GROWTH, BIOMASS, CARBON STOCK AND ECONOMIC EFFICIENCY OF AN 11.5-YEAR-OLD ACACIA HYBRID PLANTATION IN QUANG TRI PROVINCE, VIETNAM

Abstract

This study assessed the growth, aboveground biomass, carbon stock and economic efficiency of an 11.5-year-old acacia hybrid (Acacia mangium × A. auriculiformis) plantation in Quang Tri province, Vietnam. Data were collected from nine 500 m² sample plots, 27 sample trees felled for biomass and stem analysis, and 45 quadrats of 4 m². Current stand density was 907 ± 66 trees ha⁻¹, mean diameter at breast height (DBH) 17.50 ± 0.50 cm, mean total height 21.19 ± 0.33 m and standing volume 224.98 ± 6.24 m³ ha⁻¹, corresponding to a mean annual increment of standing volume of 19.56 ± 0.54 m³ ha⁻¹ yr⁻¹. DBH was more variable than height; 73.0% of trees reached DBH ≥ 15 cm but only 25.9% reached ≥ 20 cm. Total aboveground dry biomass was 176.1 ± 2.9 t ha⁻¹ (tree layer 88.5%, understorey vegetation 5.9%, litter 5.6%); at the individual-tree level, biomass was concentrated in stemwood (76.7%), branches (11.6%), bark (8.4%) and foliage (3.3%). Component dry biomass was closely related to DBH through the power function W = a × DBH^b (R² = 0.88 - 0.96; p < 0.001). Aboveground carbon stock was 82.8 ± 1.4 t C ha⁻¹ (roots and soil organic carbon excluded), equivalent to 303.8 ± 5.0 t CO₂ ha⁻¹ and to 26.4 ± 0.4 t CO₂ ha⁻¹ yr⁻¹ when averaged over stand age. At a 7% discount rate, grading the harvest by log small-end diameter class returned a net present value (NPV) of 106.2 million VND ha⁻¹, an equivalent annual value (EAV) of 13.7 million VND ha⁻¹ yr⁻¹, a land expectation value (LEV) of 196.4 million VND ha⁻¹, a benefit–cost ratio (BCR) of 2.70 and an internal rate of return (IRR) of 32.4%, exceeding the all-woodchip option by 32.3 million VND ha⁻¹ in NPV (43.6%). At age 11.5 years the stand still maintained good growth and positive financial performance, indicating potential for extending Acacia hybrid rotations towards sawlog production.