Tier 2 above-ground biomass expansion functions for South African plantation forests

Abstract

Carbon stocks estimated for greenhouse gas emission reporting in South African plantation forests require improvement for local relevance. We developed biomass expansion and conversion factor (BECF) models to convert enumeration data to above-ground biomass (AGB) for major genera. Local biomass data and allometric models were aggregated across major taxa and scaled to utilisable stem and non-utilisable (bark, branch, foliar) biomass on 1 735 permanent sample plots. Models were developed for each genus to estimate BECF from utilisable volume to overcome bracket stepping in the Intergovernmental Panel on Climate Change (IPCC) lookup tables. To enable more accurate biomass estimates for growers who record tree volume and bone-dry wood density, an alternative calculation was developed that capitalises on stem-wood comprising 70% to 90% of AGB at rotation age. AGB is the sum of utilisable stem and non-utilisable components. Stem mass can be calculated as the product of volume and wood density. Models to estimate non-utilisable biomass from utilisable volume were developed for each genus. Local models estimated lower hardwood biomass and marginally lower softwood biomass than IPCC estimates. Use of local estimates compared to default values result in lower carbon stock estimates and reduced fluxes using the stock change method. Developing further allometric models and BECFs at the species or hybrid level will have little value due to rapidly changing genetic deployment and current work has indicated non-significant differences between genotypes within each genus. Further work should focus on improved tree volume and wood density estimates as well as on below-ground biomass estimates.