Recovering tropical forests grow back nearly twice as fast with nitrogen

(Press-News.org) Young tropical forests play a crucial role in slowing climate change. Growing trees absorb carbon dioxide from the air, using photosynthesis to build it into their roots, trunks, and branches, where they can store carbon for decades or even centuries. But, according to a new study, this CO2 absorption may be slowed down by the lack of a crucial element that trees need to grow: nitrogen. 

Published in Nature Communications and coauthored by Cary Institute of Ecosystem Studies ecologist Sarah Batterman, the study estimates that if recovering tropical forests had enough nitrogen in their soils, they might absorb up to an additional 820 million metric tons of carbon dioxide each year for a decade. 

“Nitrogen is limiting how quickly young forests can regrow,” said Batterman, senior author on the paper. “When we added nitrogen to the soil, forests grew back almost twice as fast in the first 10 years. Faster growth rates mean faster absorption of carbon dioxide, which can help to give us a few more years to reduce our carbon emissions.”

Rather than fertilizing young forests, the scientists recommend planting nitrogen-fixing trees in regenerating forests and, when possible, prioritizing forest restoration on lands that receive nitrogen pollution from farms and factories.

A giant study About 50% of tropical forests are recovering from disruptions such as logging, wildfire, and agriculture — all processes that can cause nitrogen to leak out of the soil. Phosphorus is also thought to be a limiting nutrient in tropical forests. 

Scientists, led by Wenguang Tang at the University of Glasgow, wanted to test how adding nitrogen and phosphorus fertilizers would affect the growth rates (and therefore carbon absorption rates) of tree trunks and branches in recovering tropical forests. The experiment encompassed 76 hockey-rink-sized plots in Panama, each covering 1600 square meters. The plots ranged in maturity at the start of the experiment, including newly regenerating forests, middle-aged forests that had been regenerating for 10 and 30 years, and mature forests with limited human disturbance for hundreds of years. The plots received additional nitrogen, phosphorus, both, or none. Some of the sites have been monitored since 1997.

“Our work represents the world’s largest and longest nitrogen and phosphorus addition experiment of its kind,” said Tang. “Each of our 76 plots has been censused at least five times, and in each census, data were collected from more than 20,000 trees. Maintaining high-quality, consistent census data over such a long period and across so many individuals was extremely challenging.”

The nitrogen effect The team found that adding nitrogen caused the forest to regrow a whopping 95% faster in recently abandoned agricultural fields, and 48% faster in forests that had been recovering for 10 years. 

“It was pretty amazing to see,” said Batterman. “The plots with added nitrogen looked so much bigger than the ones where we didn’t add nitrogen — the trees were just huge. We were surprised how quickly the forest grew back and how strong the effect of nitrogen was.”

For the forests 30 years and older, adding nitrogen had no effect, likely because nitrogen had built up in the soil over time, thanks to nitrogen-fixing trees. These trees cooperate with bacteria to pull nitrogen gas out of the atmosphere, converting it into a form of nitrogen that plants can use. 

A phosphorus puzzle Contrary to scientific expectations, adding phosphorus to the soil made no difference to forest growth rates at any age — a striking result, said lead author Tang. “This result challenges the long-standing theory that tropical forest carbon sinks are fundamentally constrained by phosphorus availability.”

It is possible that phosphorus addition did result in changes to the trees’ roots or fruits, which were not measured in this study. Another explanation is that trees in these forests have evolved creative ways to overcome phosphorus limitations. The scientists hope to investigate this thread further, to better understand what strategies trees use to maintain high productivity despite low phosphorus in the soil.

“Future work should also examine how consistent these patterns are in other tropical forests, including in Africa and Asia,” said Tang. “However, we expect nitrogen limitation in young tropical forests may be quite common. It’s likely becoming increasingly important, too, as forest disturbances increase and carbon dioxide levels rise in the atmosphere.”

Putting the research into practice If nitrogen limitation is indeed widespread, the team estimates it may prevent recovering tropical forests from absorbing an additional 470 to 840 million metric tons of carbon dioxide per year. That’s roughly equivalent to taking 142 million gasoline-powered cars off the road each year. 

To achieve those gains, the team does not advocate for adding fertilizer to overcome nutrient limitations. Nitrogen fertilizer is expensive and energy intensive to produce; it can also pollute waterways and lead to emissions of nitrous oxide, a powerful greenhouse gas. Instead, the team recommends being more strategic about where to focus on forest regeneration and which tree species should be planted at these sites.  

“Ideally, forest stewards could make sure that some of the trees in a regrowing forest are nitrogen-fixers,” said Batterman.

Another strategy recommended by the team is to prioritize reforestation in areas where there is high nitrogen pollution from agriculture, factories, and transportation. This way, the trees can clean up the nitrogen pollution before it clogs waterways or turns into greenhouse gases, and the forests will grow back faster. 

“These practices could increase how quickly these recovering forests take in carbon dioxide,” said Batterman. “In the long-term, the forests are not going to sequester extra carbon, but in that first 10 years, they can do the job faster, and 10 years is what we really need right now. We need to make big changes to reduce our fossil fuel emissions, such as shifting to clean energy and swapping out our gas guzzlers for electric vehicles, and unfortunately, that switch is taking longer than we need it to. Reforestation is one tool that can buy us more time to decarbonize and delay the worst effects of climate change.”

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Authors Wenguang Tang, University of Glasgow, University of Leeds

Jefferson S. Hall, Smithsonian Tropical Research Institute

Oliver L. Phillips, University of Leeds

Roel J. W. Brienen, University of Leeds

S. Joseph Wright, Smithsonian Tropical Research Institute

Michelle Y. Wong, Yale University, Cary Institute of Ecosystem Studies, Smithsonian Tropical Research Institute 

Lars O. Hedin, Princeton University

Michiel van Breugel, National University of Singapore, Smithsonian Tropical Research Institute

Joseph B. Yavitt, Cornell University

Phillip M. Hannam, Cary Institute of Ecosystem Studies

Sarah A. Batterman, Cary Institute of Ecosystem Studies, University of Leeds, Smithsonian Tropical Research Institute (*corresponding author)

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This study is a contribution of the Agua Salud Project and the Gigante Fertilization Experiment. Research was funded by the Heising-Simons Foundation, the United Kingdom Natural Environment Research Council, the Andrew W. Mellon Foundation, and Agua Salud core donors, among other sources.

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Cary Institute of Ecosystem Studies is an independent nonprofit environmental research organization. Since 1983, our scientists have studied the complex interactions that shape the natural world and how climate change affects ecosystems. Our research supports effective resource management, evidence-based policy, and environmental literacy. Our staff are global leaders in the ecology of forests, freshwater, soils, disease, and cities.


 

 

 

 

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