At the United Nations Climate Change Conference in 2015, countries around the world committed to striving towards net-zero greenhouse gas emissions by the middle of the 21st century. But achieving this goal is difficult, even for countries boasting extensive forests that could, in principle, act as important carbon sinks if deforestation were halted.
Now, a study in Frontiers in Forests and Global Change has shown that in Peru, national forest regulations will need a substantial review if the country is to meet its goal of achieving net-zero by 2050. In particular, the largest trees in the Peruvian Amazon require priority protection, concluded the authors. These findings are likely also valid for other countries across the region.
"Here we show that felling trees with a diameter at breast height of at least 41cm, depending on species, releases disproportionately large amounts of carbon into the atmosphere, making the net-zero goal difficult or even impossible to achieve," said Dr Geomar Vallejos-Torres, the corresponding author and a principal investigator at the National University of San Martín, Peru.
"Protecting these trees would also help conserve biodiversity and forest microfauna, buffering forest microclimates against future climate change."
Peru has the ninth greatest forest cover in the world and the second largest within the Amazon. The amount of carbon stored above ground in the Peruvian Amazon is enormous: approximately 6.9 million tons of CO2 equivalents. But official monitoring has shown that 150,602 hectares of Amazonian forest were lost in 2024 alone, highlighting the continued pressure on forest ecosystems.
Peru's official forest policy allows trees to be harvested once they reach a minimum diameter at breast height. In the Peruvian Amazon, this threshold currently ranges from 41 to 61cm, depending on species, meaning that the largest trees are currently prioritized for selective timber logging.
Finally seeing the trees for the forest
Here, Vallejos-Torres and colleagues set out to determine which trees store the greatest amounts of carbon across five representative forests in the San Martín department of the northern Peruvian Amazon: Ojos de Agua, Huallaga, Bajo Huallaga, Alto Mayo and Alto Roque.
These forests cover a combined area of 3,802 km² between 382 and 2,086 meters above sea level. The researchers studied 100 sampling plots of 500 m² each, in which 535 trees were measured. For each tree, variables such as diameter, height, crown area, and wood density were recorded and used to estimate above- and below-ground biomass and stored carbon.
The results showed that each forest contains between five and 18 tree species, with a total of 59 species across the studied plots. Tree species were relatively evenly distributed, with no single one strongly dominating. Tree biodiversity was highest in Alto Mayo and lowest in Bajo Huallaga and Alto Roque.
Among the five forests analyzed, the Huallaga forest showed the highest carbon densities, at 331.06 tons per hectare of above-ground carbon and 47.41 tons per hectare of below-ground carbon, while Alto Mayo showed the lowest values: 86.83 tons per hectare above-ground and 15.18 tons per hectare below-ground.
Large trees catch much carbon
For most species, carbon stocks increase disproportionately with increasing trunk diameter, both above- and below-ground. The authors identified whether a tree reached at least 41cm in diameter at breast height as a key predictor of carbon storage.
For example, the species Brosimum alicastrum stored 11.4% of its total above-ground carbon in trees below this cut-off point, compared to 88.6% in larger trees, while Manilkara bidentata stored 7.9% versus 92.1% of its below-ground carbon in trees thinner versus wider than 41cm.
Across the forests studied, carbon storage reached up to 331 tons per hectare above-ground and 47 tons per hectare below-ground. The majority – between 88% and 93% of carbon, depending on species – was concentrated in the largest trees.
Taken together, the main forest species analyzed (considering only large trees) stored approximately 198 tons of carbon per hectare above-ground and 30 tons per hectare below-ground, confirming the disproportionate role of large trees in the forest carbon balance.
The authors concluded that current regulations are strongly counterproductive, as they prioritize the selective logging of the very trees that store the most carbon. Instead, they recommend changing forest policies to prioritize the protection of large trees as a key condition for achieving net-zero emissions by 2050.
"The current resolution could generate serious problems for forest conservation and carbon storage," Vallejos-Torres warned.
"Our results show that these largest trees are not expendable resources, but an integral part of the climate solution."
