Results and discussion Directory UMM :Data Elmu:jurnal:A:Agriculture, Ecosystems and Environment:Vol82.Issue1-3.Dec2000:

234 K.R. Tate et al. Agriculture, Ecosystems and Environment 82 2000 229–246 proved grassland 0.70, tussock grassland 0.80. This assumption was based on data for fine root pro- duction and respiration in temperate forests Ryan et al., 1996; Keith et al., 1997, where R a was esti- mated to be roughly equivalent to below-ground NPP. In improved established grasslands, root biomass shows little temporal variation Saggar et al., 1997, suggesting root biomass is near steady state; under these conditions, a 1 : 1 distribution of total respi- ration between R a and R h also appears reasonable, based on the estimates of R a total soil respiration for grassland soils summarised in Hanson et al. 2000. 2.3. National soil erosion National estimates of C losses from soil erosion were based on regional sediment-yield data Glasby, 1991, initially assuming that landsliding 0.85 m depth was the key erosion process. Average soil C concentration was estimated at 1.89 average soil C concentration to 0.85 m depth for all pedons in the National Soils Database; McDonald et al., 1988. Potential scaling errors associated with estimating national soil C loss by erosion were then investigated by quantifying the contribution of sediment and C from different erosion processes shallow landslides soil slips, gully, and sheet erosion in the Waipaoa River basin 2200 km 2 and the Lake Tutira catch- ment 32 km 2 located in the east coast region of the North Island Page et al., 1994a,b; Trustrum et al., 1998, 1999. In the Waipaoa River basin, estimates of sediment production for the period 1920–2000 i.e. 80 years following land clearance by different erosion processes were derived from detailed analysis of individual catchment sediment budgets Trustrum et al., 1998, 1999. Soil C removed by each process was estimated using average soil C content for a range of disturbed and undisturbed, but deforested, soil profiles M. McLeod, pers. comm.. For shallow landsliding and gully erosion assumed to be confined to the upper 0.85 m of the soil profile, an average soil C content of 1.89 was assumed see above. For sheet erosion assumed to erode the upper 0.1 m of soil, a soil C value of 5.25 M. McLeod, pers. comm. was used to estimate C losses. The amount of C sequestered on the floodplain in the lower reaches of the basin in the same 80-year period was estimated on the basis of the volume of the alluvial sediments and their average C content 1.3 as determined by coring. Likewise, C stored on the continental shelf was estimated using an average C content of 1.0.

3. Results and discussion

3.1. NPP 3.1.1. NPP for major land-cover types National NPP estimates based on light-use effi- ciency values ǫ constrained by temperature and moisture Potter et al., 1993 were generally lower than those based on the New Zealand-derived ǫ-values, ranging from no difference in unimproved grasslands to over two fold lower estimates in indigenous forests Table 2. Using the ǫ-values from Potter et al. 1993, New Zealand’s NPP was estimated to be 69 Mt C per year. This is well below the total NPP value of 128 Mt C per year obtained from the six land-cover types Table 2 using ǫ-values that included New Zealand-derived ǫ-values for exotic and indigenous forests, and scrub Table 2. These land-cover types represent nearly half of New Zealand’s land area, so their combined NPP has a major influence on the national NPP estimate. Over 50 of NPP for New Zealand is contributed by the remnant indigenous forests and improved grasslands. Although the area of exotic forests con- tinues to expand, and they are a major C sink for New Zealand MfE, 1997, they contribute only ca. 10 to simulated NPP. Low NPP values for tussock grass- lands are expected for two reasons: they include large areas of land degraded by overgrazing and burning Ross et al., 1997, and large areas that occur above ca. 1300 m elevation and are frequently interspersed with bare rock and scree. This comparison between national NPP estimates using the two different sets of ǫ-values Table 2 in- dicates the importance of using appropriate ǫ-values for local vegetation and environmental conditions. Parametric models using remotely sensed NDVI to estimate NPP Ruimy et al., 1999 are particularly sensitive to variations in ǫ. The poor agreement be- tween simulated NPP estimates for some vegetation types Table 2 is most likely to have