Vegetation carbon in New Zealand wetland ecosystems

Wetlands are a critical, though vulnerable, global carbon store, but the carbon stored in vegetation has not been quantified at a national scale for New Zealand wetlands. We undertook a literature review to assess vegetation carbon density in wetlands and used meta-analysis to estimate means and uncertainty for both vegetation structural classes and wetland type. We then used a combination of derived vegetation carbon densities alongside spatial extrapolation to estimate the amount of carbon stored in wetland vegetation in New Zealand. Our area of interest was the “Wetland - Vegetated non forest” class mapped by the Land Use Map (LUM), which is the authoritative layer for New Zealand carbon accounting. Within our area of interest, we used weighted aggregated land cover classes signifying vegetation to calculate a grand mean density (C ha⁻¹) for New Zealand wetlands mapped by the LUM in 2008, 2012, and 2016, and total vegetation carbon stocks for each of those years. We found that among wetland vegetation structural classes, total above-ground carbon varied from a mean of 4.8 Mg C ha⁻¹ for sedgelands to 23.8 Mg C ha⁻¹ for tall mangroves. For wetland types, total above-ground carbon varied from a group mean of 3.6 Mg C ha⁻¹ for pakihi to 19.5 Mg C ha⁻¹ for mangroves. Estimates of below-ground biomass were uncommon (n = ten estimates from four studies) but the limited data available suggest that below-ground carbon density was less than total above-ground carbon density for three herbaceous structural classes (below-ground divided by above-ground ranged from 0.70 for reedland to 0.86 for rushland), while for mangroves below-ground density varied: below-ground divided by total above-ground was 0.99 for tall mangroves and 2.09 for dwarf mangroves. We estimated a total carbon stock of 5 868 710 (1 467 178–10 270 243) Mg C for our area of interest, based on a density of 27.080 (6.68–47.48) Mg C ha⁻¹, and an area of 216 717 ha in 2016, the most recent estimate available. Using selected literature, we confirmed our estimates were broadly consistent with international values. There is a clear information gap regarding carbon densities in forested wetlands in New Zealand, and little ability to assess vegetation by wetland type due to the lack of spatial data around structural classes in New Zealand. We set out some recommendations to address this and other data gaps.