- A pair of recent studies show that rising temperatures are shortening the lives of trees in tropical forests and reducing their capacity to absorb carbon dioxide from the atmosphere.
- This phenomenon is already being observed in parts of the Amazon, where the temperature has already crossed a critical threshold of 25°C (77°F); by 2050, the same may happen in the Congo Basin, the world’s second-biggest tropical rainforest.
- Forests play a major role in fighting global warming, but the authors of the recent studies say we shouldn’t be overly reliant on them as a solution, given their diminished capacity to absorb carbon dioxide.
- Instead, they say that cutting emissions is more urgent than ever.
“If a tree lives 500 years, it carries the carbon assimilated and stocked for the last 500 years,” says Giuliano Locosselli, a researcher at the University of São Paulo (USP) in Brazil. “If instead, the tree lives 300 years, it means the carbon will be stocked by 200 years less. So we are accelerating the carbon cycle, and the result is that we have more carbon in the atmosphere.”
Trees have always been our main allies in the fight against global warming, thanks to their capacity to take the carbon dioxide out of the air and store it for dozens or even hundreds of years in their trunks, branches, leaves and roots. Our recklessness, however, has sabotaged this capacity. That’s the conclusion of two studies published at the end of last year, which show that rising temperatures, resulting from our runaway greenhouse gas emissions, are reducing the longevity of the trees in many forests worldwide, including in the Amazon, the largest tropical forest on the planet.
The studies — one led by Locosselli and published in the , and the other by Roel Brienen of the University of Leeds in the U.K., published in Proceedings of the National Academy of Sciences (PNAS) — look at the links between rising temperatures and tree growth and mortality rates. Locosselli and Brienen have worked together for many years and are co-authors on both studies, alongside 20 other researchers from Brazil, Chile, Argentina, Canada, the U.S., the U.K., Germany, France, Italy and Finland. Both studies use data from the International Tree-Ring Data Bank, the world’s largest public archive of this type, maintained by the U.S. National Oceanic and Atmospheric Administration (NOAA). The rings that appear in cross-sections of tree trunks provide crucial information about the individual tree’s age, growth rate, and the prevailing environmental conditions.
In 2015, Brienen had already observed a change in the dynamic of growth and mortality of Amazonian trees. He analyzed data collected in 321 different parts of the forest during the past three decades. “We label all the trees, identify the species and measure their diameters. Then we come back some years later, measure them again, calculate how fast they have been growing, how many new trees have been recruited, and how many trees are dying,” he says.
Looking at the data, Brienen realized that the trees grew faster in the 1980s and the 1990s. On one hand, that’s good news: the faster a tree grows, the more carbon it can take out of the atmosphere. But on the other hand, the faster a tree grows, the sooner it dies. “It is the ‘grow faster, die young’ phenomenon,” Brienen says. “If they grow too fast, they quickly reach a certain diameter and a certain height at which they die, because the leaves can’t pump all the water they need from the roots up to the canopy. Then they die of hydraulic failure.”
In the 1980s and 1990s, however, this trade-off between growth and mortality played in our favor. Accelerated growth made up for the trees’ early deaths when it came to the net amount of carbon captured. From 2000 to 2010, however, the situation began to reverse. “The increase in the growth rates have flattened, but the mortality rates continued to increase. It means that over recent times the forest is taking up less carbon from the atmosphere than it did before,” Brienen says.
The studies show that this change was propelled by rising temperatures. Scientists compared trees from temperate zones with those from tropical zones and realized that, the higher the temperature, the faster the growth. But there’s a limit beyond which the longevity-to-growth ratio goes out of whack: 25° Celsius (77° Fahrenheit).
“The growth rates in the tropics are already operating close to the limit,” Locosselli says. “So if the temperature rises, it doesn’t have a big impact on their growth rates. But if you look at longevity, it falls drastically when the temperature goes above 25°C.”
