Submission on proposed changes to permanent forest sink rules, 2022

Background

Aotearoa has so far failed to make substantial progress in its response to climate change, and proposed changes to rules for permanent forest carbon sinks will further undermine progress in meeting our net greenhouse gas (GHG) emissions commitments for 2030 and 2050. That our nation is one of the worst greenhouse gas polluters is beyond doubt. Climate Action Tracker provides an assessment of our performance and rates it as highly insufficient (Figure 1).

Figure 1 – An assessment of our national performance at climate change mitigation, available on-line at https://climateactiontracker.org/countries/new-zealand/ (Accessed on 14/4/22)

According to our Ministry for the Environment, Aotearoa emits about 80 million tonnes of GHGs annually. These are known as “gross emissions”. In 2018 we emitted 16.9 tonnes of CO₂-e per capita. This level of emissions placed us 16^th worst among all countries[1], and is far above both the Organisation for Economic Cooperation and Development average of 10.83 and the global average of 6.45 tonnes of CO₂-e per capita[2].

Aotearoa has agreed to two international commitments. Firstly, we have agreed to a “Nationally Determined Contribution (NDC) to keep our net GHG emissions at 50% of gross 2005 emission levels between 2021 and 2030. Secondly, and more importantly, we have pledged to get our net emissions down to zero by 2050, with perhaps some exceptions for methane emissions from agriculture.

Recent progress has been made by setting up a Climate Change Commission, providing incentives to purchase electric vehicles, and attempting to negotiate with the farming lobby. In addition an emissions trading scheme has been set up so that for a bit less than half the country’s emitters there is a price on greenhouse gas emissions. The price has recently risen to as high as $85/tonne of CO₂, leading to some investment in carbon (C) forestry, among other things, and some concern from hill country farmers about whole-farm conversions to C forestry.  Conversion of whole, mostly hill country farms, has become a political issue, prompting some lobby groups to push for legal constraints on conversions. Typically these concerns are about impacts of whole-farm C forests on hill country farming as a way of life. The farming lobby has been joined by an anti-exotic species lobby that questions what might happen to exotic C forests that remain unharvested and would much prefer to see indigenous species in C forests.

The role of forestry

Creating new forests is the most efficient way we currently know of to extract CO₂ from the atmosphere. Trees absorb solar energy and create sugar from CO₂ and water. Some of the created sugar is used to provide energy for living functions, and some is stored for longer periods in biomass. Typically half the dry-weight biomass in wood is elemental C, and amounts of CO₂ extracted from the atmosphere (“sequestered”) by trees can be calculated by multiplying the mass of stored elemental C by 44/12.

New forests are called “sinks” for CO₂ because they extract far more CO₂ from the atmosphere than they emit through respiration, but forests do not remain sinks forever. The name “permanent forest sink” can therefore be misleading for those who are unfamiliar with forestry. Eventually forest sinks become simple carbon reservoirs. Those that are repeatedly harvested and re-established typically retain about 60-70% of their maximum C content at harvest in long-term average storage, while unharvested forests eventually reach the point where they are emitting as much CO₂ through respiration and decay of dead biomass as they absorb. Their long-term storage may be punctuated by small- and large-scale disturbances such as wildfires or windthrow that reduce their average long-term storage just as periodic harvesting can reduce average long-term storage.

Establishing new forest sinks to absorb GHGs we emit can only, therefore, be a temporary solution, with additional new forest sinks providing a cheap way to extract our GHG emissions from the atmosphere, achieving net GHG neutrality while we develop ways to reduce our GHG emissions to zero. If we wished to rely on forest sinks to achieve GHG neutrality on a permanent basis then we would need an unlimited supply of unforested land on which to establish new forests each year. This fact was clearly recognised by authors of the “Globe” study, a multi-partisan, parliamentary-initiated study designed to explore how Aotearoa could reach GHG net neutrality by 2050 (Vivid_Economics, 2017).

