Carbon stocks across Brazilian macro-ecosystems: ecological structure, methodological uncertainty, and implications for carbon accounting

Tropical and subtropical ecosystems are major carbon reservoirs, yet robust carbon accounting across those landscapes remains limited by environmental heterogeneity, uneven representation of pools, and inconsistent measurement methods. Brazil provides a globally relevant model system for tropical carbon accounting due to its diversity of macro-ecosystems and intense land-use transitions. We synthesized 1,299 standardized records from 74 peer-reviewed studies across six Brazilian macro-ecosystems to examine how carbon pools are distributed among ecosystem types, how these patterns compare with global expectations, how much reported variation reflects ecological versus methodological drivers, and which data gaps most constrain reliable carbon accounting. We integrated records for aboveground biomass, belowground biomass, soil organic carbon, and ecosystem-level carbon, standardized all values to Mg C ha⁻¹, and evaluated variability using hierarchical mixed-effects models with climatic moderators. Carbon storage followed a clear ecological hierarchy: humid forests had the highest aboveground biomass, coastal wetlands had the greatest soil organic carbon, and ecotones had the highest ecosystem-level carbon. Relative to agricultural baselines, peak native-system values were approximately five- to sixteen-fold higher across major pools. The best-performing model included climate moderators and explained most of the observed variation (conditional R² = 0.972), although study-level clustering remained high (intraclass correlation coefficient = 0.965), indicating substantial methodological influence on reported estimates. Sampling effort was also strongly unbalanced: soil organic carbon was the most frequently reported pool (n = 581), whereas belowground biomass remained severely underrepresented (n = 45). These findings provide quantitative baselines for Brazilian macro-ecosystems and support the adoption of standardized, multi-pool, depth-explicit protocols to improve monitoring, reporting, and verification of carbon stocks. Methodological heterogeneity remains a dominant source of variation in tropical carbon‑stock estimates, reinforcing the need for consistent approaches to support credible Nature‑based Climate Solutions (NbCS), robust monitoring, reporting and verification (MRV) systems, and carbon‑based mitigation strategies.