Spatial comparisons of carbon sequestration for redwood and radiata pine within New Zealand

Accurate spatial characterisation of plantation carbon will become increasingly important as countries look to offset anthropogenic emissions and growers seek fair payment for carbon. Although much research has characterised carbon in existing forests, estimates are often required for unplanted areas to gain insight into the suitability of greenfield sites for carbon forests. Ideally, these predictions should be made for multiple species as matching of species to site is critical for maximising carbon sequestration. Using a novel prediction method that leverages stand level carbon prediction models, and utilises previously developed geospatial productivity surfaces, our objectives were to (i) develop national surfaces for New Zealand of carbon sequestration for redwood (Sequoia sempervirens) and radiata pine (Pinus radiata) and (ii) compare carbon sequestration between these two species across a range of ages (30, 40 and 50 years) and stand densities (planting densities of 400, 650 and 900 stems ha⁻¹). Spatial predictions of volume productivity show that suitable areas for radiata pine covered, respectively, 98% and 71% of the total land area in the North and South Islands of New Zealand. The suitable range for redwood was more restricted, and a subset of the radiata pine range, with 87% and 28% of total land area suitable for redwood, respectively, within the North and South Islands. Within regions suited to both species (i.e. the redwood range), the mean carbon for redwood growing for 40 years at 650 stems ha⁻¹ (mid-range scenario), exceeded that of radiata pine by 23% (636 vs. 519 tonnes C ha⁻¹) and 35% (540 vs. 399 tonnes C ha⁻¹), respectively, within the North and South Islands. When compared across the larger range suitable for radiata pine, mean carbon for redwood, under the mid-range scenario, was 12% higher (569 vs. 508 tonnes C ha⁻¹) than radiata pine within the North Island but 49% lower (204 vs. 402 tonnes C ha⁻¹) within the South Island, as large areas of the South Island are too cold or dry for redwood, but still suitable for radiata pine. Averaged across all regions, within the area suitable for both species, percentage gains in carbon were markedly higher for redwood than radiata pine between ages of 30 and 50 years and between stand densities of 400 and 900 stems ha⁻¹. Our results highlight the potential of redwood for further afforestation as this species is capable of high and sustained rates of carbon sequestration on suitable sites within New Zealand.