After extensively studying Aotearoa’s GHG-emitting and forestry sectors, the authors of the Globe study stated that in their opinion we could not reach net GHG neutrality by 2050 simply by reducing gross GHG emissions to zero because new technologies had to be developed, and resistance to rapid change would be strong. They recommended that new forest sinks be used to fill the gap between what we wished to achieve by 2050 and what could realistically be achieved by gross GHG emission reductions. This situation is clearly shown in a graph that quantifies the gap in our accounts that we need to fill with sequestration of CO₂ by forest sinks while we reduce our gross emissions to zero (Evison & Mason, Forthcoming) (Figure 2).

Figure 2 – The Globe study’s “Innovative” scenario of gross GHG emission reductions (red), the path Aotearoa has committed itself to for net GHG emissions (green), and the gap in our national C accounts that needs to be filled by forestry (blue dashed line) (Evison & Mason, Forthcoming). The graph and an accompanied analysis of our options will soon be submitted to a peer-reviewed scientific journal.

Reality is a bit more complicated than the situation represented by Figure 2 because we have not consistently planted the same area of new forests in Aotearoa each year. The last large-scale afforestation programme occurred during the 1990s, and many of those forests will be harvested in the 2020s, effectively reducing our carbon storage in forests, and this needs to be taken into account if we wish to genuinely reach net GHG neutrality by 2050. I’ll show that later, but for now let’s consider what factors influence the rate of CO₂ sequestration/hectare and the maximum amounts of CO₂-e storage in new forest sinks.

C sequestration by forests

Three factors overwhelmingly influence both rates of forest sink sequestration and maximum storage in forest reservoirs. These three factors are:

1.       The fertility, soils and climate on sites where the forests are established;

2.       The species established on those sites; and

3.       The ways that forests are managed. We call this management “silviculture”.

Impacts of site are easily illustrated, but are complicated by the fact that, by definition, no forests are currently growing on candidate sites for new sinks. We need to estimate potential productivity by examining the impacts of soils and climate on tree physiology (Figure 3). 

Figure 3 – Megajoules/m² of solar radiation (of which ~50% is photosynthetically active) potentially useable by a species like radiata pine over the 10 years between June 2008 and June 2018 across Aotearoa. This geographical information system layer has approximately 3 million pixels, each representing 9 ha. Green = more productive, and white = unsuitable. Scales are NZTM eastings and northings.

Such a map would be subtly different for each tree species, because species differ in their responses to site conditions and pests.

If we ask which sites currently have no forest, and are not prime farmland (land use classes 5 and 6), we get a map like that shown in Figure 4.

Figure 4 – Areas in land use classes 5 and 6 that are currently unforested, coloured by likely productivity as shown in Figure 3.

Estimates of forest CO₂ sequestration rates and storage per hectare vary widely (Table 1).

Table 1 – Example carbon dioxide sequestration rates and storage by forests in Aotearoa. The first 10 entries were in natural stands, while the other examples were in plantations. In some cases below-ground C was considered while in others it was not.

