Seasonal dynamic and influencing factors of coarse woody debris respiration in mid-subtropical evergreen broad-leaved forest
Carbon dioxide released from the respiration of coarse woody debris(RCWD) is an important component of carbon budget in forest ecosystem with moderate to large amount of CWD.Accurately estimating the fluxes of RCWD may thus be important for assessing the current and long-term C balance of forest ecosystems.Though CWD pool has been quantified in many forest ecosystems,direct measurement of RCWD is limited,especially in mid-subtropical evergreen broad-leaved forest.Thus,the importance of RCWD in most forest ecosystems is unknown.Accurate RCWD measurements are challenging because the decomposer communities may be highly sensitive to change in temperature and water content,and to natural or anthropogenic disturbances.In early studies,RCWD was measured by soda lime traps,in which the physical disturbance to CWD can be avoided during measurement process.However,the traps are well known to underestimate respiration rates and measure efflux only at the surface though decomposability of CWD varies over the cross-section of a log.The infrared gas analysis(IRGA) method provides a precise measurement on respiration from the entire cross-section of the CWD,but may involve physical disturbances such as removing it from its environment and cutting a sample.In this study,the IRGA method with LI-8100 automated soil CO2 flux system was used to measure RCWD and its seasonal dynamic.The objectives were to determine how environmental factors(mainly substrate temperature and water content) and decay class influence rate of RCWD,which may provide valueable information for greenhouse gas inventories of CWD and construction of forest carbon cycle models.The results show that: substrate temperature is commonly the most important environmental factor influencing RCWD,but substrate water content(WCWD) interacted with substrate temperature(TCWD) on RCWD across a broad WCWD gradient(from 13.65% to 153.86%).RCWD generally increased with increase in substrate temperature and water content until to a certain level,and then tended to decline.Differences in RCWD among decay classes were due to variations in substrate water content and the sensitivity of RCWD to environmental conditions.Rates of RCWD of all decay classes showed distinct seasonal variation with a single peak,with the maximum rate of 9.69 μmol CO2·m-2·s-1 in August and the minimum rates of 0.60 μmol CO2·m-2·s-1 in February.There were significant differences in RCWD rates among different decay classes.The comparisons indicated that the RCWD for ClassⅠ was significantly lower than that for Class Ⅲ and Class Ⅳ(P0.05),and the RCWD for Class Ⅲ and Ⅳ did not differ.Rates of RCWD were significantly positively correlated with substrate temperature(P0.01),which can explain 70.2%—85.6% of seasonal variations in RCWD.The correlation between RCWD and substrate water content was not significant(P0.05).The sensitivity of RCWD to seasonal substrate temperature,evaluated as Q10,ranged from 2.46 to 2.83 and increased with decay classes.