been caused by the higher values of ǫ assigned to New Zealand’s indigenous forests and improved grasslands. These K.R. Tate et al. Agriculture, Ecosystems and Environment 82 2000 229–246 235 Table 2 Estimates of NPP of the major land-cover types using different models and values for light-use efficiency ǫ Land-cover type Light-use efficiency ǫ g C MJ − 1 PAR NPP kg C m − 2 per year Area Mha Total NPP Mt C per year Potter et al. 1993 New Zealand CASA model New Zealand Forest indigenous 0.35 0.82 a 0.32 0.75 5.9 42 Forest exotic 0.35 0.82 b 0.48 1.12 1.3 13 Scrub 0.30 0.37 c 0.27 0.34 3.7 13 Grassland unimproved 0.30 0.29 d 0.29 0.28 3.7 15 Grassland improved 0.23 0.45 e 0.28 0.55 7.0 39 Grassland tussock 0.23 0.29 d 0.1 0.13 4.1 6 a White et al. 2000. b Coops et al. 1998. c D. Whitehead unpublished data. d Bartlett et al. 1990. e Schapendonk et al. 1998. ecosystems experience few climatic constraints, in contrast to those used by Potter et al. 1993 where single vegetation ‘classes’ spanned a wide latitudinal range. In New Zealand, mean annual soil tempera- tures range between 8 and 15 ◦ C over 75 of the land area, and nearly 90 of New Zealand experiences no prolonged soil moisture deficit on a regular basis. 3.1.2. Comparison of simulated NPP with site-specific NPP estimates Individual site NPP estimates Table 1 generally agreed well with predicted NPP values from the CASA model using 1 × 1 km 2 grid cells Fig. 1. Using the New Zealand-derived ǫ-values Fig. 1a, predicted NPP compared favourably with site esti- mates R 2 = 0.47, and the slope of the regression 0.97 was close to 1. Results from ANOVA sug- gested there was no significant within-land use effect or bias between the values predicted by the model and site-specific NPP estimates. Although predicted NPP using ǫ-values from Potter et al. 1993 correlated more closely with site-specific NPP R 2 = 0.55, the slope of the regression 0.31 suggested a sig- nificant bias between predicted and measured results Fig. 1b. The biggest discrepancy between predicted and measured NPP was at the tussock-grassland site, where the undisturbed grassland was in an area dom- inated by scree and bare ground, resulting in a low simulated NPP of 0.13 kg C m − 2 per year. Overall, these results suggest that the national NPP esti- mate of 128 ± 14 Mt C per year based on our New Zealand-derived ǫ-values is reasonable. 3.1.3. NPP estimates using different models A national estimate of New Zealand’s terrestrial NPP has not previously been reported. The magnitude of this C flux can, however, be calculated from esti- mates of the total stocks and turnover time of soil C, assuming steady-state conditions. For New Zealand, an NPP of ca. 140 Mt C per year was calculated by combining the soil C stock 0–1 m depth of 4260 ± 190 Mt C Tate et al., 1997 with a turnover time of ca. 30 years Tate et al., 1995 at 10 ◦ C the mean annual temperature for New Zealand. The agreement between our estimate of 128 Mt C for total NPP using the CASA model Table 2 and the value of ca. 140 Mt C per year described above is encouraging but not confirmatory. Large uncertainties exist in the national NPP estimate due to the large grid cell size 1 × 1 km 2 , the areal extent of different land-cover types, and limited information on light-use efficiency values and below-ground C allocation for the major New Zealand vegetation types. More robust APAR values are also needed, because the spatial vari- ability of predicted NPP is determined largely by this parameter Ruimy et al., 1999. Further studies us- ing more sophisticated process-modelling approaches at different spatial scales are now being made; these should provide a more reliable test of New Zealand’s first national NPP estimate based on this simple para- metric approach. 3.1.4. Temporal scaling of NPP estimates As fine temporal-scale variation in NPP is obscured by aggregating monthly values, we calculated annual 236 K.R. Tate et al. Agriculture, Ecosystems and Environment 82 2000 229–246 Fig. 1. Site specific NPP kg C m − 2 per year estimates for indigenous forests, scrub, exotic forest, and grassland improved, unimproved and tussock, based on the use of the productivity portion of the CASA model and NOAA–AVHRR imagery, and on plot-scale measurements and models. Light-use efficiency ǫ values used for a were derived for New Zealand ecosystems, and b were from Potter et al. 1993. NPP for New Zealand by estimating NDVI values at different temporal scales. First, four seasonal