Respiration of downed logs in four natural evergreen broad-leaved forests in subtropical China

Background and aims Carbon (C) loss from coarse woody debris (CWD) may be important in forest ecosystem C budgets, yet few studies have assessed CWD respiration in natural evergreen broad-leaved forests in subtropical China. The objectives of this study were (1) to quantify the respiration rates of downed logs (R-log) and Q(10) of different tree species at various stages of decay, (2) to assess the effect of microclimatic (log temperature and moisture) variables on R-log and (3) to estimate annual C flux of the CWD logs in four natural evergreen broad-leaved forest types of Altingia gracilipes Hemsl. (ALG), Tsoongiodendron odorum Chun (TSO), Castanopsis carlesii (Hemsl.) Hayata (CAC) and Cinnamomum chekiangense Nakai (CIC) in southern China. Methods A dynamic chamber method was used to measure R-log in four decay classes (DCs) (ranging from freshly felled logs in DC 1 to highly decomposed logs in DC 4). The effects of changes in log temperature (T-log) and log moisture content (M-log) on R-log were determined and annual R-log estimated. Results The R-log exhibited a distinct seasonal pattern, and it was predominantly controlled by the log temperature. The temperature sensitivity of R-log to log temperature as indicated by Q(10) ranged from 1.82 to 2.86 and was variable among decay classes and forests. Significant relationships between moisture content of logs and R-log were only observed for logs in the ALG (P=0.001), TSO (P=0.002) and CIC (P<0.001). The log temperature in combination with the moisture of logs explained over 60 % of the variation in R-log. The effects of decay class (F=3.707, df=3, P=0.013) and tree species (F=8.705, df=3, P<0.001) on R-log were significant. Annual C fluxes ranged from 652 kg C ha(-1) year(-1) in the CIC to 1,243 kg C ha(-1) year(-1) in the TSO, with varying contributions from each decay class. Conclusions At these subtropical natural forests, R-log was a significant component of overall heterotrophic respiration (about 10-20 % on the annual basis). With increasing climate change-associated tree mortality in our mature forests, CWD respiration is expected to increase. The results indicate that species, decay stage and microclimatic conditions should be considered when predicting CWD decay rates.