Deforestation Carbon Stock Tables
Results
Modelled volume yield tables
The volume yield tables (total stem under-bark volume) for the two radiata pine regimes and for Douglas-fir, Other Softwoods, and Hardwoods from the age of nine years onwards are shown in Figures 1, 2 and 3.
Figure 1: Volume Yield table graph for the pruned Radiata pine regime for the nine growth model regions (unscaled).
Large version [10KB GIF]
Figure 2. Volume yield table graph for the unpruned Radiata pine regime for the nine growth modelling regions (unscaled).
Large version [10KB GIF]
Figure 3. Volume yield table graph for Douglas fir, hardwoods and other softwoods (unscaled).
Large version [10KB GIF]
The volume yield tables for all species, growth model regions and regimes are compared graphically with NEFD yield tables in the Appendix.
Modelled Carbon yield tables
Carbon yield tables for aboveground live biomass, below ground biomass, dead wood litter (coarse woody debris), and fine litter were calculated from the stem volume yield tables using C_Change, and are shown in the Appendix. Total carbon in these four pools (which excludes mineral soil carbon) is also shown along with carbon dioxide equivalent, calculated by multiplying total carbon by 44/12.
The carbon yield tables for radiata pine and other softwoods start from age 9 and end at age 50. Carbon yield tables for Douglas-fir start from age 9 and end at age 80 and for hardwoods at age 9 and end at age 25.
MAF stated that for stands were trees are harvested at age 8 or younger, the deforestation emission is calculated as if the trees were at the age of those last harvested from the land.
As an example, predicted carbon pools for Bay of Plenty unpruned radiata pine are shown in Figure 4. Aboveground and below ground biomass carbon both generally increase with stand age. However when stands are thinned a decrease in these live pools occurs, because a portion of live tree biomass is relocated into the dead wood litter and fine litter pools.
Figure 4: Modelled changes in carbon pools and total carbon in a 2nd rotation pruned Radiata pine stand for the model region Bay of Plenty (unscaled).
Large version [10KB GIF]
Representation of thinnings in the yield table
The nominated thinning regimes were spread over 3 years to better represent stocking reductions expected across a large number of stands. The effect of spreading thinning events across 3 years instead of thinning in one year on the stand total carbon stocks is shown in Figure 5. Spreading thinning events over time considerably reduces the ephemeral drop in carbon stocks that is evident when thinning occurs all in one year (Figure 5).
Figure 5: Difference in Volume and total Carbon over time between multiple years thinning (‘average’ stand) and one thinning event (individual stand).
The Auckland radiata pine unpruned regime is shown as an example with stem volume and carbon stocks shown to stand age 15.
Large version [10KB GIF]
Comparison between PSP data and modelled yield
To test the representativeness of the modelled yield tables and to check for possible anomalies, the modelled yield curves were compared with estimates based on a nationwide database of permanent sample plots (PSPs). Plots were selected to represent regimes similar to the two radiata pine regimes used in this report. A smoothing spline function was used to estimate the mean volume yield curve of the PSP data and this was superimposed on the predicted growth curves for Gisborne/East Cape and Canterbury regions which represented the highest and lowest productivity regions according to the modelled yield tables. Figures 6 and 7 show the PSP data, the mean spline curve fitted to the PSP data, and the modelled yield curves for respectively the unpruned and pruned radiata pine regimes.
Except for the period before the heavy thinning event the two regional yield curves modelled for the unpruned radiata pine regime, lie above and below the mean yield from the PSP data. For this regime, PSPs were selected from the database on the basis of stocking at age 20 (PSPs with 350 to 450 sph at age 20 were selected). The differences between the PSP data and model at young ages can be explained by the thinning regime (most PSPs were thinned earlier than the specified regime – see Figure 8). Also, it is possible that unpruned higher-stocked stands such as those represented by this regime tend to be found on poorer sites. This may explain why the mean PSP yield for this regime tends to be slightly closer to the yield table for the poorer productivity region than to that for Gisborne, especially at older stand ages. In addition PSP data at older ages represent unimproved stands (low level of genetic improvement and site preparation), which can be expected to reduce volume yield of PSP plots at older ages compared to modelled yields.
