New Zealand uses a national forest inventory to assess carbon in live and dead pools in its natural forest. Coarse woody debris (CWD) carbon is estimated by multiplying volumes of dead stems and fallen pieces with wood and bark density, a density modifier based on a 5-level decay class scale, and the carbon fraction. Two cycles of the inventory have been completed with both providing CWD carbon estimates of approximately 20 t C ha-1 indicating the CWD pool is not changing over time. Independently, exponential decay constants have been determined for common tree species from measurements of fallen stems. However, predicted annual decay loses from the CWD pool based on these decay constants are substantially lower than inventory estimates of tree mortality. This discrepancy provides evidence that the inventory substantially underestimates the CWD pool. In the current study, decay constants were estimated using an alternative approach based on the proportion of matched pieces of CWD transferring across decay classes between inventory measurements. The objective of the study was to compare inventory estimates of CWD carbon stock changes with stock changes predicted using decay constants and measured mortality, and identify likely causes of discrepancies.
Results/Conclusions
Exponential decay constants for CWD in New Zealand’s natural forest were estimated from the proportion of matched pieces transferring across decay classes between inventory assessments, and the quantity of stocks in each decay class. The volume-weighted average decay constant was 0.0275, almost identical to an earlier estimate of 0.0267 based on measurements of fallen stems obtained independently of the inventory. Based on these decay constants, the level of CWD required to ensure that losses from decay are in balance with gains to the CWD pool from tree mortality, estimated from the inventory to be 0.82 t C ha-1 yr-1, are about 30 t C ha-1. This is substantially greater than the inventory estimate for the CWD pool of 20 t C ha-1. The analysis was repeated for plots classified into broad forest classes based on species composition, and similar results obtained for each class. It is likely that the underestimate of CWD by the inventory is due to multiple factors. In particular, the measurement protocols exclude wood that is heavily decayed, bark and sapwood that has sloughed off stems, and dead wood buried in the forest floor. and it is probable that the density modifiers over-state the loss in density of decayed wood.