Papua New Guinea (PNG) has become the focus of climate change mitigation initiatives such as reducing emissions from deforestation and forest degradation, but defensible estimates of forest carbon are lacking. Here we present a methodology for estimating aboveground forest carbon, and apply it to a large Permanent Sample Plot system maintained by Papua New Guinea Forest Research Institute. We report the first estimates of forest carbon in lowland tropical forest in PNG. Average aboveground carbon in stems 4 10 cm diam. for 115 selectively harvested 1-ha plots in lowland tropical forest was 66.3 AE 3.5 Mg C/ha (95% CI) while for 10 primary forest plots the average was 106.3 AE 16.2 Mg C/ha. We applied ratios based on field observations, in-country studies, and the literature to estimate unmeasured pools of aboveground carbon (stems o 10 cm diam., fine litter and coarse woody debris). Total aboveground carbon was estimated at 90.2 and 120.8 Mg C/ha in selectively harvested and primary lowland forest, respectively. Our estimate for primary tropical forest is lower than biome averages for tropical equatorial forest, and we hypothesize that frequent disturbances from fire, frost, landslides, and agriculture are limiting carbon stock development. The methodology and estimates presented here will assist the PNG government in its preparedness for mitigation initiatives, are of interest to communities that are seeking to participate in voluntary carbon markets, and will encourage transparency and consistency in the estimation of forest carbon.

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Preliminary assessment of carbon in forest soils of Papua New Guinea (PNG) was done for 53 sites. Simple soil-landscape model was constructed to explain how soil carbon relates to landscape position and landform processes. Despite limitations of the soil survey, sufficient data have been obtained to indicate that the amount of carbon in forest soils surveyed so far is related primarily to their carbon-complexing capacity (CCC) which, in turn, depends on their geological parent material and parent rock. Soils were grouped into six categories based on their carbon content. The six categories ranged from very low 0–50 t/ha to extremely high 601–1,000 t/ha. Amalgamating the areas of mapped geological units on the basis of their likely CCC and their median carbon content has enabled the total carbon content of all forest soils in PNG to be estimated as 7,727 × 106 t. This figure is approximate and will be modified as more results are obtained and more sophisticated GIS-based landscape analysis is undertaken. The total carbon in forest biomass, as measured in this National Inventory and estimated for roots, is 4,006 × 106 t. Thus, the total carbon in PNG’s forests at present is estimated to be 11,733 × 106 t, with 66% stored in the soil and 34% in the biomass. The high proportion of ecosystem carbon held in the soil emphasises the importance of implementing soil conservation measures to maintain the total carbon resource in PNG’s forests.