New Zealand has enormous potential to use forests to sequester carbon dioxide as part of its contribution to mitigating climate change. This preliminary report sets out results of an analysis requested by the Green Party of New Zealand to examine the carbon sequestration potential of a range of alternative afforestation strategies. Erosion-prone land was chosen as a target because re-establishing forest greatly reduces the likelihood of further erosion, and such land is often relatively unproductive as farmland. The report was delivered in August 2015. As a reference, New Zealand typically emits approximately 80 million tonnes of CO2-e per year.
To produce estimates of potential carbon dioxide sequestration rates
by new forests we used Geographic Information System layers describing land titles[1],
Land Use Capability classes from the New Zealand Land Resource Inventory[2],
and Erosion Susceptibility classes defined in a study by Mark Bloomberg et al. (2011). Erosion susceptibility classes are
shown in Figure 1.
Figure 1 – ESC
classes by Bloomberg et al.
(2011)
We also created a mask that provided estimates of “Kyoto
compliant” land, that is, land that is not currently in forest but that could
reasonably be planted in forest. This
was based on the LCDB v4.0 dataset. The classes that were explicitly included
in our analysis were:
Gorse and/or broom
High producing exotic grassland
Low producing exotic grassland
Short-rotation cropland
Landslides
In addition, broad radiata pine
productivity classes were conservatively defined by mean annual rainfall and
mean annual temperature. Land was allocated into four productivity categories:
Unsuitable for radiata pine; low productivity; medium productivity; and highly
productive. This analysis was constrained by time, and we recommend that the
analysis be extended to define productivity using a physiological model
developed by Euan Mason. This latter model takes into account variation in
radiation, soil characteristics, rainfall, temperature and vapour pressure deficit on a monthly time step, and so would provide more
secure estimates of potential productivity across New Zealand than the quick
estimates we have used for this report.
The draft potential productivity categories are shown in Figure 2.
Figure 2 – Draft potential
productivity categories for radiata pine in New Zealand defined by rainfall and
temperature. Green=Highly productive,
Yellow=moderately productive, brown=low productivity, and white=unsuitable.
Table 1 summarises land that is
either highly or extremely erodible (based on erosion susceptibility classes of 'high' or 'very high')
and that might be planted in carbon forests by productivity classes.
Approximately 1.3 M ha are available in low, medium or high productivity
categories, or 5% of New Zealand’s land area.
Areas shown are in hectares. Most
of the erodable land was in Land Use Capability classes VI to VIII, with the
majority in class VII.
We stress that this is a draft
analysis, and numbers are subject to revision.
The areas identified as erodible and
available for planting are shown in Figure 3.
Table 1 – Provisional summaries of
highly erodible land available for planting (ha)
Threatened
environment class[1]
|
----------Productivity
class--------
|
TOTAL
|
||
Low
|
Medium
|
High
|
||
0
|
89
|
39
|
27
|
155
|
1
|
51457
|
112189
|
34435
|
198081
|
2
|
34545
|
97540
|
129380
|
261465
|
3
|
22914
|
59242
|
98758
|
180914
|
4
|
87952
|
38753
|
72864
|
199569
|
5
|
38508
|
9429
|
38038
|
85975
|
6
|
131892
|
65396
|
188505
|
385793
|
TOTAL
|
367357
|
382588
|
562007
|
1311952
|
Threatened
Class
|
Threatened
Class
|
0
|
No Data
|
1
|
Acutely Threatened
|
2
|
Chronically Threatened
|
3
|
At Risk
|
4
|
Critically Underprotected
|
5
|
Underprotected
|
6
|
Less Reduced and Better Protected
|
Three points in the landscape were selected that were roughly representative of average conditions in the low, medium and high productivity classes shown in Figure 2. Forecaster software created by Scion Research was used to run the 300 Index model at each location for a silvicultural regime that comprised planting 1000 radiata pine stems/ha and leaving them to grow, and also a regime that is considered typical in pruned crops of radiata pine, with 3 pruning lifts and low final crop stockings in the range of 300 stems/ha after early thinning to waste. Model C_CHANGE was employed to estimate carbon dioxide sequestration in these regimes. The plant and leave option was adjusted downwards for the following reasons:
1) We don't
have any data for a long-rotation plant and leave option, and so the growth and
yield model we used was extrapolating for that option.
2) Estimation of carbon contents of large trees was based on a tiny dataset.
3) Examination of model outputs suggested that the growth and yield model may have been underestimating tree death from competition during the simulation period.
4) The adjusted model brought data more into line with published data (Woollons & Manley, 2012) we do have over long rotations, and so we know the values can be achieved.
2) Estimation of carbon contents of large trees was based on a tiny dataset.
3) Examination of model outputs suggested that the growth and yield model may have been underestimating tree death from competition during the simulation period.
4) The adjusted model brought data more into line with published data (Woollons & Manley, 2012) we do have over long rotations, and so we know the values can be achieved.
Figure 3 – Areas that were available
for afforestation and also judged to be at high or very high risk of erosion.
In addition, C yields for the plant
and leave options were further reduced to allow for attrition of whole stands
after 30 years. Nevertheless, estimates of sequestration for the radiata pine plant and
leave regimes remain highly uncertain.
Sources estimating sequestration
rates for native species were consulted[4],
and yield tables of carbon sequestration for typical native species were
created that mirrored the tables for radiata pine but with a slower
development. The mean sequestration rates used for native species ranged from 3.8
to 9.1 tonnes of CO2/ha/year depending on site type. Figure 4 shows the accumulated CO2
over sixty years for each option.
Figure 4 – CO2 accumulated per hectare over 60
years for each option simulated, with three lines representing high, medium and
low productivity. Plant and leave options
have been adjusted downwards from the original simulation.
