Environmental controls on carbon dioxide flux from black spruce coarse woody debris

Carbon dioxide flux from coarse woody debris (CWD) is an important source of CO2 in forests with moderate to large amounts of CWD. A process-based understanding of environmental controls on CWD CO2 flux (RCWD) is needed to accurately model carbon exchange between forests and the atmosphere. The objectives of this study were to: (1) use a laboratory incubation factorial experiment to quantify the effect of temperature (TCWD), water content (WC), decay status, and their interactions on RCWD for black spruce [Picea mariana (Mill.) BSP] CWD; (2) measure and model spatial and temporal dynamics in TCWD for a boreal black spruce fire chronosequence; and (3) validate the RCWD model with field measurements, and quantify potential errors in estimating annual RCWD from this model on various time steps. The RCWD was positively correlated to TCWD (R2=0.37, P<0.001) and WC (R2=0.18, P<0.001), and an empirical RCWD polynomial model that included TCWD and WC interactions explained 74% of the observed variation of RCWD. The RCWD estimates from the RCWD model excellently matched the field measurements. Decay status of CWD significantly (P<0.001) affected RCWD. The temperature coefficient (Q10) averaged 2.5, but varied by 141% across the 5–42°C temperature range, illustrating the potential shortcomings of using a constant Q10. The CWD temperature was positively correlated to air temperature (R2=0.79, P<0.001), with a hysteresis effect that was correlated to CWD decay status and stand leaf area index . Ignoring this temperature hysteresis introduced errors of –1% to +32% in annual RCWD estimates. Increasing TCWD modeling time step from hourly to daily or monthly introduced a 5–11% underestimate in annual RCWD. The annual RCWD values in this study were more than two-fold greater than those in a previous study, illustrating the need to incorporate spatial and temporal responses of RCWD to temperature and water content into models for long-term RCWD estimation in boreal forest ecosystems.