Total plant biomass, carbon stock, and species- and age-specific allometry for 12 of New Zealand’s early-colonising indigenous shrub and tree species to 5 years of age

Abstract

Background: Changes in biomass and carbon stocks of open-spaced plantings are important considerations in the restoration of biodiversity and ecological function of New Zealand’s degraded (non-forested) landscapes and streamside margins. Species- and age-specific allometric relationships of biomass and carbon stocks were developed for a diverse range of indigenous seral shrub and tree species during their juvenile growth period.

Methods: Whole plants of Coprosma robusta Raoul (karamu), Plagianthus regius (Poit.) Hochr. (lowland ribbonwood), Sophora tetraptera J.S.Muell. (eastern kōwhai), Pittosporum eugenoides (Cunn.) (lemonwood), Pittosporum tenuifolium (Gaertner) (kōhūhū), Hoheria populnea A.Cunn. (North Island lacebark), Myrsine australis (A.Rich) Allan (māpou), Pseudopanax arboreus (Murray) Philipson (fivefinger), Cordyline australis (G. Forst.) Endl. (cabbage tree), Knightia excelsa (R.Br.) (rewarewa), Leptospermum scoparium (J.R.Forst. & G.Forst (mānuka), and Coriaria arborea (Linds) (tree tutu) were planted as 2-year-old nursery-raised seedlings in a plot-based field trial. Plants of each species were destructively sampled annually over 5 consecutive years and species-specific allometric equations developed for foliage, branch, stem, total above-ground biomass (AGB), total below-ground biomass (BGB), and whole plant biomass to calculate carbon stocks.

Results: In each year and for each of the 12 test species, root-collar diameter over bark (RCD) gave the best fit for plant height, foliage, branch, stem, total AGB, total BGB, and whole plant biomass. At age 5 years, Coriaria arborea and Cordyline australis accumulated significantly more AGB (P = 0.05) than the remaining species, none of which were not significantly different from one another and each species had allocated more than 20% of their total biomass to BGB, with BGB:AGB ratios of between 0.24 and 0.44. Species-specific mean tree biomass values at this age ranged from 0.5 to 14.0 kg per plant. The planting of equal numbers of the seven best-performing species as 2-year-old seedlings could potentially amass a carbon stock of 13.1 t CO₂ 1000 stems ha^-1 within 3 years after establishment as open-spaced plantings.

Conclusions: RCD proved to be a reliable predictor of plant height, foliage biomass, branch biomass, stem biomass, total ABG, total BGB, and whole plant biomass. Optimisation of the carbon potential amassed by mixed plantings of indigenous seral plants will require the development of allometric equations for a wider range of species best suited to restoring ecological integrity to degraded landscapes and to improve the accuracy of existing biomass and carbon stock inventories.