The largest trees in the Peruvian Amazon store a disproportionately greater amount of carbon than smaller ones. The problem is that they are also the most likely to be cut down, releasing that carbon into the atmosphere and reducing the forest’s ability to act as a carbon sink, according to a new study.
Nearly 60% of Peru’s territory is covered by forest, the vast majority in the Amazon region, which accounts for about 11% of the entire Amazon rainforest. Peruvian forestry law allows selective logging once trees reach a minimum diameter that ranges from 41 to 61 centimeters (16 to 24 inches), depending on the species.
Why the biggest ones fall
The terrain of the Peruvian Amazon makes access and removal difficult, so logging companies prefer large trees: they yield more timber and cut transport, labor and time costs. They also tend to be older and more mature, with denser, more stable wood. But those giants are precisely the ones holding most of the carbon, and removing them returns much of it to the atmosphere, explains Geomar Vallejos-Torres, co-author of the study and an agricultural scientist at the National University of San Martín in Peru.
The team measured hundreds of trees across five forests, recording diameter, height, crown area and wood density to estimate biomass and stored carbon. They found that carbon storage, both above and below ground, rose disproportionately with trunk diameter, with a key threshold at 40.6 cm (16 inches).
Up to 93% of the carbon in the giants
The forests studied sequestered up to 331 metric tons per hectare above ground and 47 below ground. Between 88% and 93% of that carbon, depending on the species, was concentrated in trees larger than 40.6 cm in diameter. In the breadnut tree (Brosimum alicastrum), for example, only 11.4% of the carbon sat in trees below that threshold, versus 88.7% in the larger ones.
The work, published in Frontiers in Forests and Global Change, shows that Peru’s forestry policy targets exactly the trees that store the most carbon. Vallejos-Torres argues the country should change that policy to protect them.
Size is not everything
Not everyone agrees. Ulf Büntgen, a professor of environmental systems analysis at the University of Cambridge who was not involved in the research, notes that carbon residence time matters more than size, and that in the tropics that time is usually short. Vallejos-Torres counters that large trees keep accumulating carbon for centuries, while smaller ones grow too slowly to make up the difference.
Martin Perez Lara, of the World Wide Fund for Nature (WWF), considers the diameter-carbon relationship empirically valid, but warns that focusing only on size is not enough to design sustainable forest management.
A forest that is starting to release
The study lands at a critical moment: evidence suggests the Amazon is shifting from a global carbon sink into a net source. Historically its trees stored roughly 650 billion tons of carbon dioxide. Deforestation, illegal logging and climate-driven fires have damaged that capacity, and in some years the ecosystem already releases more carbon than it absorbs.
Vallejos-Torres is not optimistic about legal reform: protecting the largest trees would directly affect the interests of the timber sector, which depends on those high-value specimens and carries weight in the country’s forestry decisions.
Reporting: Anne Silva / Amazonia Mag. Source: Frontiers in Forests and Global Change; National University of San Martín; WWF; Live Science.