Coarse woody debris decay rates for seven indigenous tree species in the central North Island of New Zealand

The decomposition rate of stem and branch coarse woody debris (CWD, >10 cm in diameter) was assessed in natural forests located in the central North Island of New Zealand. CWD samples had originated from windfalls associated with cyclone Bernie, and had been decaying for approximately 20 years on the forest floor. Species-specific decay rates were estimated from the density of CWD samples relative to the density of live tree samples from the same stands. Decay rates were determined for rimu (Dacrydium cupressinum), matai (Prumnopitys taxifolia), tawa (Beilschmiedia tawa), miro (Prumnopitys ferruginea) and kahikatea (Dacrycarpus dacrydioides) in podocarp forest at Whirinaki, and red (Nothofagus fusca) and silver beech (Nothofagus menziesii) in beech forest at Kaimanawa. The average decay rate for these seven species, expressed as the time taken to lose 50% mass (t_1/2), was 30 years. Larger trees (90 cm diameter at breast height, dbh) decayed more slowly (t_1/2 = 38 years) than smaller trees (30 cm dbh; t_1/2 = 21 years). After adjustment for dbh, P. taxifolia (t_1/2 = 39 years), N. fusca (t_1/2 = 38 years), N. menziesii (t_1/2 = 31 years) and B. tawa (t_1/2 = 26 years) decayed significantly more slowly than D. cupressinum (t_1/2 = 18 years). D. cupressinum decayed more slowly than P. ferruginea (t_1/2 = 16 years) and D. dacrydioides (t_1/2 = 14 years), although these differences were not statistically significant because the CWD sample size was small for the latter two species. An attempt to expand the range of species studied using data from in-ground durability tests was not successful as species decay rankings from these tests were inconsistent with natural forest CWD rankings. Stems heavily colonized by the common decay fungus Ganoderma cf. applanatum decayed more rapidly (t_1/2 = 20 years) than those which were occupied only by other decay fungi (t_1/2 = 40 years). A tree species and dbh-dependent decay constant, λ, was derived for estimating carbon loss from CWD due to fungal decay and insect activity in indigenous forests. Future research will aim to improve these decay equations by investigating the decomposition processes associated with tree species and basidiomycete populations present at other sites in New Zealand.