Effects of differences in aboveground dead organic matter types on the stand-scale necromass and CO₂ efflux estimates in a subtropical forest in Okinawa Island, Japan

Dead organic matter (DOM), which consists of leaf litter, fine woody debris (FWD; <3 cm diameter), downed coarse woody debris (CWD_log), and standing or suspended coarse woody debris (CWD_snag), contributes to forest carbon cycling; however, few studies have considered effects of differences in DOM types on the stand-scale estimates of carbon stocks of necromass and CO₂ efflux (R_stand). This study investigated characteristics of necromass and R_stand in a subtropical forest in Okinawa Island, Japan, to quantify the effect of DOM type on total necromass, total R_stand, and estimate error of total necromass and R_stand. The CWD_snag accounted for the highest proportion (54%) of total necromass (1499.7 g C m⁻²), followed by CWD_log (24%), FWD (11%), and leaf litter (11%). Leaf litter accounted for the highest proportion (37%) of total R_stand (340.6 g C m⁻² year⁻¹), followed by CWD_snag (25%), CWD_log (20%), and FWD (17%). The CWD_snag was distributed locally with 173% coefficient of variation for necromass, which was twofold higher than those of leaf litter and FWD (72%–73%). The potential error in CWD accounted for 99% and 79% estimate error for total necromass and R_stand, respectively. For the estimates of CWD_snag and CWD_log necromass with the estimates error of <10% in this study area, sampling areas of ≥28,750 m² and ≥2058–42,875 m², respectively, were required. Our results showed that CWD considerably contributed to stand-scale carbon stocks and efflux among aboveground DOM in this forest, resulting in a major source of errors in the stand-scale estimates.