The effect of canopy structure on soil respiration in an old-growth beech-oak forest in central Japan

Soil respiration (R_s) is a key component in the estimation of the net ecosystem production (NEP) of old-growth forests, which are generally thought to have ceased carbon accumulation. The objectives of the present study were to characterize the spatial and temporal patterns of R_s, and to identify the determinants of the spatial and temporal variability of R_s, using general linear mixed models (GLMM), in an old-growth beech-oak forest. GLMM analyses identified monthly effect as a significant explanatory variable for temporal variation, as well as gap/canopy and soil water content (SWC) as explanatory variables for spatial variation, in R_s. The complexity of vertical structure in the forest was reflected in the spatial pattern of R_s, which was higher in canopy areas than in gap areas during the growing season, except in November. This spatial pattern was not affected by soil temperature. Moreover, SWC did not differ between gap and canopy areas, although SWC partially explained the spatial heterogeneity in R_s. The carbon:nitrogen ratios of soil organic matter in canopy areas were significantly higher than those in gap areas. Fine root biomass was 1.7-fold greater in canopy areas than in gap areas, likely because of the higher R_s in canopy areas, and root respiration made a much large contribution to R_s than heterotrophic respiration. The different patterns of fine root biomass between gap and canopy areas mainly control the spatial heterogeneity in R_s; thus, it is worth considering the gap/canopy variability in R_s when evaluating annual efflux in old-growth forests.