Allometric scaling of above and below ground biomass of the critically endangered agarwood (Aquilaria malaccensis Lam.) in homegardens

Carbon sequestration through tree-based systems has been well recognized due to its potential to mitigate climate change. The lack of site-specific and species-specific biomass estimation models is a challenge to accurately estimating forest biomass at local and regional scales. Agarwood (Aquilaria malaccensis) trees form an essential component of agroforestry systems in Assam and Tripura state of India. In this study, trees with stem girth range of 10–80 cm and stand age of < 10 to > 20 years old in smallholder agarwood stands classes were destructively harvested to develop biomass estimation models. Different allometric relationships were compared using either diameter alone or a compound variable, including diameter and height, to predict above ground biomass (AGB), below ground biomass (BGB) and total biomass (TB). Based on the model fit criteria (R², RMSE, AIC and BIC), the best models for estimating above ground, below ground and total biomass of the agarwood trees were lnAGB =  − 3.13 + (0.89 × lnD²H), lnBGB =  − 2.40 + (1.49 × lnD) and lnTB = 0.29 + (0.96 × lnAGB), respectively. Stand biomass in the study area showed a significant increase from 6.92 Mg ha⁻¹ in < 10 years old stands to 65.90 Mg ha⁻¹in > 20-year-old stands, and is proportionally related with the increase in tree density of the stands ranging from 930–4470 stems ha⁻¹. The continuous harvesting of various-sized trees and the re-plantation and regeneration of agarwood trees in the study area affected the increase in stand-level biomass and the distribution of biomass carbon across the size classes. The proper implementation of the Assam Agarwood Promotion Policy 2020 and similar policies of Tripura need to be supported to promote the cultivation and conservation of the species across the region.