Strategies to estimate national forest carbon stocks from inventory data: the 1990 New Zealand baseline

An estimate of live tree carbon stored in New Zealand forests at 1990 was made to partially satisfy New Zealand's international obligations under the Framework Convention for Climate Change. A national database was compiled of 4956 forest inventory plots measured as recently as possible to 1990. Plot biomass estimates were obtained by applying species allometric relationships derived from harvested stands. Forest areas and classes were taken from a 1987 national map of vegetation cover. Regularly spaced grids, based on an initial 1 km x 1 km grid, were overlaid on the total forest area and plots were tested for bias against site characteristics at the grid points. As grid point density and sample size increased, bias was minimal in regional sampling intensity and in total annual precipitation. Differences in mean elevation and annual temperature remained stable as grid point density increased, and showed little correlation with stem biomass. This sampling method gave a measure of precision not available from previous estimates. An efficient sample size to estimate the mean within a 5% level of precision (at 95% probability) required a sample of 574 plots selected from a 4-km grid. This strategy generated a mean estimate for the 1990 New Zealand forest carbon biomass of 179.3 +/- 4.9 Mg ha(-1) (+/- SE), totalling 919.1 +/- 25.1 Mt for the 5.1 million ha mapped forest area. The mean was 6-10% lower than previous estimates, and was within the range reported for other countries. Within forest classes, mean carbon biomass ranged from 105 Mg ha(-1) in pure podocarp forest to 215 Mg ha-l in mixed lowland podocarp-broad-leaved-beech forest. Of the major taxa groups throughout the forest estate, beech (Nothofagus) contributed 60% of the national forest carbon biomass reservoir, 26.7% was in other hardwoods, 13.2% in conifers, and 0.1% in other taxa (e.g. tree ferns).