raster images were produced for solar radiation and NDVI by combining months as described in Section 2. These seasonal NDVI images were then used to run the CASA model, which gave an annual average NPP of 0.25 kg C m − 2 . Second, the solar radiation and NDVI image for the middle month of each season was se- K.R. Tate et al. Agriculture, Ecosystems and Environment 82 2000 229–246 237 lected, and the estimated NPP for that month was multiplied by 3 to ‘simulate’ seasonal NPP. These re- sults were combined to produce an annual NPP of 0.27 kg C m − 2 . Finally, images representing averaged over all months solar radiation and NDVI were pro- duced, and the model was then re-run to estimate the ‘averaged’ NPP value. This monthly average was mul- tiplied by 12 to give an annual NPP of 0.23 kg C m − 2 . Annual NPP values estimated by these different ap- proaches are similar to the value of 0.27 kg C m − 2 obtained by aggregating monthly NPP estimates, and suggest that, for 1993, four seasonal images would have been sufficient to obtain a credible annual NPP estimate based on the CASA model Potter et al., 1993. 3.1.5. NPP comparisons at different spatial scales Global NPP models Kicklighter et al., 1999 pre- dict NPP at a relatively coarse spatial scale often 1 ◦ × 1 ◦ latitude × longitude pixels. Whether pre- dictions at this spatial scale are relevant to national C budgets is not clear. Although the CASA model has been used for global-scale simulations Potter et al., 1993, the model was run for New Zealand with input data available at much finer spatial resolution than is available or practical for global-scale simulations. For New Zealand, global-scale simulations predict NPP for about 26 pixels at a 1 ◦ × 1 ◦ resolution Nielson and Running, 1996. Using average NPP values simu- lated with satellite imagery obtained in 1987 mapped Table 3 Sites used to estimate A values based on soil respiration measurements; all data collected since 1995 Land-cover types n a r b A c R 10 t C ha − 1 per year Site Refs. Indigenous forest beech 362 0.86 3160 12.8 Allen et al. 1997 Indigenous forest mixed 11 0.89 3290 13.4 Ross et al. 1999b Exotic forest Puruki 376 0.80 1460 5.9 Ross et al. 1999b Exotic forest Tikitere 100 stems ha − 1 141 0.81 2290 9.3 Yeates et al. 2000 Exotic forest Tikitere 400 stems ha − 1 62 0.72 2120 8.6 Yeates et al. 2000 Exotic forest Himatangi 128 0.88 1480 6 Ross et al. 1999a Exotic forest Balmoral 378 0.76 1910 7.7 Arneth et al. 1998 Scrub manukakanuka 567 0.92 3250 13.2 Scott et al. 2000 Improved grassland Puruki 232 0.86 3060 12.4 Ross et al. 1999b Improved grassland Tikitere 73 0.92 6170 25 Yeates et al. 2000 Tussock grassland Benmore, Old Man 47 0.79 1150 4.7 N.A. Scott unpublished data a Number of respiration measurementssite. b From linear regression with zero constant intercept. c Data-set dependent variable. as a range of NPP, NPP for New Zealand was esti- mated to be about 251 Mt per year. A value of 0.46 was assumed to convert NPP dry matter to C; this value is the mean of accepted values for herbaceous vegeta- tion 0.42 and woody vegetation 0.50 Schlesinger, 1991. This gives an annual NPP of 115 Mt C per year for New Zealand, which compares favourably with our estimate of 128 Mt C per year. 3.2. Respiration Soil respiration resulting from the contemporane- ous oxidation of soil organic matter by heterotrophic microorganisms R h and autotrophic root respiration R a Raich and Schlesinger, 1992 is a major C flux in the global C cycle. Accurate, unbiased estimates of soil respiration rates have been achieved across a wide range of ecosystem types and soil tempera- tures using an Arrhenius-type relationship Lloyd and Taylor, 1994. We therefore used this relationship to estimate annual soil respiration for New Zealand’s major land-cover types. Estimates of A Eq. 2 based on actual soil respi- ration measurements varied greatly among sites and land-cover types, ranging from about 1100 in dry, high country tussock grasslands to over 6100 for an im- proved pasture Table 3. Land-cover type accounted for a substantial fraction of the between-site variabi- lity corrected R 2 = 0.54, but there was substantial residual variation between sites within a land-cover 238 K.R. Tate et al. Agriculture, Ecosystems and Environment 82 2000 229–246 type. The most dramatic example of this is the differ- ence between the two improved grassland sites. Val- ues of A were generally higher at warmer sites, and differed significantly across land-cover