Moso bamboo expansion impacts on root respiration of Japanese cedar forest and secondary broad-leaved forest in a socio-ecological production landscape

Soil respiration and its root-derived component are important pathways for carbon release from forest ecosystems, but they remain poorly understood in expanding bamboo forests. We quantified root respiration and its drivers in an abandoned Moso bamboo forest (BF), a Japanese cedar forest (CF), and a mixed evergreen and deciduous broad-leaved forest (DF) in southwestern Japan during eight campaigns from April 2024 to March 2025. Using soil coring, we measured respiration from extracted living roots ( R a ) together with fine- and coarse-root biomass. Nonlinear mixed-effects models describing R a responses to soil temperature ( T s ) and soil moisture ( θ ) were constructed. Mean R a was highest in BF (0.285 g CO 2 m −2 h −1 ), followed by DF (0.255 g CO 2 m −2 h −1 ) and CF (0.057 g CO 2 m −2 h −1 ). Fine-root temperature sensitivity and mass-specific respiration were lower in BF, indicating that physiological activity alone did not explain the elevated R a . Instead, R a was strongly influenced by fine-root biomass, which was approximately three times greater in BF than in DF and fifteen times greater than in CF. As a result, biomass-driven enhancement increased R a in BF relative to CF, whereas lower mass-specific respiration in BF offset its biomass advantage over DF. Shapley-based contribution analysis supported these interpretations by revealing that fine-root biomass was the dominant positive driver of differences in R a , whereas physiological factors partly offset the biomass effect. These local comparisons highlight that root respiration is shaped by combined biomass and physiological effects, whose relative importance differs among adjacent forest stands.