Potential for carbon forestry on highly erodable land in New Zealand - Euan Mason & Justin Morgenroth

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

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[1]

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.

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

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.

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

Hall, G. M. J. (2001). Mitigating an organisation's future net carbon emissions by native forest restoration. Ecological Applications, 11(6), 1622-1633.

Woollons, R. C., & Manley, B. R. (2012). Examining growth dynamics of Pinus radiata plantations at old ages in New Zealand Forestry, 85(1), 79-86.

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[1] https://data.linz.govt.nz/layer/804-nz-property-titles/

[2]  https://lris.scinfo.org.nz/layer/76-nzlri-land-use-capability/

[3]

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