Development of Biomass Expansion Factors for Major Conifer Species and Conifer Forests of Bhutan

Dorji Wangdi; Kinley Dem; Dawa Zangpo

doi:10.63363/aijfr.2026.v07i05.7786

Development of Biomass Expansion Factors for Major Conifer Species and Conifer Forests of Bhutan

Author(s)	Mr. Dorji Wangdi, Ms. Kinley Dem, Mr. Dawa Zangpo
Country	Bhutan
Abstract	Accurate assessment of forest biomass and biomass-related carbon emissions is critical for climate change mitigation, forestry, and REDD+ accounting. An aggregate conifer Biomass Expansion Factor (BEF) of 1.292 was used in compiling Bhutan’s Forest Reference Emission Level (FREL) in 2020, but this factor has not been empirically validated. This study estimated species- and size-specific BEFs and Biomass Conversion and Expansion Factors (BCEFs) using destructive sampling data from 357 trees representing eight major conifer species in Bhutan. A Type II analysis of variance (ANOVA) was used to assess the effects of species and 10-cm diameter classes on BEF. The main analysis applied a basal-area-weighted linear model to account for the unequal representation of trees among sampling groups and the greater contribution of larger trees to forest biomass. The weighted model showed a significant effect of species on BEF (F₇,₂₉₄ = 4.22, p < 0.001), whereas diameter class was not significant (F₇,₂₉₄ = 1.44, p = 0.199). A significant species × diameter-class interaction was also detected (F₄₉,₂₉₄ = 1.57, p = 0.013), although this effect was not supported by the unweighted log-transformed sensitivity analysis (p = 0.251). The sensitivity analysis nevertheless detected a significant independent effect of diameter class (F₇,₂₉₄ = 12.64, p < 0.001). The aggregate weighted BEF was 1.292 ± 0.025, closely matching the factor used in Bhutan’s FREL. Species-specific BEFs ranged from 1.108 for Cupressus corneyana to 1.720 for Larix griffithii, while BCEFs ranged from 0.498 to 0.807 Mg m⁻³. Applying the derived BEFs and BCEFs to 2025 wood removals resulted in estimated emissions of 258,193.68 tCO₂, 25.72% lower than the corresponding FREL estimate, primarily due to differences in wood density. The results support the continued use of the aggregate BEF where species-specific wood-removal data are unavailable, while species-specific BEFs are recommended where such data are available.
Keywords	biomass expansion factor, conifer, biomass, wood density
Published In	Volume 7, Issue 5, September-October 2026
Published On	2026-09-25
DOI	https://doi.org/10.63363/aijfr.2026.v07i05.7786

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Development of Biomass Expansion Factors for Major Conifer Species and Conifer Forests of Bhutan

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