Across the tropics there is growing financial investment in activities that aim to reduce emissions from deforestation and forest degradation, such as REDD+. However, most tropical countries lack on-the-ground capacity to conduct reliable and replicable assessments of forest carbon stocks, undermining their ability to secure long-term carbon finance for forest conservation programmes.

Clear guidance on how to reduce the monetary and time costs of field assessments of forest carbon can help tropical countries to overcome this capacity gap. The authors provide such guidance, based on assessments of four different carbon pools – above-ground, dead wood, litter and soil – in 224 study plots distributed across two regions of eastern Amazonia. The studies revealed 31-fold differences in estimated sampling costs between the most expensive carbon pool, soil, and the cheapest, litter. Large live stems (10 cm DBH), which represented only 15% of the overall sampling costs, were by far the most important component to be assessed, as they store the largest amount of carbon and are highly sensitive to disturbance. If large stems (10cm DBH) are not taxonomically identified, costs can be reduced by a further 51%, while incurring an error in aboveground carbon estimates of only 5% in primary forests, but 31% in secondary forests.

For rapid assessments, which are necessary for prioritizing locations for carbon-conservation activities, sampling of stems 20cm DBH without taxonomic identification can predict with confidence whether an area is relatively carbon-rich or carbon-poor—an approach that is 74% cheaper than sampling and identifying all the stems 10cm DBH. We use these results to evaluate the reliability of forest carbon stock estimates provided by the IPCC and FAO when applied to human-modified forests, and to highlight areas where cost savings in carbon stock assessments could be most easily made.

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