Two somewhat related matters in New Zealand have influenced our thinking about including native forests in this discussion. 

Firstly, the exceptional rates at which the exotic radiata pine (Pinus radiata) grows in this country. A combination of a rather extraordinary tree backed up by sixty years of research, improved technology, and a very experienced and capable team of practitioners, has resulted in a tree and a forest with outstanding production and therefore carbon assimilation properties. 

Secondly, as already noted, native forest was removed from most, if not all, the fertile soils of the lowlands and our current impression of native forest growth is consigned to our most difficult country and reverting secondary forest.

Remarkably limited measures of carbon sequestration by native forests have been made in New Zealand. However, Tāne’s Tree Trust (TTT) has produced its Carbon Calculator for Planted Native Forest based on the Trust’s Indigenous Plantation Database (Bergin and Kimberley 2012). This database of measurements of planted native forest and shrubland is based around a comprehensive survey completed in 2010 from throughout New Zealand but also includes earlier measurements collected over several decades. 

Although many of the planted native stands represented in the database are small and have not been well managed, this database constitutes the largest and best available information on growth rates of plantations of native trees that we have in New Zealand with over 10,000 measured native trees and shrubs. Stand densities are relatively high with surveys targeting mostly single-species tree plantations with an average density of 1450 stems per ha. Shrubs were mixed-species plantings with an average density of 4075 stems per ha. 

Photo credit Alistair Guthrie @alistairguthrie

Measures of carbon sequestration in trees and forests are derived from the annual increase in stem, branch, foliage, and root mass, which includes a high proportion of molecular carbon. While different species of tree, and even the same species growing in different locations, may have somewhat different wood densities (and therefore different quantities of stored carbon), it is relatively easy to relate volume production in forests to tonnes of CO₂ stored per hectare. Methods for doing this are provided by Beets et al. (2012, 2014). Dividing this total by its age provides the growth and thus sequestration rate as a Mean Annual Increment (MAI) in units of tCO₂ ha^-1 yr^-1.

Because growth varies through the life of a tree, growth at a particular point in time is referred to as the Current Annual Increment (CAI) and is derived from measurements made over shorter intervals of time. As will be shown, this can increase markedly as trees become established.

In any situation or on any site there is a range of production that can be achieved by forests. At the low end (such as in very young forest or forests of slow growing species), annual rates of only 1-4 tonnes of growth per ha may be typical. With fast-growing species values significantly greater than 30 tonnes can be achieved. A range of 4-30 tonnes of CO₂ sequestration per ha per annum is probably typical of MAIs for New Zealand situations although rates outside this range do occur.

The Look-up Tables published by The Ministry for Primary Industries are used by small forest growers to estimate carbon stock for the Emissions Trading Scheme(ETS). The Look-up Table for radiata pine lists its carbon MAI over 50 years as between 21 and 27 tCO2 ha^-1 yr^-1 depending on region, which is recognised as at the higher end compared with what forests in other parts of the world can achieve. 

Only one Look-up Table is provided for indigenous forest, and this lists the carbon MAI over 50 years as 6.5 tCO₂ ha^-1 yr^-1. It is this value that many associate with native forest sequestration including planted native forest. However, this table was derived from measurements of naturally regenerating shrubland. A recent study of native forest carbon sequestration rates undertaken by Kimberley and Bergin (2021 in preparation) covers  various forest types ranging from naturally regenerating native scrub through to planted and managed native forest stands. This latter work represents the only measured native forest that is in any way comparable to the many thousands of biomass and carbon measurements made for radiata pine in New Zealand.

Importantly, this study clearly demonstrates that planted and managed native trees and forests exceed the Look-up Table rate of 6.5 tCO₂ ha^-1 yr^-1 by significant margins and it is clear that the Look-up Tables need to be adjusted to reflect this situation. The following sections summarise the results of this study firstly for naturally regenerating shrubland and forest, and then for planted native forest.

