A Multi-Scale Analysis of a National Terrestrial Carbon Budget and the Effects of Land-Use Change

New Zealand is developing a national inventory of carbon (C) sinks and sources for reporting under the Kyoto Protocol and the United Nations Framework Convention on Climate Change. The first cycle of inventory is due for completion in 2005. To support this inventory, we estimated rates of net C change by scaling site-specific measurements and models of C accumulation to national levels using spatial datasets of landcover and climate. The resultant estimate of national aboveand below-ground annual net primary production (NPP) is 190 Tg C y–1. This is reasonably consistent with an independent estimate of 155 Tg C y–1 calculated from national soil C stocks and turnover times, under the assumption soil C is at or near steady state. National net ecosystem production (NEP) was calculated as the difference between NPP and soil heterotrophic respiration, with heterotrophic respiration given by the difference between total soil respiration and root autotrophic respiration. Total soil respiration was estimated using an Arrhenius-type rate equation, with scaling of site-specific measurements to national levels using spatial datasets of landcover and mean annual temperature. For each landcover class, soil autotrophic respiration was estimated from the fraction of NPP allocated to roots. This gives a very small loss of 1 Tg C y–1 for national terrestrial NEP, with emissions of 8.8 Tg C y–1 from national fossil fuel usage and cement production giving a total national net C balance of about –10 Tg C y–1. This is a significant change from the value calculated for the first estimate of the New Zealand net C balance, –55 Tg C y–1, due primarily to improved calculation of NPP for two major landcover classes: improved grasslands, and scrubland. However, the net C balance figure does not yet include any C losses to the atmosphere arising from the transport of c. 3 Tg C y–1 of eroded soil to the ocean, although such losses may not be large given the short source-to-ocean distances and high mean flow velocities of New Zealand’s rivers. Overall, the data presented suggest New Zealand is a small net C source. The estimates of NEP by landcover class also suggest an increase in the net terrestrial C balance due to Kyoto-relevant land use change in the decade since 1990 of about 1 Tg C, not quite sufficient to offset increases in emissions from increased fossil fuel use of about 2 Tg C. However, this situation is soon expected to change considerably, with large net gains in terrestrial C as exotic forests planted since 1990—Kyoto forests—begin reach peak C accumulation rates.