Land cover	Sequestration t CO2/ha/year	Storage t CO2/ha	Below ground?	Reference
Native forest average (from national vegetation survey)		525	No	(Hall, 2001)
Native woody scrub		128	No	(Tate et al., 1997)
Manuka/kanuka shrub, ~25 years	10	238	No	(Scott et al., 2000)
Manuka/kanuka shrub, ~35-55 years		554	No	(Scott et al., 2000)
Manuka/kanuka shrub, 40 year span	7-9		Yes	(Trotter et al., 2005)
Lowland native podocarp-broadleaf forest		1238	No	(Tate et al., 1997)
Mature beech-podocarp forest		1287	No	(Beets, 1980)
Mature beech-podocarp forest		1290	No	(Tate et al., 1997)
Hard beech forest		1172	No	(Tate et al., 1997)
Mountain beech forest		938	No	(Tate et al., 1997)
Kauri, Northland, aged 67, 492 stems/ha	13.8	926		(Kimberley et al., 2014)
Kauri, Fred Cowling Reserve, aged 38, 1402 stems/ha	10.9	413		(Kimberley et al., 2014)
Kauri, Fred Cowling Reserve, aged 51, 11256 stems/ha	12	614		(Kimberley et al., 2014)
Kauri, Fred Cowling Reserve, aged 69, 1325 stems/ha	18.9	1306		(Kimberley et al., 2014)
Kauri, Taranaki, Brooklands Park, aged 50, 630 stems/ha	13.3	663		(Kimberley et al., 2014)
Kauri, Taranaki, Brooklands Park, aged 71, 630 stems/ha	14.5	1027		(Kimberley et al., 2014)
Kauri, Taranaki, Brooklands Park, aged 83, 630 stems/ha	13.4	1116		(Kimberley et al., 2014)
Kauri, Hawkes Bay, aged 48, 1700 stems/ha	20.1	966		(Kimberley et al., 2014)
Kauri, Northland, aged 36, 650 stems/ha	10.9	393		(Kimberley et al., 2014)
Totara, Northland, aged 102, 1225 stems/ha	17.4	1770		(Kimberley et al., 2014)
Totara, Northland, aged 102, 1825 stems/ha	13.3	1357		(Kimberley et al., 2014)
Totara, Northland, aged 58, 816 stems/ha	6.5	376		(Kimberley et al., 2014)
Totara, Hawkes Bay, aged 48, 1975 stems/ha	8	382		(Kimberley et al., 2014)
Totara, Waikato, aged 30, 2831 stems/ha	6.1	182		(Kimberley et al., 2014)
Kahikatea, Waikato, aged 30, 2831 stems/ha	9.6	289		(Kimberley et al., 2014)
Puriri, Bay of Plenty, aged 69, 588 stems/ha	15.2	1046		(Kimberley et al., 2014)
Red Beech, Waikato, aged 16, 738 stems/ha	9.2	147		(Kimberley et al., 2014)
Red Beech, Southland, aged 14, 1579 stems/ha	6.2	87		(Kimberley et al., 2014)
Black beech, Southland, aged 14, 1508 stems/ha	7	98		(Kimberley et al., 2014)
Pasture without grazing		11	Yes	(Ford-Robertson et al., 1999)
Pruned radiata pine on a good site, 400 stems/ha (modelled), average over three 28 year rotations		814	Yes	(Ford-Robertson et al., 1999)
Pruned radiata pine on a poor site, 250 stems/ha (modelled), average over three 28 year rotations		550	Yes	(Ford-Robertson et al., 1999)
Pruned radiata pine, 250 stems/ha to age 28 Central North Island (modelled)	33	918	Yes	(Robertson et al., 2004)
Untended radiata pine, aged 15, 2500 stems/ha, site index=23	38	571	Yes	(Yallop, 2021)
Untended radiata pine, aged 15, 1250 stems/ha, site index=23	34	514	Yes	(Yallop, 2021)
Untended radiata pine, aged 15, 625 stems/ha, site index=23	27	401	Yes	(Yallop, 2021)

 

Rules for the national emissions trading scheme (ETS) specify that forests planted on land that was unforested in 1990 can earn carbon credits called New Zealand Units (NZU). One NZU is meant to represent 1 tonne of CO₂ removed from the atmosphere as trees grow. Land areas larger than 100 ha can be measured at various times and the tonnes of C stored can be estimated. However, if a forest owner’s land area is less than 100 ha then they are required to use default “lookup” table for sequestration. For some species, such as radiata pine, the tables vary with region, but for others there is simply one table. Tables tend to be conservative.

There is one lookup table for all native forests, which is a simple Gompertz yield equation based on data from 52 sites that rises to an asymptote of 445 t CO₂-e/ha assuming no water deficit (Payton et al., 2010). Clearly this table has too low an asymptote for many of the indigenous forests quoted in Table 1, and it was intended to be used for young forests established after 1990. Almost all the 52 sites measured contained manuka, kanuka and/or gorse with a few emergent native hardwoods.

