Within the formerly cleared or farmed areas of Great Barrier, the pattern of manuka on exposed ridges and upper northerly slopes, and remnant broadleaf forest patches in the gullies and lower south-facing slopes, is common. The mid-slopes are usually dominated by kanuka, which is currently the commonest tree on the island. Manuka is the pioneer species invading open grassland on the cessation of burning or grazing, usually followed by kanuka a decade or so later. Because it is longer-lived, and grows taller than manuka, kanuka comes to dominate the mixture. This process of replacement of manuka by kanuka is much slower on the ridges, probably because manuka is more tolerant of the low soil fertility and generally harsher conditions there. Moreover, where kanuka replaces manuka on the better soils of mid and lower slopes, it is itself replaced by other tree species, which germinate and grow in its protective shade. Consequently, stands of kanuka are frequently ‘nurse crops’ for more diverse forest communities, the nature of which will depend to some extent on the site history, topography and aspect, and to some extent on the proximity of seed sources and transporters such as pigeons. These processes of succession, and the survival of remnant forest patches in areas protected from fire, readily explain the landscape pattern in much of the lowland zone of Great Barrier.

In mature forest, dominated by kauri, podocarps, or broadleaved trees such as kohekohe, puriri, tawa or taraire, the rate of growth (photosynthesis) is balanced by the rate of loss in respiration and dead plant parts. Over any reasonable time span there is no net change in biomass and no net carbon gain. In a young forest however, such as the widespread manuka and kanuka stands, growth represents a real gain of carbon as biomass increases.

Carbon sequestration refers to the ‘locking up’ of carbon in vegetation – largely wood. Atmospheric carbon dioxide is converted into organic products such as lignin and cellulose (the main constituents of wood) via the process of photosynthesis. Wood is roughly 50% carbon. If these plant products remain undecomposed for long periods, then this process effectively removes (‘sequesters’) carbon from the atmosphere. Many tree species can live for centuries, while even after death their woody remains decay (returning carbon to the atmosphere) only slowly. Thus trees, especially fast growing trees such as pines and eucalypts, can remove a lot of atmospheric CO2, and this has been seen as a way of mitigating CO2 emissions. The proposed emissions trading scheme in New Zealand will give ‘carbon credits’ for tree plantings and natural regeneration, larger than a certain area, and occurring since 1990.

Much of the tea tree (manuka and kanuka) regeneration on Great Barrier commenced growth earlier than 1990, but it is still growing rapidly and accumulating carbon. In view of the large extent of this type of forest it seems prudent to make some estimates of the carbon content and rate of accumulation, in anticipation of the subject becoming more important to the world, and hence national, economy in the near future.

The ‘Kyoto protocol’ (the international agreement to which New Zealand is committed) relates only to ‘forest’ established since January 1990. It is proposed that the clearance of forest, or tea tree scrub, established since then will invoke a cost dependent on the tonnes of carbon present in the cleared vegetation. Actually it is the carbon dioxide equivalent (CO2e), which is relevant. This is obtained by multiplying 50% of the estimated dry weight of organic matter per hectare by 3.667.

A carbon credit, or ‘emissions trading unit’ (ETU), is 1 tonne CO2e/ha/annum. If the proposals go ahead – they are currently stalled – owners of sufficiently large areas of scrub, reverting since 1990, will be able to claim credits for the ETU’s gained by the scrub. However, these units are measured, not over the whole time period since 1990, but over the “first commitment period” (CP1), which is the five years from Jan. 2008 to Dec. 2012.

The rate of sequestration by tea tree has been set at a ‘default’ value of 3 tonnes C02e/ha/annum – considerably lower than some estimates of the actual C02 sequestration rates. It is proposed that when actual (plot based) estimates are available, the sequestration rate accepted for valuation will be based on the ‘lower 90% Confidence Interval’ of the mean estimate. This likewise results in a very conservative estimate, so that the government has a very small chance (1 in 20) of paying out for more CO2 than has actually been sequestered (Ref. 5).

So what we need to know is how much C02 has been sequestered by kanuka stands on GBI since 1990, how many ETUs will be sequestered per annum between 2008 and 2012, what area of the island is covered in such forest, and how much one ETU will be worth in dollars. Although such information is only of theoretical interest at the moment, because the Carbon Emissions Trading Scheme is not yet finalised, it could well become important in future.

