Decay rates of above- and below-ground coarse woody debris of common tree species in New Zealand’s natural forest

Abstract Decomposition rates of above-ground coarse woody debris (CWD) (>10 cm diameter) were determined for nine common New Zealand indigenous tree species, and of below-ground CWD for two species. CWD samples were obtained from fallen trees originating from known windfall events which had been decaying for 10–29 years on the forest floor. Species-specific decay rates were estimated from the density (dry mass divided by original volume) of CWD samples relative to the density of samples from live and freshly fallen trees. The estimated half-life ( t 1/2 ) or time to lose 50% mass for above-ground CWD across all species was 25.7 years but varied between species and increased with increasing stem diameter. Adjusted to a common diameter at breast height of 60 cm and mean annual temperature of 10 °C, Prumnopitys taxifolia and Fuscospora cliffortioides were the slowest to decay with a t 1/2 of about 40 years, Fuscospora fusca , Weinmannia racemosa , Beilschmiedia tawa , Lophozonia menziesii and Prumnopitys ferruginea the next slowest with a t 1/2 of 27–34 years, while the remaining species ( Dacrydium cupressinum and Dacrycarpus dacrydioides ) decaying more rapidly with a t 1/2 of less than 20 years. A model was derived for predicting the exponential decay constant for the above-ground CWD of individual stems as a function of diameter at breast height and mean annual temperature adjusted for species. This model can be used to predict above-ground CWD carbon stock changes in New Zealand’s natural forests following tree mortality. Below-ground CWD decay rates did not differ significantly from those of above-ground CWD for D. cupressinum , but were significantly faster for L. menziesii .