Photo credit Alistair Guthrie @alistairguthrie

Carbon sequestration rates by regenerating native shrublands are available from an inventory of New Zealand natural post-1989 forest carried out by the Ministry for the Environment in 2012 (Beets et al. 2014). The carbon MAI of measurement plots in this inventory ranged from 1.6 to 17.7 tCO₂ ha^-1 yr^-1 and averaged 7.0 tCO₂ ha^-1 yr^-1, close to the Look-up Table value. This inventory covered natural shrubland that has established since 1989 and therefore consisted of stands mostly less than 25 years old. 

Carbon sequestration rates in regenerating native shrubland covering a wider age range are provided from data collected by Bergin et al. (1995) in naturally regenerating manuka/kanuka shrubland in the East Coast of the North Island. Manuka dominates these shrublands until about age 15 years and then declines with kanuka becoming dominant. Carbon sequestration averaged about 10 tCO₂ ha^-1 yr^-1 over the first 20 years but then slowed considerably with little additional sequestration occurring beyond age 30 years (Figure 1).

While sequestration in manuka/kanuka shrubland declines rapidly after 30 or so years, regenerating climax species such as totara can often naturally enter the mix, eventually outcompeting the shrub species. Such species can continue to sequester carbon over many decades. Data from a study of naturally regenerating fully-stocked totara-dominant stands sampled in Northland (Bergin 2001), show carbon sequestration rates averaging 10 tCO₂ ha^-1 yr^-1 continue to be maintained over the 120-year age range covered by the study (Figure 1).

Figure 1. Carbon sequestration in regenerating manuka/kanuka shrubland on the East Coast of the North Island and in regenerating totara-dominated forest in Northland.

As most of the focus is on carbon assimilation by radiata pine in New Zealand, and with the data invariably from planted and managed stands, it is essential to compare like with like in an objective analysis. Data from the Tāne’s Tree Trust Indigenous Plantation Database  provides the most robust information available for planted native forest. 

Growth and yield information in planted stands containing the two most commonly planted native tree species, totara and kauri, show a trend of gradual increase as stands age. Growth is generally slower initially but increases at least up until the maximum ages of stands in the study, 70 years for kauri and 100 years for totara (Figure 2). 

On average, kauri stores carbon more rapidly than totara with an average MAI of 16.4 tCO₂ ha^-1 yr^-1 at age 50 years. Totara has a somewhat slower increase in carbon sequestration than kauri at a comparable age, with the MAI averaging only 10.0 tCO₂ ha^-1 yr^-1 at age 50 years, although this increases to 15.1 tCO₂ ha^-1 yr^-1 by age 100 years. The lower carbon sequestration rate of totara is in part due to its lower wood density compared to many other native conifers. 

Figure 2. Carbon sequestration in planted kauri and totara forests.

Of course, this data is still limited as planting is often still in a “trial” capacity, landowners have planted species of interest to themselves with less regard for provenance, the appropriateness of the site or the sort of silviculture that should be applied subsequently. Nevertheless, some work has been done which demonstrates that with attention to forest husbandry some even more impressive results can be obtained. 

Further analysis of the TTT database provides the table below which gives a summary of MAIs for a range of New Zealand species. Sequestration rates for planted kahikatea, rimu, puriri and beech are comparable to those of totara and kauri, with MAIs averaging between 10 and 15 tCO₂ ha^-1 yr^-1 at age 50 years (Table 1). For comparison, the Look-up Tables for radiata pine give average sequestration rates of 21 to 27 t CO₂ ha^-1 yr^-1 at age 50 years.

Sequestration rates of planted native tree species are low during the first few decades after planting with MAIs averaging only 3-6 tCO₂ ha^-1 yr^-1 at age 20 years. However, plantings of mixed native shrub species have much higher rates averaging 17.8 tCO₂ ha^-1 yr^-1 at age 20 years, partly because of the high stockings they are generally planted at, but also because many of these species have fast early growth. It is common practice to establish mixed plantings of native tree and shrub species with the shrub component boosting the early carbon sequestration until the tree species become dominant after several decades.

