A study of an Atlantic Forest fragment found soil held most of the carbon, more evenly mixed tree communities stored more of it, and steep terrain carried leaf litter away.
When people talk about forests and climate change, they usually talk about trees. Trees pull carbon dioxide from the air and lock carbon into their trunks, branches and roots, and forest carbon programs often focus on measuring them. But much of a forest’s carbon lies elsewhere, in the leaves and twigs on the ground and in the soil beneath.
A study of a forest fragment in southern Brazil underlines how much that matters. Published in Environmental and Biogeochemical Processes, the research found that forest structure, the evenness of tree species and the steepness of the terrain together determine how carbon is divided among living vegetation, leaf litter and soil.
“Carbon storage cannot be fully understood by measuring trees alone,” said corresponding author Vinicius Costa Cysneiros of the Federal University of Santa Catarina. “Our results show that vegetation, litter, soil, biodiversity, and topography must be considered together to reveal how a forest functions as a carbon sink.”
A forest named for a pine
The study took place in a mixed ombrophilous forest, better known as Araucaria forest, named for the Brazilian pine, Araucaria angustifolia. This tall, candelabra-shaped conifer, whose seeds are a traditional food in southern Brazil, once dominated vast highland areas. Logging and land clearing during the twentieth century reduced those forests to scattered remnants, and the species is now listed as critically endangered on the IUCN Red List.
The Brazilian pine is also an ecological keystone. Its large, starchy seeds, known locally as pinhão, feed many animals, among them the azure jay, which buries seeds for later and helps the trees spread. The trees can live for centuries, and old stands form a distinctive high canopy above a dense understory of broadleaf trees and shrubs.
Araucaria forest is part of the broader Atlantic Forest, one of the world’s great biodiversity hotspots, which once stretched along much of Brazil’s coast and inland. Only a small fraction of its original extent remains, much of it in fragments surrounded by farmland and towns.
The researchers studied an 8.3-hectare fragment in Curitibanos, in the state of Santa Catarina. The region has a humid temperate climate with cold winters and frequent frost, and the fragment is surrounded by pasture, agriculture and commercial forestry.
Counting carbon from canopy to soil
The team surveyed nine permanent plots, recording the diameter, height and species of trees and the structure of the forest. They estimated the carbon stored in aboveground vegetation, collected litter from the forest floor and measured soil carbon to a depth of 30 centimetres.
Sampling soil to 30 centimetres matches the default depth used in many carbon accounting guidelines, including those of the Intergovernmental Panel on Climate Change, though deeper layers can hold substantial additional carbon.
Carbon storage varied considerably between plots. In most locations, soil held the largest share of total carbon, though vegetation exceeded soil in several plots. On average, aboveground vegetation stored about 69 metric tons of carbon per hectare, and soil about 73 metric tons per hectare. Litter held much less, but it plays an important role as the bridge through which carbon moves from plants into the soil.
The prominence of soil is consistent with the global picture. Worldwide, soils store more carbon than the atmosphere and all living vegetation combined, which makes them a crucial and often under-measured part of the carbon cycle.
Diversity and gravity
Several patterns emerged. Plots with greater basal area, the cross-sectional area of tree trunks per unit of land, and more aboveground carbon generally had more litter carbon, because larger, denser stands drop more leaves, twigs and other organic material.
Terrain complicated that relationship. On steeper slopes, the link between forest biomass and litter carbon weakened, likely because rainfall, runoff and gravity carry litter downhill and away from the trees that produced it.
Biodiversity mattered as well. Plots with higher species evenness, meaning trees were spread more evenly among species rather than dominated by a few, had larger soil carbon stocks. That fits a broader body of research suggesting that more diverse plant communities can use resources more completely and feed more varied organic matter into the soil.
Evenness is different from species richness, the simple count of how many species are present. A plot with ten species in which one accounts for most of the trees has low evenness, while a plot with ten species in roughly equal numbers has high evenness. The finding suggests that how a forest’s trees are distributed among species, not just how many species it holds, may influence how much carbon ends up underground.
Surprisingly, the team found no direct relationship between the amount of carbon in the litter and the amount in the soil. The authors suggest that sampling in winter, when cold temperatures slow decomposition, may explain the gap. Soil carbon also accumulates over much longer periods than litter, so the two may simply operate on different timescales.
Lessons for climate programs
The study has limits that the authors acknowledge. It covered a single fragment with a limited number of plots, and shallow, stony soils prevented deeper sampling, which means soil carbon may be underestimated. The researchers call for studies across seasons and a wider range of climates, soils and terrains.
Even so, the findings carry a practical message. Forest-based climate strategies, from carbon offsets to restoration programs, often estimate carbon from tree measurements alone. The study suggests that approach can miss a large share of a forest’s carbon and the factors that control it.
Protecting tree diversity, supporting natural regeneration and accounting for terrain could all improve carbon estimates and the conservation programs built on them. For the fragmented remnants of Brazil’s Araucaria forest, that may strengthen the case for protecting what remains, not only for its endangered pines and wildlife but for the carbon stored quietly in its soil.
Notes
- Theodoroski, G. N., Cysneiros, V. C., Schmitt, D. E., & Topanotti, L. R. “Carbon partitioning in a mixed ombrophilous forest fragment in southern Brazil.” Environmental and Biogeochemical Processes 2 (2026): e013. Published 13 July 2026. DOI: 10.48130/ebp-0026-0009.
- Shenyang Agricultural University Collaborative Journals. “Forest diversity and terrain shape how carbon is stored in southern Brazil’s Atlantic Forest.” EurekAlert!, 27 July 2026. https://www.eurekalert.org/news-releases/1137759.