Measuring forest carbon

The Ministry for the Environment measures the carbon in New Zealand forests to help it track greenhouse gas emissions and removals from forestry. This page explains the role of forests and how forest carbon is measured.

Role of forests

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.

Definition of a forest

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.

Classification of forests for reporting purposes

For reporting of greenhouse gas emissions, New Zealand’s forests are divided into those that already existed before 1990 (pre-1990 forests) and those established since 1990 (post-1989 forests). Under the Kyoto Protocol, post-1989 forests are eligible to be accounted for as ‘carbon sinks’ and can be used to offset emissions from other sectors of the economy.

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 30 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.

Post-1989 forests

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

How carbon forest is measured

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 modeling techniques are used in the calculations for natural and planted forests respectively.

Airborne scanning laser (LiDAR) data are used in conjunction with field measurements to improve the precision of the carbon stock and stock change estimates for planted forests.
These techniques enable the conversion of plot data to units of carbon per unit of forest area.

Figure 1: Location of forest sampling plots in pre-1990 forests in New Zealand

Figure 2: Location of forest sampling plots in post-1989 forests in New Zealand

Calculating national forest carbon stocks 

The number of hectares of each forest class in New Zealand is calculated from geospatial 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

For pre-1990 natural forest plots, additional observations about vegetation present at each site and the condition of the landscape are collected at the same time as forest carbon data. These observations are used by organisations such as the Department of Conservation for:

  • monitoring biodiversity and forest condition
  • building sampling frameworks for regional and local vegetation monitoring
  • assisting reporting on environment, biodiversity and sustainable forest management.

Find out more

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

Forest, soil and LiDAR data are available to central and local government agencies, research institutes, universities and other organisations.