Paula Yarur Thys (2021) measured C storage in planted native forest stands on Banks Peninsula, Canterbury up to 59 years after planting, and compared their C storage to those estimated by the lookup table assuming a water deficit (Figure 5). She found that data were highly variable, that they more or less agreed with the lookup table for young stands, but older stands had C storage exceeding that shown in the table. Moreover, a nationwide survey by Beets et al. (2009) demonstrated that many natural stands exceeded the asymptote in the lookup table (Figure 6).

Figure 5 – Measured CO2-e storage (blue triangles) versus age, and the Ministry for Primary Industry’s carbon sequestration lookup table for native forests on dry sites in Canterbury (red line).

Figure 6 – C storage in Aotearoa’s native forests estimated by Beets et al. (2009)

Kimberley et al. (2021) reported some new observations of sequestration rates in plantations of native species, including a number from Kimberley et al. (2014), but some from 2014 appear to be missing from the 2021 graph and so I have added them (Figure 7). Some of the implied sequestration rates they found are quite high for native species, and so this is encouraging. However, as sequestration rates vary also with site and stand management, I asked where their plots were in the landscape and how the stands were managed. Mark Kimberley replied, “They are scattered across the country, more in the North Island than South Island.” He also assured me that full details will be provided when a paper is prepared for peer review. If these plots were repeatedly measured then families of curves might be fitted in order to represent sequestration rates and carbon storage on a wide range of sites and stand management practices. It is difficult to judge how the reported rates might apply across the range of sites available for carbon forestry in the absence of detailed plot information, and repeated measures would provide us with a more realistic appraisal of variation in sequestration rates.

Figure 7 – Estimates of CO2-e storage in kauri and totara plantations on a range of sites at a range of stems/ha Kimberley et al. (2021) (triangles) and some extra points from Kimberley et al. (2014) (circles). Lines are fitted to the 2021 data.

Radiata pine sequestration rates can be estimated by using growth and yield models of stem dimensions combined with a carbon estimation model called C_CHANGE (Beets et al., 1999) or by applying individual tree biomass models (Moore, 2010) to stem measurements in tree lists from inventories. Thousands of permanent sample plots are available for the construction of growth and yield models, and thousands of inventory plots are established in Aotearoa’s plantations each year. A small number of very large trees have been assessed for biomass, however, which limits the applicability of C_CHANGE and Moore’s biomass models. Growth and yield measurements of stems are more sparse for other species, and comprehensive biomass data are rarely available.

Lookup tables for exotic species may greatly underestimate actual sequestration and storage. A study undertaken in a 7.5 ha experiment at Rolleston, Canterbury, across a range of stems/ha, combined with destructive harvesting of 476 local trees for biomass estimation showed that the Canterbury lookup table underestimated CO₂ sequestration of radiata pine by up to 63% (Yallop, 2021) (Figure 8). For reference, this site has a very low site index of 23, and site indices over 40 have been recorded in other parts of Aotearoa. A study currently underway at the School of Forestry, University of Canterbury, will examine how assessed sequestration rates compare will directly measured rates on a wider range of sites and with a wider range of stand management practices.

Figure 8 – C sequestration and storage by radiata pine over 15 years at 2500, 1250, and 625 stems/ha and with two levels of weed competition control: 2 years (N) versus 4 years (H) on a poor site in Canterbury (Site index=23) compared to the Ministry for Primary Industry’s default carbon sequestration lookup table for Canterbury (in purple) (Yallop, 2021).

In summary, forest sequestration rates and carbon storage vary with site quality, species, and stand management. Sequestration rates in highly stocked stands of some native species on highly productive sites can approach 2/3 of those observed in radiata pine stands at lower stockings on average sites, but in many cases sequestration rates and maximum storage of C in native forests appears to be much lower than that achieved by our most rapidly-growing exotics. Moreover, native plantations appear to take longer to reach their highest rates of sequestration. The lookup tables for exotic species may be very conservative, and those for native forests need to be more diverse, reflecting the wide range of sequestration rates and storage values recorded in plots. The lookup table for natives may be roughly right for some shrubs such as manuka & kanuka or young stands of trees, but the level of maximum storage (the asymptote) clearly underestimates what has been observed in some older, high forest stands.

