Carbon storage along a stand development sequence in a New Zealand Nothofagus forest

Abstract

Indigenous forests cover about 6.4×10 6 ha or 24% of the land area of New Zealand and thus form a major carbon (C) pool, but there are few estimates of C storage for these forests, especially at different stages of stand development. C storage and distribution in stemwood, coarse woody debris, forest (floor litter (L) + fermentation /humus(FH)-layers) and upper (0–0.1 m) mineral soil were measured along a stand development sequence in montane zone mountain beech ( Nothofagus solandri var. cliffortioides) forest in the Craigieburn Range, central South Island. C storage in all pools peaked in 125-year-old pole stands (219 Mg/ha) and declined as the stands matured (>150 years old, 192 Mg/ha), and were replaced by seedling stands (10 years old, 152 Mg/ha) following windthrow, to reach a low point (114 Mg/ha) in 25-year-old sapling stands. Coarse woody debris provided an important pool for C storage following catastrophic forest disturbance, and varied reciprocally with stemwood along the development sequence. Above-ground components showed a much greater range (88 Mg/ha) with stand development than forest floor and mineral soil components (16 Mg/ha). Stemwood and coarse woody debris components varied by 136 and 76 Mg/ha, respectively, while litter, the FH-layer and upper mineral soil C varied by 4, 17, and 12 Mg/ha, respectively. The sum of forest floor and mineral soil C ranged between 53 Mg/ha (sapling stage) and 69 Mg/ha (pole stage), and did not differ significantly with age. The litter and FH-layers, and mineral soil were re-sampled after a period of 8 years in the sapling and pole stands. C storage in the litter layers did not change significantly over this period. C storage in the FH-layer declined significantly by 9.7 Mg/ha in the pole stage, but showed little change in the sapling stage. It is hypothesised that C loss resulting from ongoing decomposition in the FH-layer was balanced by continuing inputs of coarse woody debris in the sapling stage, but not in the pole stage, where the coarse woody debris resource was relatively small. There was no change in mineral soil C storage over the 8-year period. It is concluded that, in the absence of disturbance, C storage in the forest floor may vary substantially over periods of <10 years, in contrast to the mineral soil where it appears relatively stable over decadal time intervals.