New allometric equations for quantifying tree biomass and carbon sequestration in seasonally dry secondary forest in northern Thailand

ABSTRACT: As tropical deforestation and forest degradation accelerate, carbon-credit trading could provide a financial incentive to preserve and regenerate forests. Since carbon trading relies on the accurate quantification of carbon stocks, allometric equations are urgently needed to derive above-ground dry biomass (AGB) from easily measured variables. Few allometric equations have been published for regenerating secondary forests in Southeast Asia. This study established new allometric equations, to determine AGB and carbon in regenerating secondary forests on shifting cultivation fallows in northern Thailand. To develop the equations, data were collected from 78 trees (136 felled individual stems, including coppices) ranging in size from 1 to 32.9 cm diameter at breast height (D). The dependent variable was AGB. The independent variables were D, tree height (H) and wood density (WD). Wood density varied significantly among species (p<0.05). Consequently, including WD in the function D2H´WD predicted AGB most accurately, compared with other functions that excluded it. Mean carbon concentration in stems, branches, and leaves was 44.84%±1.63 of dry mass, but it varied significantly among tree species (p<0.05). The new allometric equation revealed that tree ABG was highest in secondary forest, followed by the 7-year-fallow and the 4-year-fallow: 105.3, 38.3 and 10.3 Mg ha-1, respectively, while above-ground carbon was 47.7, 17.4, and 4.6 Mg C ha-1, respectively. Natural regeneration accumulated carbon slower than reported for active restoration, suggesting that managed restoration is preferable to passive regeneration on fallows in northern Thailand. The allometric equations, derived from this study, can be used to accurately determine tree ABG and carbon storage in regenerating secondary forest, with higher precision than has hitherto been possible, thus satisfying the monitoring requirements of REDD+ and other carbon-trading schemes.

Keywords Aboveground biomass, allometric model, secondary succession, tropical forest, wood density