Measuring forest carbon

Forests have an important role to play in the reduction of greenhouse gas emissions. Trees absorb carbon dioxide, one of the main greenhouse gases, from the atmosphere. They convert the carbon component to sugars, which are solid forms of carbon, and release oxygen back to the atmosphere. The carbon is stored in trees’ leaves, branches, stems and roots and accounts for about half of the dry weight of a tree.  

As forest covers about 37 per cent of New Zealand’s land area, it has a significant impact on the carbon cycle in New Zealand.

Carbon is measured in all forests. New Zealand has defined forest land as land:

• on which the tree species reach a height of at least 5 metres
• that is greater than 1 hectare
• with greater than 30 per cent canopy cover.

Forest land can also be land with the potential to reach the above parameters under its current management.

For reporting of greenhouse gas emissions

New Zealand’s forests are divided into those that were:

• already forest as 1 January 1990 (pre-1990 forests)
• established since 31 December 1989 (post-1989 forests).

Under the Kyoto Protocol

All forests were accounted for, but the rules differed depending on when the forest was established.

• Forests that had been established since 1990, known as post-1989 forests, were eligible to be fully accounted for as ‘carbon sinks’ and can be used to offset emissions from other sectors of the economy.
• Forests that already existed as at 1 January 1990, known as pre-1990 forests, are accounted for against a business-as-usual reference level.

Under the Paris Agreement

Post-1989 forests

Post-1989 forests continue to be accounted for as they were under the Kyoto Protocol. However once they attain their long-term average carbon stock, taking into account all carbon pools and activities, the forest will transfer to the Forest management category. In this category the forest is accounted for under a business-as-usual reference level. New Zealand continues to account for all deforestation emissions.

Post-1989 planted forest

Post-1989 planted forest is planted forest that has been actively established on non-forest land after 31 December 1989. It covers about two per cent of New Zealand’s land area.

Post-1989 natural forest

Post-1989 natural forest is forest arising from natural regeneration of indigenous tree species as a result of:

• management change after 31 December 1989
• or self-sown exotic trees established after 31 December 1989 (where managed as forest).

This forest type covers less than one per cent of New Zealand’s land area. It provides an important carbon sink due to the high rate of sequestration of the young forest.

Pre-1990 forests

Pre-1990 forests continue to be accounted for under a business-as-usual reference level, as per the Kyoto Protocol.

This is to address the:

• dynamic effects of age structure resulting from activities and practices before the reference year
• ongoing cycles of forest harvest and regrowth that occur as part of normal, sustainable forest management in production forests.

Forests are further divided into natural and planted forest as the carbon cycles in these two forest ecosystems are different from each other and require different ways of estimating carbon stock change.

Pre-1990 natural forests

These are natural forests that already existed prior to 1 January 1990. Natural forest consists predominantly of native species originating from the primary forest cover. There are just under eight million hectares of natural forest left in New Zealand. They cover about 29 per cent of New Zealand’s land area.

Pre-1990 planted forests

These are forests that were established prior to 1 January 1990 for wood supply or soil conservation purposes. More than 1.4 million hectares of plantation forest were established in New Zealand between 1920 and 1990. These forests are predominately privately owned and cover about five per cent of the country’s total land area.

To determine carbon stocks in forest land and change in those stocks over time, the Ministry for the Environment uses inventory-based methods and country-specific conversion equations and models.

• Standard-sized sampling areas (forest plots) are located on a sampling grid set across forest land. The plots are permanent and monitored over time. Figure 1 shows the location of forest plots in pre–1990 forests. Figure 2 shows the location of forest plots in post–1989 forests.
• Measurements are taken of both living trees and dead wood, along with other plot specific data.
• These forest plot data are converted to carbon by applying methodologies developed specifically for the purpose. These methods vary between natural and planted forests. Allometric equations and modelling techniques are used in the calculations for natural and planted forests respectively.

These techniques enable the conversion of plot data to units of carbon per unit of forest area.

The number of hectares of each forest class in New Zealand is calculated from land use mapping. This information is used with measurements of carbon per unit of forest area to calculate the carbon stored in New Zealand forest land at a national level. 

Forest condition and biodiversity

Forest data and landscape observations are collected at each site. These data are used by organisations such as the Ministry for the Environment, Department of Conservation and local government organisations for:

• monitoring biodiversity and forest condition
• building sampling frameworks for regional and local vegetation monitoring
• reporting on forests in Environment Aotearoa
• reporting on sustainable forest management in the Montreal Process Criteria and Indicators report
• contributing to the UN Global Forest Resources Assessment.

Guidance the Ministry for the Environment uses when measuring forest carbon.

• 2006 IPCC Guidelines for National Greenhouse Gas Inventories [Intergovernmental Panel on Climate Change website]
• 2013 Revised Supplementary Methods and Good Practice Guidance Arising from the Kyoto Protocol [Intergovernmental Panel on Climate Change website]

See also Forest, soil and LiDAR data and Mapping land use.