Tropical rainforests help slow climate change by pulling carbon dioxide from the air and storing carbon in wood. Fast-growing forests matter even more because rapid growth means faster carbon capture.
Many tropical regions now contain young forests that regrow after farming, logging, or fires.
Scientists want to understand what controls growth speed during early recovery. New research shows that nitrogen plays a central role in determining how fast young tropical forests store carbon.
Nutrients shape forest recovery
Tree growth depends on sunlight, water, and soil nutrients. Nitrogen supports leaf growth and photosynthesis.
After land clearing for cattle or crops, soil often loses nitrogen through rain runoff and gas release.
Low soil nitrogen slows plant growth even when sunlight and water remain available. As a result, carbon capture also slows.
Ecologists predicted that forest age might change nutrient needs. Early recovery stages may face nitrogen shortages, while older forests may face limits from other nutrients such as phosphorus.
Testing that idea required long-term experiments across forests of different ages.
Measuring growth across ages
A research team led by Wenguang Tang from the University of Leeds and the University of Glasgow designed a large experiment in Panama.
Study sites included 76 large forest plots across the landscape in Panama. Some plots started as open cattle pasture.
Other plots represented forests recovering for 10 years, 30 years, or mature forests that had experienced minimal human disturbance for centuries.
Scientists added nitrogen, phosphorus, both nutrients, or no nutrients at all. Tree growth and death were measured for tens of thousands of trees across many years.
Rapid growth needs nitrogen
Results showed a clear pattern. Nitrogen strongly limited growth in very young forests.
Added nitrogen increased aboveground biomass growth by 95 percent in recently abandoned pasture and by 48 percent in forests with about 10 years of recovery.
Carbon storage rose quickly as tree trunks and branches grew faster.
“Nitrogen is limiting how quickly young forests can regrow,” said Sarah Batterman, senior author from the Cary Institute of Ecosystem Studies.
“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 give us a few more years to reduce our carbon emissions.”
Added nitrogen mainly boosted growth rather than reducing tree death. Faster-growing trees competed more strongly for light and space, which caused some natural thinning, yet overall carbon storage still increased.
Older forests respond differently
Forests older than 30 years showed no response to added nitrogen. Soil nitrogen builds up naturally over time as nitrogen-fixing trees grow.
Such trees partner with bacteria that convert nitrogen gas from the air into usable soil nitrogen. As recovery continues, nitrogen shortages fade.
Research also found no clear phosphorus limitation at any forest age. That result challenged long-standing ideas about tropical soils.
Tropical trees appear to use clever strategies to access phosphorus locked in soil minerals or organic matter.
Roots release enzymes that free phosphorus, allowing continued growth even when soil phosphorus levels appear low.
“This result challenges the long-standing theory that tropical forest carbon sinks are fundamentally constrained by phosphorus availability,” said Tang.
Carbon nitrogen feedback in forests
Forest recovery creates a feedback loop between carbon and nitrogen. Rapid carbon uptake during early recovery increases demand for nitrogen.
Nitrogen-fixing species respond by adding nitrogen to soil. Over time, nitrogen availability rises, growth accelerates, and nitrogen fixation slows again. That cycle explains why nitrogen limits early growth but not later stages.
Models often assume phosphorus limits tropical forest growth. Evidence from Panama suggests nitrogen deserves greater attention during early recovery. Nutrient limits shift as forest structure and species composition change.
Climate benefits of faster regrowth
Nitrogen limitation may prevent young tropical forests from storing 470 to 840 million metric tons of carbon dioxide per year worldwide.
That amount roughly equals emissions from more than 140 million gasoline-powered cars. Faster regrowth during early years matters most because global emissions remain high.
Scientists do not support fertilizer use in forests. Fertilizer production consumes energy and releases greenhouse gases. Runoff also harms rivers and oceans. Instead, researchers suggest smarter forest management.
“Ideally, forest stewards could make sure that some of the trees in a regrowing forest are nitrogen-fixers,” said Batterman.
Restoring forests near pollution
Another approach focuses on restoring forests near farms or cities where nitrogen pollution already exists. Growing trees absorb excess nitrogen before pollution reaches waterways or forms harmful gases.
“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.”
Young tropical forests cannot replace clean energy or emission cuts. Still, faster forest recovery can buy precious time while societies shift away from fossil fuels.
The study is published in the journal Nature Communications.
