Biomass and carbon estimation for three browsable native shrubs in New Zealand

Background: New Zealand’s native shrublands have declined markedly over the past 180 years, yet shrub species on hill-country farms provide multiple co-benefits, including slope stabilisation, biodiversity enhancement, carbon sequestration, and potential for supplementary fodder. Despite this potential, robust allometric models for estimating biomass and carbon stocks remain limited, particularly for native shrubs with forage value. Methods: Above-ground biomass (AGB) was estimated by destructively sampling 54 individual shrubs of three native shrub species (Coprosma robusta Raoul, Pittosporum crassifolium Banks et Sol. ex A.Cunn. and Melicytus ramiflorus J.R.Forst. & G.Forst.) across two contrasting sites in New Zealand (Limestone Downs and Massey University). Species-specific, site-specific, and generalized allometric models were developed for foliage, branch, and stem biomass using both stem diameter (D) and height (H) as predictors under ordinary least squares (OLS). AGB models were additionally evaluated using both OLS and seemingly unrelated regression (SUR). Carbon stocks were estimated from best-performing models and projected over a 10-year period using growth functions. Results: Species-specific models outperformed site-specific and generalized models. Bivariate models incorporating both D and H consistently produced stronger fits than univariate models. OLS performed comparably to, or better than, SUR under the available sample sizes. Site effects were evident, with Limestone Downs generally supporting higher biomass and CO₂ stocks than the Massey University site. Estimated stand-level mean annual increment (MAI) ranged from 0.8 to 16.5 t CO₂ ha⁻¹ yr⁻¹, with several species–site combinations exceeding the New Zealand ETS indigenous forest default value of 6.5 t CO₂ ha⁻¹ yr⁻¹. Conclusions: The developed allometric models provide a useful basis for estimating biomass and carbon stocks in New Zealand native shrubs, with direct relevance to emissions accounting and integration into steep hill-country farming systems.