A multi-scale analysis of a terrestrial carbon budget: Is New Zealand a source or sink of carbon?

Abstract

Interest in national carbon (C) budgets has increased following the signing of the Kyoto Protocol as countries begin to develop source/sink C inventories. In this study, specific-site measurements, regional databases, satellite observations, and models were used to test the hypothesis that New Zealand’s terrestrial ecosystems are C neutral because C uptake by planted forests and scrub is roughly balanced by C losses from indigenous forests and soils. Net ecosystem C balance was estimated from the difference between net primary production (NPP) and heterotrophic soil respiration. The productivity portion of the CASA model and NOAA–AVHRR imagery were used to estimate national NPP ( 128±14 Mt C per year). Main sources of uncertainty were the coarse spatial scale ( 1×1 km 2 grid cells), and the general lack of information on photosynthetically active radiation, light-use efficiency, and below-ground C allocation for the major vegetation types: indigenous and exotic forests, scrub, and grasslands (improved, unimproved and tussock). Total soil CO 2-C production predicted from an Arrhenius-type function coupled to climate and land-cover data was 380±30 Mt C per year, suggesting that New Zealand’s terrestrial ecosystems may be either (a) a net source of atmospheric CO 2 or (b) roughly in C balance if ca. 252 Mt CO 2-C per year (66%) can be attributed to roots. Soil moisture limitations on respiration were small, reducing the national value to 365±28 Mt C per year. Differences between NPP and heterotrophic soil respiration were −29 Mt C per year for improved pastures, −8 Mt C per year for indigenous forests, and +4 Mt C per year for planted forests; the large negative value for improved grasslands may be due to under-estimation of NPP and root respiration. Soil C losses to coastal waters, as estimated from a consideration of all the major erosion processes, were ca. 3–11 Mt C per year. These national-scale estimates of ecosystem C balance were in general agreement with those based on plot-scale data for some major ecosystems including planted forests (4 Mt C per year vs 3.7 Mt C per year, respectively) and indigenous forest (−8 Mt C per year vs ca. −2.8 Mt C per year, respectively). Poor agreement for forest regenerating after land abandonment (−17 Mt C per year vs +3 Mt C per year) was probably due to an underestimate of NPP at the national scale. Overall, the results suggest that New Zealand is a net C source, despite the fact that some ecosystems are accumulating C. For some land-use types, using the balance between NPP and soil respiration at the national scale to estimate the net ecosystem C balance may be too coarse, and studies of land-use changes at finer spatial scales are needed to reduce uncertainties in national-scale C balance estimates.