230 K.R. Tate et al. Agriculture, Ecosystems and Environment 82 2000 229–246
system 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. © 2000 Elsevier Science B.V. All rights reserved.
Keywords: Net primary production; Soil respiration; Scaling; Remote sensing; Models
1. Introduction
A ‘missing sink’ of carbon C of about 1.8 Pg per year Schimel, 1995 has arisen from attempts to close
the global C budget by balancing atmospheric car- bon dioxide CO
2
concentrations against fossil fuel emissions, land-use change, and ocean C uptake Tans
et al., 1990. Although strong evidence now exists for the terrestrial biosphere being the most likely candi-
date Rayner et al., 1999, much uncertainty still sur- rounds the temporal variation and spatial distribution
of this sink. Spatial and temporal patterns of C in- put to, and release by, soils are especially uncertain
Torn et al., 1997. Possible explanations for this sink Lloyd, 1999 are forest regrowth, CO
2
fertilisation, and N deposition effects on terrestrial ecosystems.
Interest in national C budgets has increased recently for several reasons. First, new national greenhouse
gas reductions’ targets for Annexe One countries have been set under the Kyoto Protocol. If these targets are
ratified, credible national strategies for reporting and reducing CO
2
and other greenhouse gas emissions will need to be developed, using internationally recog-
nised methodologies subject to periodic auditing. Some countries have signalled their intention to use
terrestrial sinks to reduce their CO
2
emissions IGBP Terrestrial Carbon Working Group, 1998. Presently,
New Zealand relies heavily on C uptake and storage in exotic plantation forests mainly Pinus radiata
to offset annual CO
2
emissions of 7.5 Mt CO
2
-C from energy and industrial sources MfE, 1997. This
strategy will increase the need for more integrated, multi-disciplinary approaches involving C monitor-
ing at various scales, experimentation and modelling Cannadel and Mooney, 1999. Second, projections
of the likely effects of these emissions on climate are much less certain at regional and national scales,
where assessments of societal impacts of changing climate are more pertinent than at the global scale.
Third, strategies for achieving multiple environmental aims are likely to be more effective at the national
scale, including reducing greenhouse gas emissions and biodiversity loss, and protecting forests and pro-
ductive soils. Finally, current efforts by signatories to the Kyoto Protocol to make C emissions-reduction
targets binding may be a prelude to more compre- hensive full C-accounting IGBP Terrestrial Carbon
Working Group, 1998. This could be best achieved at the national scale, where databases, monitoring
networks and historical knowledge are likely to be well coordinated Tian et al., 1999.
Accordingly, we are using field and laboratory measurements, databases, satellite remote sensing and
models to assess New Zealand’s national C balance at various spatial scales. We tested the hypothesis
that New Zealand’s terrestrial ecosystems are in C balance, and that the substantial uptake of CO
2
by planted forests and aggrading scrublands is roughly
balanced by potential C losses from indigenous forests and soils, by comparing NPP and soil heterotrophic
respiration. The approach was first to establish a na- tional baseline of soil and vegetation C, against which
to estimate changes Tate et al., 1997. Second, as annual NPP is balanced by soil CO
2
emissions in ecosystems at steady state, each of these C fluxes was
estimated nationally for the major vegetation types: indigenous and exotic forests; scrub; and grasslands
improved, unimproved, and tussock. Collectively, these ecosystems cover about 90 of New Zealand’s
land area Tate et al., 1997. Third, national-scale C balance estimates were compared with plot-based es-
timates of the C balance for indigenous forests, exotic forests, and scrub. Fourth, the potential impact of soil
erosion on national C balance estimates was exam- ined at different scales. Finally, information gaps, and
major sources of uncertainty are discussed.
2. Methods