These findings aren’t just a snapshot of what’s happening now; they also highlight a long-term pattern. In Brazil, the higher mortality rates observed in trees in the northern and central Amazon are expected to spread to the southern portion of the rainforest by 2050. The forecast is also worrisome for the Congo Basin in Central Africa, home to the world’s second-greatest expanse of tropical rainforest, where temperatures across the region are expected to exceed 25°C by 2050. “It means that the capacity of forests to take the carbon out of the atmosphere and store it may be slowing over time,” Brienen says.
For Locosselli, the most important message is that we shouldn’t bet all our climate solution chips on the forests. “Trees and forests still have a major role in the control of the quantity of carbon in the atmosphere, but their capacity of storing carbon dioxide has been reduced by the rise of the temperatures and the changes in the rain pattern,” he says. “So we have to reforest and protect our forests? Yes. But we also have to reduce emissions.”
Impacts on the Paris Agreement
These findings affect not only the future of tropical countries, like Brazil and the Democratic Republic of Congo, but all nations committed to the Paris Agreement, which aims to curb the average global temperature rise by 1.5°C (2.7°F) above pre-industrial levels by 2100.
Gustavo Pinheiro is the coordinator of the low-carbon economy portfolio at the Climate and Society Institute (ICS), a nonprofit that promotes sustainable industry in Brazil. He says the rise in tree mortality shows that the Paris Agreement’s goals should be more ambitious and accomplished within a shorter time frame. “There is an urgency to meet these goals since we are already seeing the effects on the hydraulic cycles and the rise of the temperatures,” he says.
But Brazil, under the administration of President Jair Bolsonaro, has gone in the opposite direction. While countries like China and the U.K. have touted more ambitious goals, the Bolsonaro government has not only reduced its goals of emission reductions but also conditioned them on payments of $10 billion per year. “Moreover, the Brazilian new nationally determined contribution” — or NDC, which is the reduction goal the country has committed to under the Paris Agreement — “is totally uncertain as it doesn’t present how the country will meet its goals,” Pinheiro says.
Brazil is also failing to deliver what would be its main contribution to the Paris Agreement: an end to deforestation. From 2004 to 2012, starting under the administration of then-president Luiz Inácio Lula da Silva, Brazil saw its annual deforestation rate drop significantly — from 27,772 square kilometers (10,722 square miles), more than half the size of Florida, to 4,571 km2 (1,765 mi2). This was thanks mainly to the Action Plan for Prevention and Control of Deforestation in the Legal Amazon (PPCDAM), implemented by Marina Silva, Lula’s environment minister.
But the rates went up again from 2013, under the presidency of Dilma Roussef, who slashed the PPCDAM budget. In 2019, the first year of Bolsonaro’s presidency, deforestation exceeded 10,000 km2 (3,900 mi2), and the result was a rise of 9,6% in the country’s carbon emissions. The far-right leader has promoted the dismantlement of environmental agencies and enlisted the armed forces to do the job. He has also encouraged illegal mining and deforestation with his rhetoric. INPE, the national institute responsible for monitoring the Amazon, is still calculating the official rate of deforestation in 2020, but it’s expected to be more than 11,000 km2 (4,250 mi2).
Locosselli, G. M., Brienen, R. J. W., Leite, M. D., Gloor, M., Krottenthaler, S., de Oliveira, A. A., … Buckeridge, M. (2020). Global tree-ring analysis reveals rapid decrease in tropical tree longevity with temperature. PNAS, 117(52), 33358-33364. doi:10.1073/pnas.2003873117
Brienen, R. J. W., Caldwell, L., Duchesne, L., Voelker, S., Barichivich, J., Baliva, M., … Gloor, E. (2020). Forest carbon sink neutralized by pervasive growth-lifespan trade-offs. Nature Communications, 11(1), 4241. doi:10.1038/s41467-020-17966-z
Banner Image: Sunrise over the Amazon rainforest. Photo by Rhett A. Butler.