types F 3,8 = 4.94, P = 0.03. Comparison of paired sites that dif- fered in land-cover type only viz. exotic forest and improved grassland at Tikitere and Puruki suggests that lower A values are associated with exotic forests. This is due to the lower proportion of NPP allocated to roots Table 1 in exotic forests. The values of 308.56 and 227.13 K for the parameters E and T Lloyd and Taylor, 1994 were used, and provided as good a fit to the data as did data-specific parameters. These Lloyd and Taylor 1994 values were therefore used with av- erage land-cover specific A values for all national soil respiration calculations. The fact that the temperature response of soil respiration measurements can largely be explained Table 3 by Lloyd and Taylor’s 1994 Arrhenius-type relationship is encouraging, and sug- gests this relationship is not scale-dependent for these non-water limited sites. Soil respiration for improved grasslands 2.3 kg CO 2 -C m − 2 per year is much higher than for the other land-cover types Table 4. This is likely to be due to both the relatively high productivity of these grasslands compared to other grasslands Table 2 and the high proportion of below-ground C allocation Table 1. However, this estimate should be treated with caution as it is based on A values for two sites that differed markedly; seasonal effects on soil res- Table 4 Soil respiration means ± SEM, total NPP, and C balance of the major land covers Land-cover type Respiration rate kg CO 2 -C m − 2 per year Respiration Mt CO 2 -C per year Total NPP Mt C per year a C balance Mt C per year b NL c L d Total L d Heterotrophic L d Forest indigenous 1.4 ± 0.2 1.4 ± 0.2 83±12 50 42 − 8 Forest exotic 0.9 ± 0.2 0.9 ± 0.2 12±3 9 13 4 Scrub 1.4 ± 0.3 1.4 ± 0.3 50±10 30 13 − 17 Grassland unimproved 1.3 ± 0.1 1.3 ± 0.1 47±6 14 15 1 Grassland improved 2.3 ± 0.4 2.2 ± 0.4 151±20 68 39 − 29 Grassland tussock 0.6 ± 0.3 0.6 ± 0.3 23±10 5 6 1 Total NA e NA 365±28 NA 128±14 NA a New Zealand estimates cf. Table 3; SEM for total estimated from ANOVA of simulated vs site NPP values see Section 3.1.2. b Total NPP minus heterotrophic respiration. c Non-moisture limited. d Moisture limited. e Not applicable. piration may have contributed to this difference. The lower respiration rate for Puruki is, in fact, similar to the rates for the native forest and scrub sites, but is considerably higher than those for the exotic for- est and tussock-grassland sites Table 3. Low soil respiration values for tussock grasslands Table 4 probably resulted from their very low NPP Table 2 and consequent low A values Table 3. Soil respira- tion tended to be lower in exotic forests as compared with native forests, most likely because of the smaller number of fine roots in the forest floor layer of the exotic forests N.A. Scott, unpublished data. Differences in respiration rates between land-cover types, and differences in A values, are in part related to the greater temperature sensitivity higher Q 10 of root respiration compared with bulk soil respiration Boone et al., 1998. Roots contribute more to total respira- tion in improved and unimproved grasslands than in forests, and are located closer to the surface. These dif- ferences in temperature response and location of roots in the soil profile are also likely to explain differences in respiration between indigenous and exotic forests Table 4. In New Zealand’s indigenous forests, large masses of fine roots are frequently found in the forest floor layer e.g., Tate et al., 1993, whereas fine root biomass in plantation forests is lower and tends to be located deeper in the soil profile Arneth et al., 1998. As for NPP, improved grasslands and indigenous forests contribute most to soil respiration nationally Table 4, largely because they represent nearly 50 of K.R. Tate et al. Agriculture, Ecosystems and Environment 82 2000 229–246 239 the land area; the smallest contribution is from tussock grasslands see Section 3.1.1. Potential temporal scaling errors in national res- piration estimates were investigated by comparing the results obtained using mean monthly tempera- tures with those that would have been obtained using mean seasonal and annual temperatures. These com- parisons showed that if mean seasonal temperatures had been used, only small errors would have resulted national range 0–2, average 0.7. If mean an- nual temperatures had been used, more significant errors would have been introduced national range 1–12, but national total soil respiration would have been over-predicted by only 4. These errors arise from the non-linearity of the Arrhenius-type temperature-response function. 