Assessing Reduction Of Cluster Size To Estimate Wood Volume In An Amazonian Forest

While the Brazilian National Forest Inventory (NFI) is in progress, there is a growing demand to understand the effect of cluster size on the accuracy and precision of forest-attribute estimation. We aimed to find the minimum cluster size (in area) to estimate merchantable volume (MV) with the same accuracy and precision as the estimates derived from the original cluster of 8,000 m(2). We used data from an inventory carried out in a forest unit (Bom Futuro National Forest) in the southwestern Brazilian Amazon, where 22 clusters were distributed as a two-stage sampling design. Three products were evaluated: (i) MV of trees with a diameter at breast height (DBH) >= 20 cm (P1); (ii) MV of trees with DBH >= 50 cm (P2); and (iii) MV of commercial species with DBH >= 50 cm and stem quality 'level 1' or 'level 2' (P3). We assessed ten scenarios in which the cluster size was reduced from 8,000 m(2) to 800 m(2). The accuracy of P1, P2 and P3 was highly significantly lower for reductions < 2,400 m(2). The precision was more sensitive to variations in cluster size, especially for P2 and P3. Minimum cluster sizes were >= 2,400 m(2) to estimate P1, >= 4,800 m(2) to estimate P2, and >= 7,200 m(2) to estimate P3. We concluded that it is possible to reduce the cluster size without losing the accuracy and precision given by the original NFI cluster. A cluster of 2,400 m(2) provides estimates as accurate as the original cluster, regardless of the evaluated product.