Using airborne LiDAR to determine total sapwood area for estimating stand transpiration in plantations

This study offers an unprecedented opportunity to estimate total sapwood area over an entire catchment (A_scat/A_g) using small-footprint light detection and ranging technology with a minimal amount of labour in field. Forty-two-year-old plantations of Japanese cypress (Hinoki; Chamaecyparis obtusa Sieb. et Zucc.) and Japanese cedar (Sugi; Cryptomeria japonica D. Don) vegetated the 2.98ha experimental catchment. Field observations identified diameter at breast height (DBH) of all trees and produced the relationship between DBH and tree sapwood area (A_stre). The sum of A_stre generated actual values of A_scat/A_g. For light detection and ranging data analyses, local maximum filtering revealed height of tree apices (H) and tree number (N) with 9% omission errors. A novel process was developed to identify tree species by their apices based on height of the apices and canopy roughness. Four methods were tested. In Methods A-C, H was converted to A_stre directly or via DBH, then, the sum of A_stre created A_scat/A_g. H-A_stre or H-DBH relationships were varied irrespective of labour-intensive measurements, and A_scat/A_g was underestimated up to 85% of actual value because of the smaller N. On the other hand, in Method D, ready-made stand density management diagrams (SDMDs) overestimated mean DBH. However, a product of overestimated mean A_stre and the underestimated N was almost identical to the actual A_scat/A_g. The estimates were 84% and 95% of the true A_scat/A_g in Hinoki and Sugi, respectively, and the former will be more precise if the SDMD is suitable for the site as indicated through sensitivity analysis.