3.2.1. Heterotrophic respiration Using the root allocation values in Table 1, R a val- ues for indigenous forests, exotic forests, scrub, and grasslands unimproved, improved, tussock were 33, 3, 20, 33, 83, and 18 Mt CO 2 -C per year, respectively. Heterotrophic respiration was then calculated as the difference between total respiration Table 4 and R a . At the national scale, heterotrophic respiration ranged from 5 to 68 Mt CO 2 -C per year for the different land-cover types Table 4; it was highest in improved grasslands due to both high respiration rates per unit area Table 4 and their large areal extent Table 2. Although uncertainty values for total respiration for the different land-cover types could be estimated Table 4, it was not possible to quantify uncertainty in R h for the whole country because few data on root allocation were available for all the land-cover types. 3.2.2. Moisture effects on soil respiration There are small areas of New Zealand east coasts of the North and South Islands, and central South Island east of the Southern Alps where prolonged summer soil moisture deficits can affect soil respiration. For example, moisture L of soil respiration was observed in a semi-arid tussock grassland following a prolonged dry period in April 1999 N.A. Scott, unpublished data. The national comparison of predicted total soil respiration under both L and NL conditions indicated that soil moisture deficit has little influence on soil respiration rates Table 4. The increasing frequency of El Nino events over the past decade has, however, caused prolonged drought over much larger areas in the eastern parts of North and South Islands. During drought years, therefore, soil respiration–temperature relationships alone will not provide reliable national soil respiration estimates. 3.3. National C balance Subtraction of heterotrophic respiration from the corresponding NPP estimate for each land-cover type Table 4 gives an estimate of the annual C balance, viz. net ecosystem production NEP. However, it must be emphasised that these C balance estimates need to be treated with caution given the uncertain- ties in the assessment of both respiration and NPP. These NEP results suggest that indigenous and ex- otic forests, and unimproved and tussock grasslands, are likely to be in C balance over a one-year time period, whereas scrub and improved pastures appear to have relatively large C losses Table 4. Vegetation in scrub ecosystems is, however, likely to be accu- mulating C nationally Scott et al., 2000 over both short and long time scales, and could be a significant C sink comparable to exotic forests in New Zealand see Section 3.4. The annual C loss suggested for scrub probably resulted from an under-estimate of NPP. Short-statured scrub dominated by gorse Ulex europaeus and broom Cytisus scoparius is exten- sive in areas of uneconomic low productivity pas- toral land, and is likely to have low ǫ-values. Older stands of manuka Leptospermum scoparium and kanuka Kunzea ericoides scrub could, however, have ǫ -values that are similar to indigenous forest because they have similar proportions of woody and canopy tissue to larger trees Scott et al., 2000. Light-use efficiency values were not available for other scrub types, so a conservative value of 0.37 g C MJ − 1 PAR D. Whitehead, pers. comm. was used, based on mea- surements at a temperature-limited site. This may have led to an under-estimate of NPP for scrub vegetation nationally. The apparently large annual net C loss indicated for improved grasslands is inconsistent with results from inter-decadel comparisons of soil C concentrations, which show little or no change over a wide range of soil types Tate et al., 1997. Present results should be viewed in light of the large discrepancy in soil 240 K.R. Tate et al. Agriculture, Ecosystems and Environment 82 2000 229–246 respiration between the two improved grassland sites. Use of the lower R 10 value of 12.4 for the Puruki site, instead of the mean value of 18.7 for the two improved grassland sites Table 3, would reduce the imbalance from −30 to −7 Mt C. The apparently large C loss may have also resulted, in part, from under-estimation of root respiration R a , based on 55 C allocation to roots Table 1. Recent research, using 13 C to trace C uptake and allocation in pastures, suggests that root C