The case for exotic tree species

Many imported species grow and sequester CO₂ much more rapidly than native species within the time frames required to meet our 2050 target. Radiata pine has been chosen as an example for the following reasons (although other species such as dryland eucalypts might do the job equally well or even better in some cases):

1)    It grows rapidly and sequesters C at a much higher rate than native species. Between 2008 and 2012, our national carbon accounts indicate that radiata pine planted after 1990 sequestered at an average rate of 34 tonnes of CO₂-e/ha/year, and rates might be even higher with silvicultural regimes aimed at maximising value from sequestered carbon credits. By contrast, estimated rates of sequestration for native species are often below 10 tonnes of CO₂-e/ha/year during the years immediately following forest establishment (Scott et al., 2000; Trotter et al., 2005), and the slower development of young native stands would mean that they would take longer to begin effective sequestration. In older indigenous stand higher rates have been reported on some sites, but not at the rates typical of radiata pine. To be fair, studies of native forest sequestration are sparse, as outlined in the previous section, but we can also get an idea of relative sequestration rates by comparing the more numerous reports of growth rates of stems of various species (Pardy et al., 1992; Silvester & McGowan, 1999), and native species often take 3-4 times longer to reach equivalent stem volumes of radiata pine plantations at harvest even at higher stockings.

2)    We are experts at producing seedlings for exotic species and they are cheap.

3)    Radiata pine 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)    Radiata pine is not a high country wilding risk (Ledgard, 2008).  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 (which is a rare condition in New Zealand). Our wilding species are commonly other, more hardy imports, such a P. contorta, P. ponderosa, P. nigra and Douglas fir. These wilding risk species should be avoided in carbon forests.  Relative to these other species and areas of plantation, radiata pine is only rarely a wilding, and this is on lowland, ungrazed sites.

5)    On warm, moist sites (either medium or high productivity categories), exotics can 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 9). Understoreys of native vegetation are common in plantations on such sites (Brockerhoff et al., 2003; Ogden et al., 1997). This issue has been much studied by a PhD graduate from the School of forestry named Adam Forbes (Forbes et al., 2015a, 2015b, 2016). In order for native forest to regenerate under pines local native seed sources are essential.

6)    Studies suggest that radiata pine will continue to sequester carbon for up to 100 years on some sites (Woollons & Manley, 2012).  This means that exotic forests could remain as sinks for some considerable time.

Afforestation with exotics and conversion to native forest

I love our ngahere, and I would be delighted to be able to recommend that all our carbon forests should comprise native species, but unfortunately afforestation with native species is very expensive, and the sequestration rates of native species are not only lower than those of cheap exotics, but they take decades longer to reach appreciable rates even on some of the best sites and at high stockings (Figure 7 and Table 1).

Figure 9 - 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, Nothofagus fusca, and N. solandri var. solandri Figure from Hall (2001).

Most of our imported, exotic forest plantation species are pioneer species, intolerant of shade, and although they can be regenerated under a canopy, the canopy needs to be exceptionally sparse before any appreciable amount of regeneration will occur. Figure 10 shows how sparse the canopy was after an attempt at continuous cover forestry with radiata pine in the foothills of Canterbury.

It therefore makes sense to consider the option of planting exotics in permanent C forests and then converting them to native forests once the exotics have completed their task of filling the gap in our national C accounts that is critical over the next few decades.

Establishment of native species in some areas of our exotic plantation forests occurs with no intervention as a transition from mostly exotic weeds in young stands to increasingly native species in old stands (Brockerhoff et al., 2003; Ogden et al., 1997), but as noted by Forbes & Norton (2021), this process is not guaranteed to occur in all stands. Proximity to seed sources, extent of small scale disturbance, lack of a moisture deficit and fertility may all promote an indigenous understorey (Figure 11).