Carbon accumulation by native trees and soils in an urban park, Auckland

Carbon storage by trees and soil in urban areas is of increasing interest as a potential greenhouse gas mitigation measure. Our objectives were to (1) quantify carbon accumulation in above- and below-ground tree biomass, organic layer and mineral soil (0–5 cm) of a 27-year-old planted forest in Auckland and (2) compare the sequestration potential of urban trees with natural shrublands and forests in New Zealand. A mixed-species allometric equation for urban-grown native trees was developed on the basis of the tree biomass of 21 trees belonging to four species (Corynocarpus laevigatus, Kunzea ericoides, Pittosporum eugenioides, P. tenuifolium). Our allometric equation and a recently developed mixed-species equation for New Zealand native forest species produced similar results. A total of 45.9 Mg C ha^–1 was stored in above- and below-ground tree biomass in our Auckland park, equating to an average annual carbon sequestration rate of 1.7 Mg C ha^–1. This rate is within the range reported for New Zealand shrublands. Despite the adverse conditions posed by some urban environments (e.g. poor-quality soil, contamination), planted urban forests can sequester carbon at similar rates to natural vegetation in New Zealand.