Towards a more accurate assessment of the vegetation carbon stocks in New Zealand grasslands

Abstract

Carbon (C) markets are well established for aboveground woody vegetation domestically, and for soil organic C overseas. By reviewing published literature, this project aims to provide a scientifically defensible framework and dataset for estimating the vegetation C stocks in New Zealand grassland ecosystems. New Zealand currently uses IPCC Tier 1 values for biomass and root-to-shoot ratios derived from global vegetation types that do not align well with our intensively managed grassland systems. Shifting the High and Low producing grassland types used in the Land Cover Database to a Tier 2 approach requires inclusion of New Zealand-specific values of aboveground and belowground biomass. From the literature search of field measurements, the overall mean root-to-shoot ratio was 1.35 ± 1.86 and 3.64 ± 2.91 for High and Low producing grassland, respectively. These differ substantially from those root-o- shoot ratios (4.0 and 2.8) currently used in National Inventory calculations. There is good reason to challenge the Tier 1 assumptions, as a higher proportion
of the C fixed by the plant is allocated belowground in environments where nutrients are scarce, such as in Low producing grassland, compared with High producing grassland. The review also identified an overall lower C concentration in plant tissue than that used in the inventory. This might be contributing to an overestimation of vegetation C stocks in grassland. We suggest the replacement of Tier 1 with Tier 2 values. Doing so provides a more accurate assessment of the
C stocks in New Zealand grassland for the purpose of inventory reporting.