Macroclimatic Drivers and Multi-Pool Carbon Allocation Across Global Forests: A Bitemporal (1990–2025) Comparison Using FAO Global Forest Resources Assessment Data

Forest ecosystems store atmospheric CO2 across five IPCC-defined carbon reservoirs: aboveground biomass (CAGB), belowground biomass (CBGB), coarse dead wood (CDW), litter carbon (CLIT), and soil organic carbon (SOC). Although these pools are dynamic and influenced by macroclimatic conditions, global assessments often examine them separately, leaving their joint climatic sensitivity under-quantified. We evaluated the distribution and interrelationships of the five forest carbon pools, assessed their associations with macroclimatic drivers, and tested whether the climatic sensitivity of SOC differed between 1990 and 2025. We analyzed standardized national inventory data from the FAO Global Forest Resources Assessment 2025 for 53 countries (106 country-year observations) together with ERA5 mean annual temperature and precipitation. Kruskal–Wallis tests followed by Dunn’s post hoc comparisons evaluated regional differences, Spearman rank correlations assessed bivariate relationships, and generalized additive models (GAMs) quantified the unique and shared contributions of precipitation and litter carbon to SOC. Standardized climatic effect sizes (β) on SOC were compared between 1990 and 2025 using a paired-difference bootstrap test (Δβ). SOC density differed significantly among regions (p = 0.0058), with Europe showing the highest values, whereas CAGB did not differ significantly (p = 0.52). CAGB and CBGB were strongly correlated (p < 0.001), while SOC showed only weak bivariate associations with living biomass pools. In the multivariate analysis, precipitation and CLIT together explained 35.9% of the spatial variation in SOC, but their contributions were almost entirely independent (unique precipitation, 17.4%; unique CLIT, 18.2%; shared fraction, 0.3%). This near-zero shared fraction indicates that precipitation and litter carbon contributed largely distinct information to national-scale SOC variation rather than acting through a strongly coupled pathway. Paired-difference testing indicated no detectable change in either temperature or precipitation effect sizes on SOC between 1990 and 2025 because the 95% confidence intervals for Δβ overlapped zero. Forest carbon pools are governed by pool-specific and only partly overlapping climatic and biological controls rather than by a single common driver. At the national scale, precipitation and litter carbon jointly contribute to variation in SOC density, while the estimated climatic sensitivity of SOC remained statistically stable between 1990 and 2025. These findings support multi-pool, climate-explicit approaches to long-term forest carbon monitoring.