Remote sensing-based estimation of global litterfall dynamics in forest ecosystems and its potential impact on soil respiration

Organic carbon flux entering the pedosphere through forest litterfall drives the spatiotemporal dynamics of soil respiration (RS[jls-end-space/]). Synthesis of 14,912 in-situ observations across 843 sites parameterized a remote sensing-driven statistical model to map global forest litterfall production, PFL[jls-end-space/], at 500 m resolution (2000–2022). Global annual average PFL reached 30.06 Pg of dry mass (95% CI: 28.91–31.22 Pg). Production density exhibited an average increase of 8.25 ± 1.37 × 10−3 t·ha−1·yr−2, with upward trends spanning 50.64% (95% CI: 49.20%–52.15%) of global forest areas. Statistically significant rises occurred across 13.87% (95% CI: 12.50%–15.10%) of these domains, predominantly within tropical evergreen broadleaf and boreal needleleaf forests. Temperature functioned as the primary driver of global PFL variability, while localized environmental factors constrained regional dynamics. Causal decoupling via asymmetric residual analysis quantified the standardized sensitivity slope of RS to PFL at 0.016 (95% CI: 0.011–0.021). Implementation of Olson's first-order decay kinetics, modeling exponential substrate decomposition over time, revealed rapid tropical turnover contrasting with profound temperate biogeochemical inertia; this lag effect yielded a 24.62% explanatory gain at a one-year lag, persisting at 2.75% after four years. Global validation across 128 in-situ manipulation experiments demonstrated that asymmetric sensitivity index, defined as the ratio of respiratory log-responses to litterfall removal versus addition, shifted systematically from −0.151 in the tropics to −0.558 in temperate regions. This confirms a mechanistic transition from acute input-dependency to robust legacy-buffering along climatic gradients. Ultimately, these findings bridge fine-scale PFL−RS coupling gaps, providing critical physical constraints for global biogeochemical models.