The biggest trees in the Peruvian Amazon store the most carbon — and they also face the greatest threat from humans

The biggest trees in the Peruvian Amazon store disproportionately more carbon than smaller trees do, a new study finds. But these larger trees are also the ones most likely to be harvested, which means more carbon is being released into the atmosphere, thereby reducing these forests' ability to act as carbon sinks, the researchers say.

Almost 60% of Peru’s land is covered by forests, the vast majority in the Amazon region where it accounts for around 11% of the total Amazon rainforest. Peru's current forestry law allows trees to be selectively harvested when they reach a minimum diameter, which ranges from 16 to 24 inches (41 to 61 centimeters) depending on the species. The terrain in the Peruvian Amazon makes accessing and removing trees difficult, so forestry companies prefer larger trees because they yield the most wood, which reduces transportation costs, labor and time. These trees also tend to be older and more mature, with denser, harder and more stable wood.

But these large trees also store the most carbon, and removing them returns much of that carbon to the atmosphere, study co-author Geomar Vallejos-Torres, an agricultural scientist at the National University of San Martín in Peru, told Live Science in a translated email.

To determine how much carbon is stored in these large trees, Vallejos-Torres and his colleagues measured hundreds of trees across five forests in the country, recording variables such as diameter, height, crown area and wood density to estimate aboveground and belowground biomass and stored carbon.

They found that carbon storage, both aboveground and belowground, increased disproportionately with increasing trunk diameter, with 16 inches being an important threshold. The forests they studied sequestered up to 331 metric tons per hectare (148 U.S. tons per acre) aboveground and 47 metric tons per hectare (21 U.S. tons per acre) belowground. The majority of that carbon — between 88% and 93%, depending on the species — was concentrated in trees larger than 16 inches in diameter. Breadnut trees (Brosimum alicastrum), for example, stored 11.4% of their total aboveground carbon in trees smaller than this cutoff point, compared with 88.7% in larger trees.

The work was published Jan. 25 in the journal Frontiers in Forests and Global Change.

The results show that Peru's forestry policy targets the trees that store the most carbon and suggest that the country should shift its forest policy to protect them, Vallejos-Torres said.

"Given the urgency of keeping carbon reserves out of the atmosphere, it is necessary to conserve trees larger than 41 cm," he said. "This also allows for the conservation of forest biodiversity and microfauna, buffering the microclimate in the face of future climate changes."

Size isn't everything

But some researchers say tree size is not always the most important consideration for carbon sequestration. Ulf Büntgen, a professor of environmental systems analysis at the University of Cambridge who was not involved in the research, said the length of time the carbon is stored is more relevant.

"The paper talks not much about tree ages and thus ignores carbon residence time, which is overall low in the tropics," he told Live Science.

Vallejos-Torres countered this argument, however, saying that the largest trees continue to accumulate carbon for centuries, while smaller ones grow too slowly to make up the difference.

"The regeneration of smaller trees is slow, uncertain and often limited by degradation, disturbance and microclimatic changes, so the lost carbon is not recovered on timescales relevant for climate mitigation," he said.

The relationship between tree diameter and carbon stocks highlighted in the study is both empirically valid and relatively intuitive, said Martin Perez Lara, director of forest climate solutions impact and monitoring for the World Wildlife Fund. But he added that focusing only on tree size is not the best way to design a climate-friendly forestry system.

"A large share of research shows that well-designed management systems, including selective harvesting forests — including some trees around 40 cm — can contribute positively to climate mitigation and reducing degradation risks, and support long-term carbon dynamics," he told Live Science.

Despite the urgent need to preserve carbon sinks in the Amazon, Vallejos-Torres does not have much faith that the necessary changes to Peru's forestry policy will be forthcoming. "A legal reform that protects the largest trees would directly affect the economic interests of the timber sector, which depends on the extraction of these individuals of high commercial value and has an important weight in forestry policy decisions in the country," he said.