242 K.R. Tate et al. Agriculture, Ecosystems and Environment 82 2000 229–246
3.4.3. Carbon storage in native forests Native forests in New Zealand occupy about 6.5
million hectares Newsome, 1987 and contain about 940 Mt C in live and dead biomass Hall et al., 1998,
making them the largest vegetation C reservoir na- tionally. Although there is currently little harvesting
of native timber, other disturbances besides natural mortality may be influencing the stability of these
forests. Over 70 million possums Trichosurus vulpec- ula
Seitzer and Gatseiger, 1992 are a major pest Rose et al., 1992, consuming ca. 21 000 t of vegeta-
tion each day Seitzer and Gatseiger, 1992. Most of this browsing is targeted towards a few species and,
where possums are common, major changes in for- est structure, composition, and biomass have occurred
Rogers, 1995.
Based on plot re-measurement data in the 1970s, some forest types in South Island appear to be losing
C from the live vegetation pool at a rate of up to 12 t C ha
− 1
per year Hall and Hollinger, 1997. Over the entire South Island, however, forest biomass ap-
pears to be constant or increasing slightly 0.30 Mt C per year, based on current estimates of biomass
change Hall and Hollinger, 1997. The data for South Island showed that the largest decreases in forest
biomass occurred in forest classes that are more com- mon on North Island. If the biomass changes for each
forest class are extrapolated to North Island, estimates suggest these North Island forests could be losing
0.7 Mt C per year. These losses could be higher, how- ever, as suggested by the national CBAL Eq. 2
estimate of −8 Mt C per year Table 4. North Island forests have suffered greater impacts from herbivores
than South Island forests Rogers, 1995. Changes in forest composition and structure will also alter
annual rates of C uptake, and could create stands of high productivity following major disturbance. Over-
all, significant uncertainty still exists in our estimates of the contribution of native forests to the national
C budget, but ongoing work in forest inventory and productivity modelling will refine these estimates.
4. Soil erosion
Soil erosion has been identified as one of the most pervasive forms of anthropogenically induced degra-
dation of terrestrial ecosystems Valentin, 1996, and could have a major impact on national C budgets
Eq. 1. New Zealand is tectonically active, has a young landscape, and is located in a climatic regime
that is periodically disturbed by high intensity rain storms. The soft-rock hill country that accounts for
more than 40 of the land area was largely defor- ested by Maori and European settlers. This deforested
area is susceptible to landsliding and accelerated ero- sion, and has greatly increased the supply of sediment
to stream channels and floodplains Trustrum et al., 1990. Though most of New Zealand catchments
are small typically a few 1000 km
2
in area, they discharge large amounts of sediment to the ocean
284 Mt per year from South Island and 103 Mt per year from North Island rivers; Glasby, 1991.
If landsliding to 0.85 m depth, with an average C concentration over this depth of 1.89 was the
main sediment source, then national C loss from soil erosion would be ca. 3 Mt C per year. However,
catchment-scale research has demonstrated the need to consider additional erosion processes when quan-
tifying robust catchment-scale sediment-C budgets Page et al., 1994a,b; Trustrum et al., 1998, 1999.
For example, in the Waipaoa River basin historical land-use change has led to enhanced runoff and soil
erosion, and the generation of ca. 15 × 10
6
t per year of sediment over 80 years Hicks et al., 2000. Al-
though gully erosion generated most of the sediment, it accounted for only about 2 of C losses; shallow
landslides accounted for about 25 of the C loss. Sheet erosion, while contributing only about 10 of
the catchment sediment yield, accounted for about 50 of C loss. The remaining 20 of C loss is gen-
erated by earthflows, tunnel gullies, bank erosion and vegetation inputs. If landsliding was assumed to be
the main sediment source, total C loss would be only about 50 of the C that has accumulated in the flood-
plain and on the continental shelf over 80 years. By attributing C losses to the different erosion processes,
we can account for about 80 of whole-catchment C losses. Clearly, national estimates of C losses from
erosion need to include erosion processes other than just landsliding.
Scaling these results up to the national-scale re- quires knowledge of the relative importance of dif-
ferent erosion processes in different catchments. The Waipaoa catchment is dominated by gully erosion, but
nationally many catchments are likely to be dominated
K.R. Tate et al. Agriculture, Ecosystems and Environment 82 2000 229–246 243
by landslide erosion. In these catchments, relative C losses by landslides and sheet erosion are likely to be
more balanced than in the Waipaoa catchment. For example, in the Lake Tutira catchment, landsliding
1.89 C generated about 70–80 of the sediment supplied to the lake; sheet erosion 5.25 C con-
tributed the balance. These results suggest that land- slides accounted for about 50 of the C losses in this
catchment Trustrum et al., 1998, 1999.
If landsliding and sheet erosion are assumed to each contribute about 40 of the national sediment yields
387 ± 130 Mt; Glasby, 1991, and the remaining 20 are related to other erosion processes, then annual C
losses by erosion of ca. 11 Mt C for New Zealand are possible; only net C losses from land to sea are con-
sidered in this calculation. This C loss by erosion is probably an overestimate, however, as it assumes that
the land surface had largely not been disturbed, so that the C concentration of the sediment from sheet erosion
was 5.25. Nevertheless, the importance of sheet ero- sion is substantiated by Lambert et al.’s 2000 finding
that C inputs to soil under hill country pastures are in- sufficient to sustain current soil C levels. Whether the
transfer of C from terrestrial ecosystems to the ocean represents a major C loss depends on the fate of the
sediment material once it has reached the marine en- vironment.
5. National C budget