In a study to investigate the use of kanuka as a firewood species in 1953, R. J. Lyttle measured five plots in the Whangaparapara area and one in the Kaiaraara (Ref. 2). Lyttle’s plots were carefully chosen to represent the typical growth of a kanuka stand from inception (mixed with manuka) to maturity, covering the age range from 2 to 70 years. They are typical of kanuka height growth throughout the island, which is rapid and linear for the first fifty-sixty years, but slows down to almost zero in the oldest stands (Fig 1). Lyttle’s data allows us to estimate the weight of carbon/hectare and the overall rates at which this has been accumulated (Table 1). The figures in column four of Table 1 are considerably lower than a ‘whole stand’ C estimate would be because they exclude roots, forest floor debris etc. Thus, they are conservative values.

The median and average values of CO2 sequestration are 9.9 and 8.95 tonnes/ha/annum respectively. These figures can be compared with values of 10—11.4 tonnes CO2e/ha/annum obtained from Landcare’s ‘Carbon Calculator’ (Ref. 1) for low – medium quality sites on Great Barrier, and with values ranging from 6.6 to 12.8 (average 7.3) tonnes CO2e/ha/annum obtained for kanuka stands by Trotter et al. (2005) and Walcroft et al. (2002) (Refs. 3 and 4). Thus, the Great Barrier figures in Table 1 are within the expected ranges. The lower 90% confidence limit on the mean given in Table 1, is 5.4 tonnes CO2e/ha/annum, and this can safely be used as a minimum value on which to base ETUs.

According to Landcare estimates (LCDB2), manuka and kanuka communities cover 14,742 ha (54%) of Great Barrier. If we assume that 10% of this can be classified as ‘ridge top manuka’ then the area of kanuka scrub on Great Barrier is approximately 13,268 ha., and at 5.4 tonnes CO2e/ha/annum this is accumulating c. 71647.2 tonnes/CO2e/ha/annum. The remaining ridge top sites are probably accumulating at c. one quarter the rate, giving a further 1990.2 tonnes, for a total of 73637.4 tonnes/CO2e/ha/annum. At $15 per tonne (the minimum suggested value of an ‘emissions trading unit’) the scrub communities on Great Barrier would have an overall value of approximately $1.1 million per annum. Even if the ‘default value’ of 3 tonnes CO2e/ha/annum is used, this still ‘values’ the kanuka/manuka scrub at c. $614,000.

However, both these figures are entirely theoretical at present, partly because the parameters of the New Zealand scheme have not yet been finalised, and also because the Kyoto agreement applies only to native forests ‘established’ after 1990. Most of the tea tree areas on Great Barrier result from abandonment of farming land, or fires, before this date. However, some areas could still qualify, if they were not ‘forest’ in 1990. What constitutes ‘forest’ has not yet been clarified, but it has been suggested that stands less than 5m tall in 1990 would not be classified as ‘forest’ at that time. Applying equations relating stand height and age derived from measurements of kanuka growth on Great Barrier, indicates that kanuka stands on slopes will have increased in height by c. 4.7m since 1990, so that arguably any stands currently less than c. 9.7m tall could qualify. Even if only 10% of the island’s kanuka stands could qualify, the default value used, and the ETU set at the minimal $15 value, this still represents a minimum value of over $60,000 per annum island wide – for land which would otherwise bring in no income. Aerial photographs could be used to establish the actual area of this younger forest with greater precision.

Kanuka was formerly regarded as either a weedy nuisance or a firewood crop. Over the last few decades, on steep low fertility hill country, unsuitable for farming or even pine plantations, it has become clear that the scrub cover helps to bind the soil and reduce run-off and flooding. It has also been recognised as important for honey production, and has biodiversity value in promoting the establishment of native forest. Use of kanuka for firewood may continue on Great Barrier, where other fuel sources are not available or expensive, although it conflicts with its role in C-sequestration and in the establishment of more mature forest types. Overall, kanuka scrub can now be promoted as a potentially significant economic benefit for Great Barrier Island, in anticipation of the time when New Zealand finally decides on the detail of how it will meet its international obligations to slow down the rate of global warming.

References etc:

(1)        Landcare carbon calculator: www.landcareresearch.co.nz[home>services>carbon-calculator].

(2)         Lyttle, R. J. 1953. Report on tea tree – Great Barrier Island. Ranger’s Proficiency Course No. 2. Auckland Conservancy A1164554. From D.o.C, Port FitzRoy.

(3)         Trotter, C. et al. 2005. Afforestation/reforestation of New Zealand’s marginal pastoral lands by indigenous shrublands: the potential for Kyoto forest sinks. Annals of Forestry Science 62: 865-871.

(4)         Walcroft, A. et al. 2002. Biomass accumulation in young regenerating shrubland ecosystems on marginal pastoral farmland. Landcare Research Contract Report LC0102/121 June 2002. 16pp.