Table 1. Average carbon sequestration mean annual increment (MAI) of commonly
planted native tree species and mixed shrub species.

The carbon sequestration curves for planted tree species (Figure 2) show a trend of productivity increasing steadily with age. This increase in growth rates from age 30 years can be seen in Table 2 which shows CAIs by species and age. At 50 years, while totara is under 20, all other conifer and hardwood tree species have CAIs between 20 and 30 t CO₂ ha^-1 yr^-1.

Table 2. Average carbon sequestration current annual increment (CAI) of commonly
planted native tree species and mixed shrub species.

Photo credit Michael Bergin @mikeb_nz

A 102-year-old stand of planted totara in Northland with a stocking of 1640 stems per ha is estimated to have sequestered 1639 tCO₂ ha^-1 since planting, representing an MAI of 16.4 tCO₂ ha^-1 yr^-1.

A grove of kauri planted in 1963 in the Holts Forest Trust Sanctuary, Hawkes Bay. At 48 years old and a high stocking of 2000 stems per ha it is estimated to have sequestered 1100 tCO₂ ha^-1 since planting representing an MAI of 22.9 tCO₂ ha^-1 yr^-1.

The widely held view that New Zealand native forests are slower growing and accordingly slower to sequester carbon, as indicated by the MPI Look-up Tables, may be discouraging landowners from planting native trees, even where it is their preference to do so. Worse, it may be leading to planting advice that is incorrect and not helpful at a time when any form of tree planting by landowners is a bonus. 

Analysis of Tāne’s Tree Trust data from planted native trees still supports the position that radiata pine is initially faster growing and simpler to manage, but the difference between carbon sequestration in radiata pine and well managed planted native forest is much less than is often suggested. And investment in research and development would benefit native forestry as it has the radiata-pine industry, i.e., result in increased growth rates and more knowledge around forest management.  

New Zealand’s Carbon Look-up Tables for the Emission Trading Scheme should include the option for planted native forest as well as regenerating native forest. The current Look-up Tables for native forest are accurate when applied to naturally regenerating shrubland. However, to achieve good levels of sequestration over a long timeframe, regenerating forest needs to include climax tree species such as totara. 

Properly sited and managed planted native tree species are a good alternative where landowners wish to sequester carbon over long time periods, as well as enhancing natural landscapes, indigenous biodiversity and cultural values.

References

Beets, P.N., Kimberley, M.O., Oliver, G.R., Pearce, S.H., Graham, J.D., Brandon, A. 2012. Allometric equations for estimating carbon stocks in natural forest in New Zealand. Forests, 3: 818-839.

Beets, P.N., Kimberley, M.O., Paul, T.S.H., Oliver, G.R., Pearce, S.H., Buswell, J.M., 2014. The inventory of carbon stocks in New Zealand’s post-1989 natural forest for reporting under the Kyoto Protocol. Forests, 5, 2230-2252.

Bergin, D.O. 2001: Growth and management of planted and regenerating stands of Podocarpus totara D. Don. PhD thesis, University of Waikato. Unpublished. 316p.

Bergin, D., Kimberley, M. 2012. Nationwide survey of planted native trees Tāne’s Tree Trust. Technical Handbook. Technical Article 10.1.

Bergin, D.O.; Kimberley, M.O.; Marden, M. 1993: How soon does regenerating scrub control erosion?  New Zealand Forestry, 38(2): 38-40.

Bergin, D.O.; Kimberley, M.O.; Marden, M. 1995: Protective value of regenerating tea tree stands on erosion-prone hill country, East Coast, North Island, New Zealand.  New Zealand Journal of Forestry Science, 25 (1):  3-19.

