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.

Gondwana disintegrates

About 80 million years ago Gondwana, that enormous southern continent, started to shed a portion of herself that moved eastwards. The large rafting piece of land, which we know now as Zealandia, carried with it a wide selection of biota originating from the mega continent with current affinities with land we now call Antarctica, South America and of course Australia.

Aotearoa arises

The islands that we know as Aotearoa are the tiny visible remnant, just 6%, of that migrating continental mass which now lies under the sea. We know that by 60 million years ago Zealandia and Aotearoa were sufficiently far from Australia to begin to develop some unique species and to lack some important Australian plants and animals. The upheavals of the land mass, tectonic, volcanic, and simple passage of time have left us with a couple of small islands, much of which is geologically recent. We know that we lie on a plate boundary as well as a massive active volcanic field, part of the Pacific Ring of Fire and the stochastic events arising have had dramatic and far-reaching effects on our landforms and our biota.

Ice ages and extinctions

However, the most recent and arguably the most dramatic effects on both plants and animals have been the extreme glacial events of the past two million years, culminating in the most recent glaciation which reached its peak just fifteen thousand years ago. The effects of these events have been dramatic, resulting in well documented extinctions of many plant species and the resulting retreat of species into ice age refugia. Whereas in continental land masses species can migrate north and south in response to climate changes, in an island there is no place to go and extinctions and disjunctions abound.

Homo enters

So what did we find when the last of the planet’s human migrations first arrived at these islands? Massive forests of conifers and beech species covered 80% of the land with only the high mountains free of forest. The plants and animals present presented a unique range of species with 80% unique to these shores. But we now know that what appears to be such a rich array of diversity is in many ways depauperate. We have but 2500 species of native plants which is just 0.6% of the world’s plant species. We have less than twenty widespread canopy tree species. Some countries, especially in the tropics, have many hundreds. We have but two native mammals and only 91 land-based bird species. While our long isolation from other land masses accounts for the uniqueness of our biota, that isolation, and to a great extent the past ice ages, have caused the loss of many species.

Homo does what Homo does

Human activity has accelerated the loss of species and has continued unabated. It started slowly some 800 years ago with initial colonisation of the land and it is shown that 6.7 million hectares of forest was burnt prior to European arrival. Then in the past 150 years the rush to convert Aotearoa into a large pasture has seen another 6.2 million hectares removed, reducing our native forest cover by some 70%. The removal of forest from our hill country has been a disaster for our soils, for our water, and for our biodiversity. In geological time hills and mountains slowly and sometimes violently erode under the strong forces of gravity to reach the lowland and eventually the sea. But by removing our forest cover we have accelerated that process, in the same way we accelerated species loss, by a thousand-fold.

The evidence of that is easy to see on a high proportion of hill country. The tiny terraces called terracettes that adorn so much of our high country are evidence of soil creep. They are formed by gravitational movement down slope, exacerbated by animal tracking, but initiated, maintained and amplified by gravity. This is the mechanism whereby our hill country soils become depleted and our lowlands enriched.

New Zealand is a very hilly country: 37% of our land mass is at a slope of 15 degrees or more, that is 10 million hectares. That of course includes our high country which is naturally erosion prone and out of production now. What we need to concentrate on is our hill country, particularly erosion-prone grazing areas.

The Ministry for the Environment (MfE) tells us there are 830,000 hectares of erosion prone grazing land in the North Island alone. There are well over a million hectares of steep land in New Zealand where a thin mantle of soil is held in place by shallow grass roots.

Even on gentle slopes the evidence of this insidious soil creep is widespread and obvious. We see the effects of this soil movement in our streams when the headwaters of our rivers are not protected. The downstream siltation and turbidity of our rivers can be traced back to this feature of our hill country. The next step, also too frequently seen, comes when this chronic problem becomes acute. The sliding creep gives way to minor subsidences that expose the bedrock and create small landslides

But there is a better way; here we see natural regeneration of mānuka on those same slopes, healing the land, providing a nursery for native trees to thrive, slowing down runoff and eventually allowing the full range of tree species to thrive.

And this is the result of natural succession on those same slopes, healed hillslope providing the full range of ecosystem services for flood control, soil stabilisation, biodiversity, and massive carbon storage.

While the image shows what a healthy hillslope in New Zealand could look like, attempts to accelerate the process by planting and management are being applied. The mānuka honey boom has inadvertently created an opportunity for these hillslopes to be clothed again, from which, given neighbouring seed trees, native forest species will arrive.

But the reclothing of our eroding hills becomes urgent as we enter uncertain times with climate change bringing higher intensity rainfall to some of these areas and the slow depletion of soil mass makes it harder and harder to force the growth of grass.

A related, but essentially similar need, is to wrap native forest around the headwaters of all our streams. These two issues go hand in hand, and we need to realise that the health of our lowlands depends on the health of our uplands for soil stability and for water quality.

The problem of soil erosion has many parallels and connections with the whole climate change issue. A slow creep finally catching up with our ability to both comprehend and act.

Does that ring some bells with our response to the totality of the climate change problem?

But serendipitously they can also be connected in a way that has a multitude of positive outcomes. As it becomes obvious that much of this eroding hill country becomes less and less productive in the traditional agronomic sense, it presents a splendid opportunity to provide one of the carbon sinks advocated by the Climate Change Commission.

Planting these slopes in trees which return the ecosystem services that these hillsides evolved into over millions of years has innumerable benefits – in soil stability, water retention and purity, biodiversity, carbon capture and landscape values. It would be so much better to have our hills clothed in native forest.

The O TātouNgāhere (Our Forest) body of work forms the basis of a plea to return at least a portion of our hill country into native forest. It is a big ask and demands that we agree to value our long-term wealth and even survival. There has been a massive retreat of farming from some of our hill country, especially when stock subsidies were removed, and acknowledging the need to remove significant carbon from the atmosphere in the long term we will need to again identify where we can site the extensive forest plantings advocated by the various environmental groups including Tāne’s Tree Trust and the Climate Change Commission.

Prof. Warwick Silvester