The modelled yield curves for the pruned radiata pine regime are in agreement with the PSP data, with a similar stocking (PSPs were selected with between 200 to 300 sph at age 20, and with pruned height greater then 3m). The mean of the PSP data lies between the two modelled yield curves which represent the upper and lower bounds of regional productivity levels.
Overall, the comparison between nationwide PSP data and the modelled regional radiata pine yield tables revealed no major deficiencies or abnormal behaviour.
Figure 6: Comparison between PSP data and two modelled volume yield curves for an unpruned Radiata pine regime.
Large version [10KB GIF]
Figure 7: Comparison between PSP data and two modelled yield curves for a pruned radiata pine regime.
Large version [10KB GIF]
Figure 8: Stocking (sph) in PSP plots compared with stocking of the two growth region models (unpruned regime).
Large version [10KB GIF]
Comparison of modelled Yield Tables with NEFD Yield tables
The modelled stem volume tables were also compared with the total stem volumes of the relevant NEFD tables. Overall the NEFD yield tables showed lower volumes than the modelled yield tables (Figures 9 & 10; further graphs are in the Appendix).
In some Growth Modelling Regions e.g. Hawkes Bay, Gisborne, Waikato-Taupo the differences between our modelled tables and the NEFD tables were small. The South Island growth modelling regions showed greater differences, with Nelson Marlborough showing the smallest difference and Canterbury the greatest difference especially at older stand ages.
The differences between the modelled yield tables and NEFD yield tables result from differences in the underlying data used for deriving the tables. For example, NEFD tables are based on areas that are not fully stocked in contrast to the generally fully-stocked PSPs that were used to estimate regional 300 Index values (radiata pine) and 500 Index values (Douglas-fir). To overcome this difference, either the modelled yield tables could be adjusted to account for stocking difference (e.g. reduced by 20%), or alternatively unstocked areas (roads, patches of scrub etc.) could be excluded when determining the deforestation areas.
Figure 9. Modelled Volume for Growth Modelling Regions Waikato/Taupo and Bay of Plenty compared with NEFD yield table data.
Scaled average yield for the modelled radiata regimes (not area weighted) is also shown.
Large version [15KB GIF]
Figure 10. Modelled Volume for Growth Modelling Region Canterbury/West Coast compared with NEFD yield table data.
Scaled average yield for the modelled radiata regimes (not area weighted) is also shown.
Large version [10KB GIF]
Another difference between NEFD and modelled tables is that, while the 300 Index (or 500 Index) represents an average productivity level for relatively recently planted stands, the NEFD table represents “snapshots” in time for each year in the current resource, and therefore is a mix between younger stands with improved stocks and silvicultural management and older stands established with less improved stock and variable management practices.
Another discrepancy might exist between the site productivity indices used for model predictions (300 Index and 500 Index) and actual site productivity of the stands used in developing the NEFD tables. The regional 300 Index values used in this study came from lookup tables provided in the Radiata Pine Calculator which were derived from mean estimates of PSPs provided by the Stand Growth Modelling Cooperative. However, the distribution of these PSPs in each region may not necessarily be completely representative of the forests in that region. Rather than relying on 300 Indices calculated from PSP data, an alternative approach would be to derive them from the NEFD yield tables (e.g., using the NEFD volume at age 20 years). This would give volume predictions that were in line with NEFD yields.
Scaled modelled Yield compared to NEFD1995 yield
The comparison of the scaled modelled yield with the 1995 NEFD yield tables shows that the scaling results in a conservative modelled yield for most of the regions, e.g. Waikato /Taupo (see Figure 9 & 10 and also Appendix). For Hardwoods the scaling resulted in a still very high modelled yield compared to the 1995 NEFD yield.
Contact for Enquiries
Sustainable Land Management and Climate Change
MAF
Pastoral House
25 The Terrace
PO Box 2526, Wellington
Tel: 0800 CLIMATE (254 628)
Contact this person