allocation varies from 39 to 51 across various man- agement regimes and seasons Stewart and Metherell, 1999. If these lower estimates of root allocation are used, the annual net C loss for improved grasslands appears even greater, at 45 Mt C per year. Alterna- tively, estimates of NPP in improved grasslands may have been too low. Defining and quantifying NPP in grazed systems is problematic McNaughton et al., 1996, and it is possible that the satellite-derived NPP estimate for New Zealand’s grazed grassland ecosystems is incorrect. This is possible because a conservative value for ǫ of 0.45 was used, which falls at the low end of the range reported Schapendonk et al., 1997 for Lolium perenne L. of 0.38–1.13. More refined NPP estimates may be achieved us- ing pasture production models e.g., Schapendonk et al., 1998 linked to national information on animal abundance. Our estimate of R h , based on the assumption that R a and root C allocation are equal, could have con- tributed to some of the large C imbalances observed e.g. in improved grasslands. An alternative approach would be to assume that R a is a constant proportion of total soil respiration. Several studies have attempted to partition total soil respiration into autotrophic and heterotrophic components e.g. Bowden et al., 1993; Haynes and Gower, 1995, with results for R a vary- ing from about 25 to 60 of the total Landsberg and Gower, 1997; Hanson et al., 2000. Use of an R a R total value of 45 for forests and 55 for grasslands Han- son et al., 2000, would give R h estimates of 46, 7, 25, 19, 60, and 9 Mt CO 2 -C per year for indigenous and exotic forests, scrub, unimproved and improved grass- lands, and tussock grasslands, respectively. These al- ternative estimates would have a small impact on the national C balance for each land-cover type, with the only change in sign i.e. switching from a gain to a loss occurring in unimproved and tussock grasslands Table 4. Clearly, this is an area where more infor- mation is required to improve the robustness of our national-scale C balance calculation. 3.3.1. Major sources of uncertainty Large uncertainties were anticipated from the out- set when estimating the C balance of New Zealand’s terrestrial ecosystems from differences between NPP and R h . This is an inevitable outcome of compar- ing two large numbers. Moreover, estimating these differences on an annual basis assumes that this is the most relevant time frame over which to measure both NPP and soil respiration. This may be reason- able for improved grasslands, where growth rates and turnover times are relatively rapid, but is questionable for indigenous forests, where NPP and soil respiration might not be synchronised on an annual basis Tate et al., 1993. However, the general agreement between national-scale NEP simulations and plot-based C bud- get estimates see Section 3.4.3 for both indigenous and exotic forests suggests that this assumption intro- duced no major uncertainty. The standard error of the mean SEM for NPP for each land-cover type could not be quantified because data for the whole population of grid cells were used. In contrast, an SEM of ca. 14 Mt C per year for the national NPP was achieved from prediction of NPP for specific sites where independent estimates had been made Fig. 1. The large number of soil respi- ration measurements from several land-cover types provided the opportunity to estimate SEMs for each land-cover type Table 4, if the assumption was made that the residual uncertainty was homogeneous across the different land-cover types. Although future work will need to include estimates of uncertainty for both NPP and soil respiration for the different land-cover types, comparisons between simulated and measured national C budget estimates see Section 3.4 are encouraging. They suggest that for most ma- jor land-cover types, the national-scale NEP estimates are in general agreement with those based on specific plots. 3.4. Land-use change and national C budgets — analysis at finer spatial scales The above comparison of net primary production and soil respiration for New Zealand suggests that the two processes are roughly in balance, given the K.R. Tate et al. Agriculture, Ecosystems and Environment 82 2000 229–246 241 rather large uncertainties associated with each C flux. This analysis is, however, based on flux estimates over just one year. Over time, New Zealand’s national C budget could be strongly influenced by factors such as land-use change that alter the ‘balance’ between NPP and soil respiration, resulting in non-steady- state ecosystems. These factors must be included in national-scale C budget calculations. In this sec- tion, estimates of C gain or loss based on data collected at finer spatial scales are compared with the national-scale C balance simulations. The veg- etation and soil C changes discussed below do not occur indefinitely, however, and their effect on the national-scale C budget will diminish as the ecosys- tems involved approach a new steady state. 