Assessing reduction of cluster size to estimate wood volume in an Amazonian forest

Nivea Maria Mafra Rodrigues,Hassan Camil David,Vinícius Augusto Morais,Emanuel José Gomes Araújo,Gabriel William Dias Ferreira

doi:10.1590/1809-4392202003131

Abstract

ABSTRACT 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 m2. 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 m2 to 800 m2. The accuracy of P1, P2 and P3 was highly significantly lower for reductions < 2,400 m². The precision was more sensitive to variations in cluster size, especially for P2 and P3. Minimum cluster sizes were ≥ 2,400 m² to estimate P1, ≥ 4,800 m² to estimate P2, and ≥ 7,200 m² 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 m² provides estimates as accurate as the original cluster, regardless of the evaluated product.

Highlights

Forest inventories are the main tool to quantify and characterize forest resources and their composition, providing useful information to forest management, conservation, and policy (Tomppo et al 2010)
There is a growing demand to understand the relationship of cluster size with accuracy and precision of estimates, i.e., what is the effect of sampling unit size on the precision and accuracy of the estimates of merchantable volume? We aimed to estimate the minimum sampling unit size that provides merchantable volume estimates as accurate and precise as the standard National Forest Inventory (NFI) sampling unit of 8,000 m2 used in the Amazon
P3 in 800-m2 SUs had a negative lower limit of the CI, which was owed to the low density of trees of commercial value with diameter at breast height (DBH) ≥ 50 cm in forest stands, which made them vary scarce in the sampled areas of 800-m2 SUs (Figure 2; Table 1)

Summary

Introduction

Forest inventories are the main tool to quantify and characterize forest resources and their composition, providing useful information to forest management, conservation, and policy (Tomppo et al 2010). Based on the degree of coverage, the inventories can be classified either as complete, when the entire forest is measured (i.e., a census), or incomplete, when only a forest sample is evaluated (Loetsch et al 1973). In the latter case, a representative sample is crucial, which depends on distinct factors, such as the dimension of sampling units and the sampling design, which define the precision of estimates. Defining minimal sampling effort that ensures both high accuracy and precision in the estimation of variables of interest will increase inventory efficiency (Gregoire and Valentine, 2007; Westfall et al 2016)

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