Tropical forests can regrow twice as fast on nitrogen-rich soils, a major long-term experiment in Central America finds. The results could reshape how countries plan reforestation to fight climate change.
Tropical forests cleared for logging or agriculture can bounce back twice as fast when their soils are rich in nitrogen, according to a major new study that could change how countries plan reforestation to tackle climate change.
A team led by the University of Leeds ran what they describe as the world’s largest and longest experiment on how nutrients influence forest recovery after deforestation. Their findings, published in the journal Nature Communications, suggest that paying attention to soil nitrogen could significantly increase how much carbon young forests pull out of the atmosphere.
“Our study is exciting because it suggests there are ways we can boost the capture and storage of greenhouse gases through reforestation by managing the nutrients available to trees,” lead author Wenguang Tang, who conducted the work as a doctoral student at Leeds, said in a news release.
Tropical forests are among the planet’s most important carbon sinks. Through a process known as carbon sequestration, they remove carbon dioxide from the air and store it in trunks, branches, roots and soils. As governments look to forests as part of their climate strategies, understanding what speeds up or slows down regrowth has become a pressing scientific and policy question.
To probe that question, the researchers established 76 forest plots across Central America, each about one third the size of a football pitch and at different stages of regrowth after being cleared. Many of these areas had previously been used for cattle ranching or other agriculture, or had been logged.
Over as long as two decades, the team tracked which trees grew and which died in each plot. They then applied different nutrient treatments: some plots received nitrogen fertilizer, some received phosphorus, some received both, and some were left without any added nutrients.
By comparing how the forests changed over time, the scientists found that soil nutrients strongly shaped how quickly the forests came back. In the first 10 years of regrowth, trees in nitrogen-rich plots recovered about twice as fast as those without enough nitrogen.
The work involved collaborators from the University of Glasgow, the Smithsonian Tropical Research Institute, Yale University, Princeton University, Cornell University, the National University of Singapore and the Cary Institute of Ecosystem Studies.
Although the team used nitrogen fertilizer as a research tool, they are not calling for fertilizer to be spread on recovering forests. Adding synthetic nitrogen at large scale can release nitrous oxide, a powerful greenhouse gas, and can damage waterways and biodiversity.
Instead, the researchers point to more nature-based ways to ensure forests have the nitrogen they need. One option is to plant more trees from the legume family, such as many species of beans and related plants, which host bacteria on their roots that naturally add nitrogen to the soil. Another is to focus restoration in areas where nitrogen has already built up, for example because of air pollution.
These strategies could help young forests lock away more carbon during their fastest growth phase, without the downsides of industrial fertilizers.
The team also explored what their results might mean on a global scale. If similar nitrogen limitations are common in young tropical forests worldwide, they estimate that a shortage of nitrogen could be holding back the annual removal of about 0.69 billion tonnes of carbon dioxide. That is roughly equivalent to two years of carbon dioxide and other greenhouse gas emissions from the United Kingdom.
The study arrives just weeks after the close of COP 30 in Brazil, where governments announced the Tropical Forest Forever Facility fund to support tropical countries in protecting and restoring forests. As money begins to flow into new restoration projects, the findings offer a way to get more climate benefit from each hectare of land.
“Our experimental findings have implications for how we understand and manage tropical forests for natural climate solutions,” added principal investigator Sarah Batterman, an associate professor in the University of Leeds’ School of Geography.
Batterman and colleagues stress that protecting intact, mature tropical forests remains the top priority, because old-growth forests store vast amounts of carbon and support rich biodiversity. But with about half of tropical forests already recovering from past deforestation or degradation, decisions about where and how to restore are increasingly important.
The new research suggests that mapping soil nutrients, encouraging nitrogen-fixing tree species and avoiding nitrogen-poor sites when possible could all help maximize the climate payoff from reforestation efforts.
More broadly, the study highlights that not all forests are equal when it comes to climate. Age, history of land use and underlying soil conditions can all influence how quickly trees grow and how much carbon they can store.
As countries refine their climate pledges and design large-scale tree-planting programs, the researchers argue that these ecological details should be built into planning. Doing so could help ensure that investments in tropical forests deliver the greatest possible benefit for the climate, while also supporting local ecosystems and communities.
Source: University of Leeds