3.4.1. Abandonment of agricultural land Based on land-cover data for 1987, abandonment of uneconomic pasture land primarily in the North Island hill country and re-growth of scrub vegeta- tion has been a major land-use change Newsome, 1987. In 1987, ca. 3.5 million hectares of New Zealand’s land area was covered by scrub vege- tation, with ca. 1 million hectares dominated by manuka and kanuka. This vegetation often estab- lishes following disturbances such as fire, and fre- quently invades pasture land when grazing ceases. These particular species grow rapidly, and stands of ca. 35 years can accumulate ca. 150 t C ha − 1 in the vegetation Scott et al., 2000. Although the CBAL Eq. 2 estimate for this land-cover type is − 17 Mt C per year Table 4, this large predicted loss may have resulted from an under-estimate of NPP Section 3.3. Depending on the age-class distribution nationally, manuka and kanuka could sequester large amounts of C each year. If a maximum biomass of 170 t C ha − 1 af- ter 50 years is assumed, average biomass accumulation would be 3.4 t C ha − 1 per year. If this scrub vegetation occupies 1 million hectares, it could be accumulating about 3.4 Mt C per year. Use of a more conservative estimate based on a maximum age of 80 years and the same maximum biomass of 170 t C ha − 1 would give an average accumulation of 2.1 Mt C per year. Using a national biomass estimate for manukakanuka scrub of 50 t C ha − 1 from a national vegetation database Hall et al., 1998, and again assuming 80 years as a maxi- mum stand age, the national estimate is only 0.6 Mt C per year. However, none of these estimates accounts for differences in scrub growth rates related to climate, soil type, and previous land-use. More work is needed to refine these estimates and assess their impact on the national C budget. 3.4.2. Planting of exotic forests At the national scale, increased planting of exotic forests primarily P. radiata is the most important land-use change occurring in New Zealand. In 1996, 84 000 ha of new forest planting occurred MAF, 1998. Carbon accumulation in these plantations varies with site quality, but when averaged over the national forest estate, is about 230 t C ha − 1 over 30 years. At harvest, about half of this C remains on the site as slash, roots, forest floor and stumps; much of it is eventually oxidised and returned to the atmosphere as CO 2 Maclaren, 1996. However, if the harvested area is replanted to another rotation, the C remaining after harvest is considered to be ‘stored’, and averages about 112 t C ha − 1 nationally Maclaren, 1996. The amount of C sequestered by plantation forests is well documented, and in 1995 about 1.5 million hectares of these forests sequestered 3.7 Mt CO 2 -C, about half of New Zealand’s total CO 2 emissions for that year MfE, 1997. The estimate CBAL Eq. 2 +4 Mt C per year; Table 4 agrees closely with this nationally reported estimate. Current national reporting of C accumulation in plantation forests includes C accumulation in the for- est floor, without considering changes in organic C in the mineral soil MfE, 1997. About 84 of the new exotic forests established in 1996 were on land previously used for pasture MAF, 1998. Compari- sons of soil C at paired sites, where land-use is the only different factor, have indicated that organic C in the surface mineral-soil layers is 17–40 lower under plantation forests than pasture Alfredsson et al., 1998; Scott et al., 1999. Similar differences in mineral-soil C were found for these two land-use types in a national soil C database linked to polygons described by soil type, climate, and land-use Scott et al., 1999. However, lower mineral-soil C under exotic forests could be offset by C accumulation in the forest floor, which averaged 20 t C ha − 1 Scott et al., 1999; this forest floor C is included in the 112 t C ha − 1 estimated to be the average exotic-forest sink. 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