Intergovernmental Panel on Climate Change. (2014). Technical Summary. In Climate Change 2013 – The Physical Science Basis: Working Group I Contribution to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (pp. 31-116). Cambridge: Cambridge University Press. doi:10.1017/CBO9781107415324.005 

Kimberley, M.O..; Bergin, D.O. 2021: Rates of carbon sequestration in planted and regenerating New Zealand native forests. Manuscript in preparation.

Reed, A.H. (1953). The Story of the Kauri. A.H. Reed & A.W. Reed, Wellington, NZ. 439 p.

Concerns about elevated atmospheric CO₂ usually focus on their biggest source –  combustion of fossil fuels. However, forests also play a critical role. Trees store enormous quantities of carbon in their stems, roots and branches and when a forest is destroyed, there is a release of CO₂ into the atmosphere. It is believed that one third of human-induced carbon emissions since pre-industrial times have come from land use change, mainly forest clearance. The flip side is that recovering forests can remove large quantities of CO₂ from the atmosphere.

Native forests and carbon in New Zealand

The historic role of forests in New Zealand is much greater than in most other countries. Before humans arrived, forests covered 85% of our land area but we have reduced this to only 38%. It is likely that the CO₂ released from historic forest clearance is greater than the CO₂ released by burning fossil fuels over the past two centuries.

We can see this by considering the immense quantity of carbon stored in our remaining native forest. Over the past two decades, the Ministry for the Environment (MfE) has conducted an inventory of New Zealand’s native forests which shows they store carbon equivalent to 6.6 billion tonnes of CO₂, more than 180 times our current annual CO₂ emissions.

However, although our native forest stores a vast quantity of carbon, MfE’s forest inventory indicates it is currently carbon neutral. This is because the forests are mature, and the CO₂ their growing trees remove from the atmosphere is in balance with the CO₂ lost by decaying wood from dead trees.

Beginning to reverse our forest loss

New Zealand’s climate and soil are well suited to forestry and beginning to reverse the forest clearance of previous centuries by planting new forests will be essential if we are to meet our target of becoming carbon neutral by 2050. Much of the focus will be on planting forests of native trees, and the current government has a goal of establishing large areas of native forest over the coming decades.

It is therefore important to have a good understanding of the rate at which planted native forests will remove CO₂ from the atmosphere. There is surprisingly little clear information on this subject although there is a widespread perception that native trees store carbon at much lower rates than many exotic tree species such as radiata pine. But how true is this?

In the rest of this article, we will refer to the ‘CO₂ removal rate’ of a forest, which we define as the weight of CO₂ removed by each hectare of forest every year. To get a better understanding of what a CO₂ removal rate means in practice, note that New Zealand’s annual CO₂ emissions are currently 35 million tonnes, or about 19 tonnes per household. Therefore, if a forest has a CO₂ removal rate of 19 tonnes, each hectare of that forest will cover the emissions of one average household. Note that a forest can continue to remove CO₂ year-on-year for many decades although the process will eventually cease once the forest is fully mature. However, so long as the forest remains intact, the carbon will remain safely stored in its trees.

Look-up tables for exotic and native forest

One obvious source of information about CO₂ removal rates is the look-up tables published by the Ministry for Primary Industries for use by small forest growers in the emission trading scheme. Note that forest growers with more than 100 hectares are required to use the Field Measurement Approach whereas small forest owners use the look-up tables.

According to the look-up tables, averaged over the 50 years from planting, the CO₂ removal rate for radiata pine forest is 25 tonnes, the rate for Douglas-fir forest is 19 tonnes, and the rate for other exotic softwoods is 13 tonnes. However, the rate for native forest is only 6.5 tonnes.

It is hardly surprising that radiata pine which has been selected by foresters for its exceptional growth rate in New Zealand conditions leads the pack.

But are native forests really such mediocre carbon storers?

How reliable are the native forest look-up tables?

