabay. Find her on Twitter @lizkimbrough
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