Grouping areas of forest with the same characteristics for calculating carbon

Why you need to group your forest into sub-areas for calculating carbon

When you complete an emissions return for the ETS, you must calculate how much carbon is in your forest land.  This is also required for some other ETS applications. To prepare for this, you must group together areas of forest that have the same characteristics, such as age and forest type. This is because there’s a different amount of carbon in trees of different species and ages.

Anyone preparing an emissions return for forestry who has more than one forest type or age needs to complete this process. You also need to do this by region if you have any radiata pine forest and:

• you have pre-1990 forest land, or
• you’re using standard carbon tables for post-1989 land.

Find out more about using carbon tables

You will need to know how to work out the forest type, age, and region of your forest. The ETS has specific rules for how to identify these.

Identifying forest type, region, and age for calculating carbon

Identifying sub-areas in carbon accounting areas on post-1989 forest land

If you have post-1989 forest land registered in the ETS, you will have set up one or more carbon accounting areas. When you group areas of forest with the same characteristics into sub-areas, do this within each carbon accounting area separately. A sub-area cannot include land from more than one carbon accounting area.

When you prepare an emissions return for post-1989 forest land it covers a certain period of time. You must calculate the amount of carbon in your forest land at the beginning and the end of this period. You must identify your sub-areas separately for these 2 sets of calculations. This is because any clearing or planting you do during the emissions return period may change the characteristics of the forest. Your initial sub-areas will usually not be the same at the end of the emissions return period.

From 1 January 2023, there is a new method of accounting for carbon in post-1989 forest land. This is called "averaging accounting". Previously, only stock change accounting was available.

Sub-areas apply to both accounting methods. However, there are some different things you must consider when you create your sub-areas, depending on which accounting method you’re using.

Find out more about accounting methods

If you're using stock change accounting for post-1989 forest land

With stock change accounting, if post-1989 forest land has been cleared in the 10 years before the date you’re calculating for, stands with a different crop or clearance history also need to be grouped separately.

Find out more about grouping areas that were cleared in the previous 10 years

If you’re using averaging accounting for post-1989 forest land

If you have post-1989 forest land under averaging accounting, there are more things to consider when you create your sub-areas. This depends on the age of the trees and the rotation the forest is on. 

The process for creating sub-areas for land under averaging accounting is the same as for stock change accounting while: 

• the forest is on its first rotation, and
• the trees are younger than the average age for the forest type.

Once the first rotation reaches the average age or is cleared, you must create your sub-areas differently. This is because, once the forest reaches this point, the age and forest type to use for carbon calculations do not change.

Find out more about averaging accounting

For your emissions returns, these areas are treated as if the amount of stored carbon doesn’t change. When forest land has reached this point, you must create your sub-areas based on:

• the forest type of the first rotation
• the region, if the forest type is radiata pine and you’re using standard carbon tables
• the age you must use for the carbon calculations, based on the average age of the first-rotation forest type or the age the first rotation was cleared (if cleared before the average age). 

Find out what age to use when calculating carbon for areas of forest under averaging accounting

For example, you may have:

• 2 hectares of radiata pine that is 16 years old (the average age for radiata pine), and
• 2 hectares of forest land on its second rotation, where the first rotation was radiata pine harvested at age 26 (and is therefore considered to have stopped gaining carbon at age 16).

In this example, all 4 hectares of your forest land will form 1 sub-area, as it is all considered to be 16-year-old radiata pine for the sake of calculating carbon.

Identifying sub-areas on pre-1990 forest land

You must identify sub-areas if you’re completing an emissions return for deforesting pre-1990 forest land. Create your sub-areas based on:

• the forest type
• the region, if the forest type is radiata pine, and
• the age of the trees in the year the forest land was cleared.

Steps to identify sub-areas in your forest land

To identify sub-areas in your forest land:

1. Divide the pre-1990 forest land or post-1989 carbon accounting area into areas by their forest type (or the forest type of the first rotation, for averaging accounting). If there are areas where trees of different species are mixed in together, work out which forest type applies using the instructions in the web page linked under step 4.
2. If the trees are radiata pine, divide the area again by region. Use the regions in the web page linked under step 4. (If you are using participant-specific carbon tables, you can skip this step.)
3. If there are areas of trees that are different ages (or ages to use under averaging accounting), divide them again by age. If trees of different ages are mixed in together, find the average age of the stand using the instructions in the web page linked under step 4.
4. You should now have separate groups in which all the forest stands have the same characteristics to use for calculating carbon. Each of these forms a sub-area. For each sub-area, you will need to know the total area of land it covers. Round the area to 1 decimal place (.05 and above is rounded up).

Identifying forest type, region, and age for calculating carbon

Identifying sub-areas if post-1989 forest land was cleared in the previous 10 years

You must take the previous forest into account when identifying sub-areas if you have:

• post-1989 forest land under stock change accounting that was cleared in the previous 10 years, or
• post-1989 forest land under averaging accounting where naturally regenerating scrub was cleared in the previous 10 years, in order to plant a first-rotation forest (and the scrub was younger than the average age for its forest type). 

This is because you have to account for the emissions from woody debris left on the land after clearing trees. This debris is considered to decay and emit carbon over 10 years. The amount of carbon left in this debris depends on the forest type and the age of the trees when they were cleared. This means you must calculate the residual carbon in them separately.

For each sub-area, the trees in the previous forest must have been:

• all one forest type
• cleared in the same year
• all the same age, and
• in the same region, if they were radiata pine and you're using the standard tables from the regulations.

For example, if you had cleared an area each of Douglas-fir and regenerating native forest and replanted it all with Douglas-fir, the forest must be split into separate sub-areas. The forest land where the 2 different species were in the last rotation must be treated separately for your carbon calculations.

Changing sub-areas to account for small areas for your emissions returns

If you have small areas of forest land that would be eligible to form separate sub-areas, but are less than a hectare in size, there is a particular way to treat them for your emissions return. From 1 January 2023, you will be able to account for these small areas of forest land by including them in existing sub-areas, or creating new sub-areas out of several small areas.

Modified sub-areas apply to both accounting methods. However, there are some different ways that you can modify your sub-areas, depending on which accounting method you’re using.

If you have questions about mapping your forest details or need assistance, email ForestryETS@mpi.govt.nz

Changing sub-areas to account for small areas for your emissions returns - guidance [PDF, 764 KB]