The answer is that they are probably not very reliable for planted native forest as they are based on data from regenerating shrublands (MPI 2017), which have less carbon storage potential than taller forests.

Examples of planted native stands

A better source of information is Tāne’s Tree Trust (TTT) whose aim is to assist New Zealand landowners plant and manage native trees. Tāne’s Tree Trust has been collecting tree measurements from stands of planted native trees for many years. Planted native stands are often small in scale, poorly managed, and on less productive land. Nevertheless, the database TTT has amassed represents the most comprehensive set of planted native tree and shrub measurements available.

By applying the same carbon models used in MFE’s forest inventory. Tāne’s Tree Trust have been able to estimate the carbon from their measurement data of planted native trees and shrubs. Here are three examples of planted native conifers from the Tāne’s Tree Trust database:

However, not all planted stands of New Zealand natives will achieve these levels of carbon storage. Analysis of the database indicates that high CO2 removal rates only occur in stands with sufficient numbers of trees per hectare, with all the above examples having more than 1,000 trees per hectare. Secondly, these examples are also from reasonably old stands, with ages ranging from 40 to 102 years and we generally find that native tree species require several decades from planting before they achieve high CO2 removal rates.

The hare and the tortoise!
The potential carbon sequestration of planted native forest can be examined in more detail using the TTT Carbon Calculator for Planted Native Forest which was developed from the TTT plantation database. The Calculator predicts that a stand of native trees planted at 1,250 trees per hectare has a CO2 removal rate of only 6 tonnes over the first 30 years after planting, but this increases to 20 tonnes between ages 30 and 60 years, and further increases to 27 tonnes between ages 60 and 90.

This growth pattern of the CO2 removal rate steadily increasing over many decades after planting for natives contrasts with the pattern of radiata pine where the CO2 removal rate peaks before age 20 years and then gradually declines. So, for a landowner wanting to plant carbon forestry, it is a matter of choosing between the hare and the tortoise – radiata pine for a quick return or natives as the long-term option.

Shrubs as a nurse
TTT suggests that one way of filling the low CO2 removal rate in the first few decades after planting is to combine tree species with shrub species such as manuka. We recommend this style of planting for other reasons; inter-planting tree species within a nurse crop of shrubs provides essential protection for tree species in the early years following planting. On many sites, this mimics natural successional processes whereby hardy fast-growing shrub species firstly colonise exposed, open sites, ameliorating them for subsequent establishment of our usually more sensitive native tree species.

Pure shrub stands are widely planted for environmental reasons often at close spacings. The Carbon Calculator predicts that a pure shrub stand planted at a 1.5 m spacing has a CO2 removal rate of 17 tonnes to age 30 years, but the rate declines sharply beyond this age. However, if every fourth shrub is replaced with a tree seedling such as a totara or kahikatea, the CO2 removal rate of this mixed tree/shrub stand is 18 tonnes over the first 30 years, 22 tonnes between ages 30 and 60 years, and 19 tonnes between ages 60 and 90 years.

So, planting mixtures of native trees and shrubs can remove CO2 from the atmosphere at satisfactory rates from an early age. However, CO2 removal rates are not the only factors to consider in carbon forestry.

Longevity and carbon storage capacity of native forest

Two other important considerations are the life expectancy of the trees, and the carbon storage capacity of the mature forest. Firstly, life expectancy is important because carbon forests are generally intended to be managed as permanent forests, with in some cases an occasional judicious thinning when stands become overstocked, providing a potential long-term timber crop. In terms of longevity, native conifers, which are exceptionally long-lived, have a clear advantage over radiata pine.

Secondly, carbon forests should ideally have a high carbon storage capacity. Here again, native conifers should perform well. It will be centuries before we know for certain the final carbon storage capacity of planted native trees. However, we can safely infer that it will be high on the basis of examples of groves of very large native conifer trees in our natural forests which can carry high levels of carbon per hectare.

Dr Mark Kimberley