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Tropical Forests Continue Carbon Storage under Moderate Warming

 Erika Berenguer at base of Brazilian tree
Researcher Erika Berenguer, one of 225 collaborators on the study, stands at the foot of a tree in Brazil. | Erika Berenguer

A long-term global assessment of the sensitivity of tropical forests to increasing temperatures brings encouraging news: in the long run, Earth's tropical forests may be more resilient to a warming world than previously thought.

Growing forests play a crucial role in maintaining Earth's atmosphere by absorbing carbon dioxide in the atmosphere and keeping levels of the greenhouse gas from rising too quickly. Tropical forests worldwide — where nearly 40% of the planet's carbon-thirsty vegetation resides — are particularly important in mitigating global temperature rise.

Some estimations of long-term heat sensitivity suggest that tropical forests and their carbon stocks are already highly vulnerable or currently being impacted under current climate conditions, although this remains one of the largest uncertainties in climate change models. According to a new study published in the May 22 issue of Science, however, the ability of tropical forests to store atmospheric carbon is likely to remain intact in moderate climate warming scenarios.

The promising findings show a previously underappreciated resiliency of tropical forests, even in the face of rising global temperatures — so long as they're not further impacted by other human disturbances such as clearance, logging or fires.

The results also reveal the limits of this resilience, revealing a critical yet not so far-off thermal tipping point of daytime temperatures exceeding 32.2°C or roughly 90°F. Below this threshold, the amount of carbon stored by forests changes little and forests remain more resistant to warming than has been predicted by most climate models.

Above this temperature, tropical forest biomes are much more sensitive. They become far less able to store excess carbon and trees are more likely to die under the hot, dry conditions — potentially accelerating climate warming overall, according to the study.

"Tropical forests may be able to resist small increases in temperature, but the safety net of this resilience is small," said lead author Martin Sullivan, a researcher from the University of Leeds. "We cannot be complacent about how forests would respond if we warm the planet much more than 2°C."

Sullivan noted that over a quarter of tropical forests are already above the 32.2°C threshold — particularly those in South America where baseline temperatures and projected future warming are highest. Even stabilizing warming at the internationally recognized targets of 1.5°C or 2°C above pre-industrial levels is likely to push the majority of tropical forests around the world beyond this critical tipping point, likely resulting in long-term, biome-wide forest lost.

"The key is avoiding further warming, as each degree of warming beyond 2°C will cause more rapid loss of carbon than each degree leading up to 2°C," said Sullivan.

According to Sullivan, roughly 2.5 acres of tropical forest contains more carbon in its trees than the weight of 2000 people — or about 124 metric tons in the space of two football fields. As tropical forests cover vast areas of Earth, even changes as small as a few tons of carbon here and there within scattered forest plots quickly scale up to a few billion tons across the entirety of the biome, said Sullivan.

To assess long-term climate controls on tropical forests directly, Sullivan and a global team of researchers evaluated biomass carbon and carbon flux across 590 globally distributed, permanent tropical forest plots. The massive, biome-wide study combined the work of more than 225 collaborators from 36 countries.

"Setting up and measuring forest inventory plots in often very remote locations is very time-consuming. Based on the number of person-days it takes to set-up and remeasure plots, it would have taken one researcher about 500 years of effort to collect all the data used in this study themselves," said Sullivan.

Unlike forests in higher latitudes, tropical forests are unique in that they are routinely exposed to the hottest of rising temperatures, said Sullivan. While short-term drying and warming are known to impact forests, their long-term sensitivity to sustained climate warming, as well as how increasing temperatures might affect land-atmosphere carbon fluxes, remains poorly understood.

While over the short term, rapidly declining tree species may appear as total forest collapse, eventually new species more tolerant to the new conditions become established, claiming the forest as their own. "Looking at variation in space allows us to see how forests respond to climate after having had time to adapt," said Sullivan.

A number of previous studies have suggested that rising nighttime temperatures are the main driver of observed year-to-year fluctuations in the amount of carbon tropical forests take in from the atmosphere. "We would therefore have expected nighttime temperature to also control spatial variation between forests and the amount of carbon they store," said Sullivan.

Instead, the researchers discovered that it is the forests with the hottest daytime temperatures that store the least carbon. Hot maximum temperatures reduce carbon storage by depressing overall forest growth rates and reducing the time that carbon resides in the ecosystem by killing trees. According to the results, these adverse effects became far more prominent when daytime forest temperatures exceeded 32.2°C.

The study underscores the importance of not only limiting continued warming, but also global tropical forest conservation efforts.

"By protecting forests that remain, and encouraging restoration to connect fragmented forests, we can realize tropical forests' full adaptation potential to climate," said Sullivan.

"This means that land managers in tropical areas do have the power to help their forests fight climate change," he said.

[Credit for associated image: Roel Brienen, University of Leeds]