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Carbon uptake in tropical forests withers in drier future: Study

Tropical flora in Wayqecha cloud forests in Andes, Peru.

Tropical flora in Wayqecha cloud forests in Andes, Peru. Image by Rhett A. Butler/Mongabay.

  • A new study incorporating satellite data on organic material, or biomass, in tropical forests with experimental data about the effects of temperature and precipitation suggests that forests may lose substantial amounts of carbon by the end of the 21st century.
  • Even with low continued carbon emissions, tropical forests, especially those in the southern Amazon, could lose between 6.8 and 12% of their aboveground carbon. With higher emissions, they could lose 13.3 to 20.1% of their carbon stores.
  • The results highlight the need to reduce global temperatures rapidly to maintain the healthy forests best able to sequester carbon from the atmosphere.
  • The team reported their findings Feb. 6 in the journal Nature Climate Change.

Warmer temperatures could significantly diminish the ability of tropical forests to siphon carbon from the atmosphere by intensifying dry seasons, according to a recent study.

The tropics pack away an enormous amount of organic matter, or biomass, in the trees and other organisms inhabiting these ecosystems. Locked up in that biomass is more than half the world’s aboveground carbon — the equivalent of around 20 years of human-caused emissions.

Scientists often use climate models to predict how a warmer future might affect these forests, including their ability to take up carbon and allay the impacts of climate change. But these models aren’t always able to incorporate some of the major impacts of escalating climate change — such as precipitation changes, heat stress or wind — on the forests.

“There are so many things going on,” said Maria del Rosario Uribe, the study’s lead author and an ecologist and postdoctoral scholar at Yale University in the U.S.

Uribe and her colleagues, in search of a more accurate rendering of how changes to the climate could alter tropical ecosystems, brought together satellite mapping of biomass along with the observed effects of temperature and precipitation from experiments going back to the mid-20th century. The resulting models revealed that, even with relatively low emissions, these ecosystems could lose between 6.8 and 12% of the aboveground carbon they held in 1950 by 2100.

In a scenario with higher emissions, these forests could lose more than 20% of that carbon, the team wrote Feb. 6 in the journal Nature Climate Change.

It turns out that surging dry season intensity (not just changes to precipitation patterns) appears to be the most important factor instigating the predicted increased loss of biomass. This impact isn’t likely to be uniform across all tropical rainforests, however. The researchers found that parts of the Amazon that shrink as a result of drier periodic conditions could be responsible for as much as 40% of this predicted biomass loss.

Clouds above the rainforests in Amazon.
Clouds above the rainforests in Amazon. Image by Rhett A. Butler/Mongabay.

The dynamics at play are linked to the way plants get the energy they need. In general, the process of photosynthesis in which plants draw in CO2 and use sunlight and water to make food tends to increase with humidity and temperature, Uribe explained. And greater concentrations of CO2 in the atmosphere could effectively “fertilize” tropical forests and help boost their growth — to a point.

However, research has shown that temperatures that are too high can curb the rate of carbon-sequestering photosynthesis. Plants tend to “shut down” once temperatures reach a certain threshold, she said, leading to drier air that’s also less favorable for the process. That’s why climate models that are “overly sensitive” to the CO2 fertilization effect may underestimate the impacts of other factors, such as dryness and rainfall, the authors write.

The research highlights “that even without any type of land use change, which is the most important impact on tropical forests, [ongoing changes to] the climate can reduce the carbon storage in these forests,” Sassan Saatchi, a senior scientist at NASA’s Jet Propulsion Laboratory in the U.S., told Mongabay.

Saatchi, who was not involved in the research, said the findings also suggest that nature-based solutions aimed at protecting standing forest and rehabilitating degraded and deforested areas should be taken “extremely seriously” because they not only offset the carbon released by human activity but they encourage healthy forests that better regulate the climate.

Over time, reduced water availability in the warming tropics could take its toll. Rainforest trees play a big role in producing rain via their uptake of water and subsequent evapotranspiration. But less moisture means less rain to keep the forest healthy, Saatchi said. That can have a cumulative negative effect, especially in places like the southern Amazon, where forests are not as well adapted to prolonged dry spells as they are in parts of the Congo Rainforest in Africa, for example.

Saatchi likened the consequences to repeated illness in humans.

“If you constantly get sick, your immune system would be shattered,” making you more susceptible to getting sick again, Saatchi said. “That really impacts your health, and this is something that we worry a lot [about] with these tropical forests.”

This study provides yet more evidence, he said, that underlines the need to protect tropical forests from degradation and destruction, which in turn increases their resilience when faced with worsening climate change.

“It basically means that we need to start quickly to reduce the temperature and keep our forests healthy,” Saatchi added, “because these [forests] are the ones that regulate the climate much better.”

Banner image: Tropical flora in Wayqecha cloud forests in Andes, Peru. Image by Rhett A. Butler/Mongabay.

John Cannon is a staff features writer with Mongabay. Find him on Twitter: @johnccannon

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Cernusak, L. A., Haverd, V., Brendel, O., Le Thiec, D., Guehl, J., & Cuntz, M. (2019). Robust response of terrestrial plants to rising CO2. Trends in Plant Science24(7), 578-586. doi:10.1016/j.tplants.2019.04.003

Mau, A., Reed, S., Wood, T., & Cavaleri, M. (2018). Temperate and tropical forest canopies are already functioning beyond their thermal thresholds for photosynthesis. Forests9(1), 47. doi:10.3390/f901004

Uribe, M. D., Coe, M. T., Castanho, A. D., Macedo, M. N., Valle, D., & Brando, P. M. (2023). Net loss of biomass predicted for tropical biomes in a changing climate. Nature Climate Change. doi:10.1038/s41558-023-01600-z

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