Why you need to calculate changes in carbon stock
If you have post-1989 forest land registered in the ETS, you need to complete emissions returns at different points in time. For all of these emissions returns, you need to calculate changes in carbon stock for the period that the return covers. This means you must do 2 sets of calculations. You must calculate:
- how much carbon was in the forest at the start of the period, and
- how much carbon was present at the end of the period.
You must then work out the difference between these amounts. This will tell you the total change in carbon stock for the period.
There are specific methods you must use for these calculations. These are explained on this page.
Find out more about post-1989 forest land
Find out more about emissions returns
Appointing a representative to manage your ETS tasks
Use the online system to do your calculations
The new online system, Tupu-ake, has been available since early 2023. Tupu-ake can calculate your emissions return for you. This calculation is known as an "input return". This means you do not have to manually calculate the values for your emissions return.
You may still calculate the values for your emissions return yourself if you prefer.
Preparing to calculate carbon
To calculate the amount of carbon in your forest land you need to have completed the following 2 steps first.
Step 1: Identify the forest type, region, and age to use, depending on the carbon accounting method
You need to know certain things about your forest land, such as the forest type and age of the trees. There are specific rules for how to work these out in the ETS.
Identifying forest type, region, and age for calculating carbon
From 1 January 2023, you may be using averaging accounting for your post-1989 forest land. The method you use for carbon accounting depends on when the land entered the ETS. If you’re using stock change accounting, you must do your calculations based on the age and the forest type of the trees on the land. Calculations for averaging accounting may be more complicated.
Once a forest under averaging accounting reaches its average long-term carbon stock, the carbon it’s considered to have is paused. You won’t earn any New Zealand Units (NZUs or units) for this forest land after this point.
Depending on the tree age and rotation of the forest, the forest type and age you must use when you do your calculations may differ from the trees on the land.
Find out more about carbon accounting
Step 2: Group similar areas of forest together into sub-areas
If your forest has trees of more than one forest type or age, or across multiple regions (if it's radiata pine), you also need to do some work to separate each carbon accounting area into multiple sub-areas. You'll do your carbon calculations for each of these sub-areas separately.
Any changes during the period of your emissions return will affect the placement of your sub-areas. This means they will be different for your calculations at the beginning of the period and your calculations at the end of the period.
Grouping areas of forest with the same characteristics for calculating carbon
Using carbon tables
When you do your carbon calculations, you need to refer to "carbon tables". These are tables that tell you how much carbon is in your forest land per hectare.
Find out which carbon tables you must use and why you need them
If your forest is older than the last age on the carbon tables for that forest type, use the value for the last age.
Using standard carbon tables
In the standard carbon tables for post-1989 forest land there are 4 tables. These are used for different situations depending on whether the land was cleared recently.
Tables of carbon stock per hectare for post-1989 forest land – NZ Legislation
The first 2 tables show carbon stock per hectare for the forest by forest type (and region for radiata pine).
The other 2 show carbon stock per hectare for residual carbon left from a previous clearing. Residual carbon is the carbon from woody debris left on the land after clearing. It is considered to decay over the following 10 years, and continues to produce emissions during that time. These 2 tables are set up in the same way as the first 2. You can ignore these if:
- your forest is on its first rotation, or
- the last clearance was more than 10 years before the start of the period your emissions return covers.
Using participant-specific carbon tables
If you're using carbon tables created specifically for you (known as participant-specific tables), the amount of carbon contained in your forest depends on the forest types on your land. For each forest type on your land, you'll have a table for forest carbon stock, and one for residual carbon stock.
Using the field measurement approach to get carbon tables
How to do your carbon calculations for an emissions return for post-1989 forest land
When you submit an emissions return, you must account for the change in carbon stock from the beginning to the end of the period that it covers. This means you need to be able to calculate the amount of carbon in your forest at each date, then work out the difference between the 2 values.
For each carbon accounting area, the carbon stock change is equal to (the carbon stock at the end of the period) minus (the carbon stock at the beginning of the period).
You must round the result to the nearest whole number (0.5 or above rounds up).
If the amount of carbon stored in the carbon accounting area has decreased, the result will usually be a negative value. If it has increased, the result will usually be positive. This number is the one you will use to fill in your emissions return.
Find out more about submitting your emissions return
Instructions for doing the calculations
The instructions in the coloured, expanding boxes below tell you how to calculate the carbon in post-1989 forest land at a particular date. There are extra calculations to do if you need to account for carbon left on the land after clearing. This is the case if:
- you're using stock change accounting and trees on the land have been cleared in the previous 10 years, or
- you’re using averaging accounting and young, regenerating scrub was cleared from the land in order to plant a first-rotation forest.
Find out what makes a forest "first rotation"
Make sure you use the correct age and forest type when you do your calculations for averaging accounting
Under averaging accounting, the age and forest type you need to use when calculating carbon may not be the same as those of the trees on the land.
Find out what you need to do when calculating carbon using averaging accounting
