Estimating carbon stock in lowland Papua New Guinean forest – low density of large trees results in lower than global average carbon stock

Papua New Guinean forests (PNG), sequestering up to 3% of global forest carbon, are a focus of climate change mitigation initiatives, yet few field-based studies have quantified forest biomass and carbon for lowland PNG forest. We provide an estimate for the 10 770 ha Wanang Conservation Area (WCA) to investigate the effect of calculation methodology and choice of allometric equation on estimates of above-ground live biomass (AGLB) and carbon. We estimated AGLB and carbon from 43 nested plots at the WCA. Our biomass estimate of 292.2 Mg AGLB ha−1 (95% CI 233.4–350.6) and carbon at 137.3 Mg C ha−1 (95% CI 109.8–164.8) is higher than most estimates for PNG but lower than mean global estimates for tropical forest. Calculation method and choice of allometric model do not significantly influence mean biomass estimates; however, the most recently calibrated allometric equation generates estimates 13% higher for lower 95% confidence intervals of mean biomass than previous allometric models – a value often used as a conservative estimate of biomass. Although large trees at WCA (>70 cm diameter at breast height) accounted for 1/5 total biomass, their density was lower than that seen in SE Asian and Australia forests. Lower density of large trees accounts for lower AGLB than in neighbouring forests – as large trees contribute disproportionately to forest biomass. Reduced frequency of larger trees at WCA is explained by the lack of diversity of large dipterocarp species common to neighbouring SE Asian forests and, potentially, higher rates of local disturbance dynamics. PNG is susceptible to the El Nino Southern Oscillation (ENSO) extreme drought events to which large trees are particularly sensitive and, with still over 20% carbon in large trees, differential mortality under increasing ENSO drought stress raises the risk of PNG forest switching from carbon sink to source with reduced long-term carbon storage capacity.