Yield tables over sixty years were
created for the following options:
1) Plant and
leave the entire area in radiata pine at a planting rate of 50,000 ha/year over
26 years
2) Plant at the same rate but prune, thin and harvest all of it at an appropriate time then replant
3) A 50:50 mixture 1 and 2
4) Plant the total area in native forest at 50,000 ha/year
5) A 50:50 mixture of 1 and 4 with 1 confined to the 3 least threatened environments
6) A 50:50 mixture of 3 and 4 with 3 confined to the 3 least threatened environments
2) Plant at the same rate but prune, thin and harvest all of it at an appropriate time then replant
3) A 50:50 mixture 1 and 2
4) Plant the total area in native forest at 50,000 ha/year
5) A 50:50 mixture of 1 and 4 with 1 confined to the 3 least threatened environments
6) A 50:50 mixture of 3 and 4 with 3 confined to the 3 least threatened environments
Figure 5 shows annual sequestration
rates for the six simulated scenarios.
Figure 5 – Millions of tonnes of CO2 accumulated per year over the entire area of erodable land for the
six planting scenarios. Year zero is the year that planting 50,000 ha/year for 26 years was initiated.
Sequestration rates for many of the scenarios would see us more than meet our international commitments for climate change mitigation. Moreover, planting 50,000 ha per year is easily achievable; during the 1990s New Zealand planted up to 100,000 ha per year. If we allow for an afforestation cost of $3000/ha, then the annual cost of such a programme would be $150 million. At $20/tonne of carbon we would need to sequester 7.5 million tonnes of carbon dioxide in any given year to cover this cost. Four of the six scenarios greatly exceed this level of sequestration after the first decade.
Why radiata pine?
Radiata pine has been chosen for the
following reasons:
1) It grows rapidly and sequesters C at
a much higher rate than native species
2) We are experts at producing
seedlings for this species and they are cheap.
3) It will grow on a wide range of
sites and we understand how to establish it on diverse sites, despite its
sensitivity to shade and frost.
4) It is not a high country wilding
risk. It is very intolerant of both
shade and frost, and would only seed naturally on moist lowland areas where
adjacent land was not intensively grazed.
5) On warm, moist sites (either
medium or high productivity categories), it would act as a nurse crop for
native forest, and the C reservoirs we establish would ultimately change to
become native forest so long as seed sources were available in the local
vicinity (Figure 6).
6) Studies suggest that radiata pine
will continue to sequester carbon for up to 100 years on some sites, but we
have assumed 60 in this analysis. This
means that the forests would remain as sinks for some considerable time.
Recent work by Adam Forbes as part of his PhD thesis at the School of Forestry, University of Canterbury, identifies the need for local seed sources in order for native forest to regenerate underneath exotic plantations. It would therefore be sound policy to make establishment of appropriate indigenous seed sources a condition for those receiving credits for carbon sequestration in their exotic forests.
Recent work by Adam Forbes as part of his PhD thesis at the School of Forestry, University of Canterbury, identifies the need for local seed sources in order for native forest to regenerate underneath exotic plantations. It would therefore be sound policy to make establishment of appropriate indigenous seed sources a condition for those receiving credits for carbon sequestration in their exotic forests.
Figure 6 - Forest biomass dynamics
after introducing the exotic pine species Pinus
radiata to the native species pool. Dynamics are modeled for a site near
Christchurch, New Zealand. Species aboveground biomass is cumulative. "Kunzea and Leptospermum" include the early colonizing species K. ericoides and L. scoparium. "Others" include the species Griselinia lit- toralis, Pittosporum
eugenioides, Aristotelia serrata, Elaeocarpus hookerianus, Fuchsia ex-
corticata, Nothofagusfusca, and N.
solandri var. solandri Figure from Hall (2001).
Important points
Planting new forests on 1.3 million hectares of erosion-prone land in New Zealand (about 5% of our land area) could sequester an enormous amount of carbon dioxide from the atmosphere, greatly helping us meet our international commitments for climate change mitigation.
Contrary to public perceptions, radiata pine is probably the most useful species for this purpose because it is unlikely to be a wilding, we can grow it very cheaply, it can be nurtured on a wide range of sites, and it sequesters carbon dioxide at a very high rate.
Establishment of local indigenous seed sources can promote ecological succession to native forest in unharvested stands of radiata pine, and so establishment of small areas of such seed sources should be a precondition to receiving credits for exotic forest carbon sequestration.
Caveats
This draft has been produced in a
short time and so we recommend further analysis, particularly a refinement of
rates of sequestration on diverse land types. Our estimates of erodible land
area available are preliminary. It is
likely that incentives to plant may well result in some planting of land that
is not highly prone to erosion.
References
Bloomberg, M., Davies, T., Visser, R., &
Morganroth, J. (2011). Erosion Susceptibility Classification and Analysis of
Erosion Risks for Plantation Forestry
Retrieved November, 2012, from http://www.mfe.govt.nz/laws/standards/forestry/erosion-susceptibility-classification.pdf
[1] https://data.linz.govt.nz/layer/804-nz-property-titles/
[2]
https://lris.scinfo.org.nz/layer/76-nzlri-land-use-capability/
Threatened
Class
|
Threatened
Class
|
0
|
No Data
|
1
|
Acutely Threatened
|
2
|
Chronically Threatened
|
3
|
At Risk
|
4
|
Critically Underprotected
|
5
|
Underprotected
|
6
|
Less Reduced and Better Protected
|
[4]
There is an excellent summary at http://maxa.maf.govt.nz/forestry/pfsi/carbon-sequestration-rates.htm
