Examples
Integrating the CO₂ storage balance into GHG balances
A comprehensive greenhouse gas (GHG) balance should include the following elements:
1. GHG emissions along the production chain
2. Impacts of wood harvest on CO2 sink capacity of forests (CO₂ storage balance)
3. Storage of CO₂ in wood products
As a fourth element, the GHG balance of the wood product, should be compared with that of alternative products that would be used instead of the wood product (substitution effects). Substitution effects are not part of the GHG balance of the wood product, but they are carried out separately and are then compared with the wood product’s GHG balance.
Harvested wood can be grouped into different so-called assortments, e.g. sawlogs, industrial wood, wood from thinning and forest residues or harvest residues. The way how the wood of a harvested tree is to be allocated to these assortments is not always straightforward and depends on the quality and dimension of the wood, but also on the demand.
In order to integrate the CO₂ storage balance into the GHG balances of products, each quantity of wood removed from the forest gets assigned a share of the CO₂ storage balance according to the relative quantity of wood. This so-called mass allocation is used in the following examples. Alternatively, it could be weighted according to the economic value of the wood products. This would mean that more valuable products, such as sawlogs, get a larger share of the CO₂ storage balance. Less valuable products, such as firewood, would get a smaller share.
The value of the CO₂ storage balance depends on a number of factors, e.g. the condition of the forest. In order to represent a range of values, the examples are calculated assuming a low and a high estimate of the CO₂ storage balance.
Example 1: Stemwood directly used as firewood for energy
Since we assess GHG emissions of wood used for energy, the CO₂ storage balance is converted from t CO₂/m³ into the energy unit Megajoule (MJ), i.e. g CO₂/MJ. The conversion is based on data provided by Fehrenbach et al. (2022 [1]).
| Element of GHG balance | Low estimate of CO₂ storage balance (g CO₂e/MJ) |
High estimate of CO₂ storage balance (g CO₂e/MJ) |
|---|---|---|
| CO₂ storage balance | 82.6 (equivalent to 0.62 t CO₂/m³) | 152.9 (equivalent to 1.15 t CO₂/m³) |
| CO₂ storage in wood product | 0.0 | 0.0 |
| GHG emissions along production chain (harvest, transport, processing) | 4.5 | 4.5 |
| GHG balance of wood used as firewood | 87.1 | 157.4 |
| GHG emissions from alternative products to replace wood | 80.0 (RED II) | 80.0 (RED II) |
| Total GHG mitigation | - 9% (no net mitigation) | - 97% (no net mitigation) |
The GHG balance in this example results in 87.1 g CO₂e/MJ if a low CO₂ storage balance is assumed. A high CO₂ storage balance results in a GHG balance of 157.4 g CO₂e/MJ. Comparing this value with the avoided emissions of 80 g CO₂e/MJ through the substitution of fossil fuels, in this example, no net GHG reduction is achieved with the use of wood for energy. For comparison, the EU Renewable Energy Directive (RED II) requires a net GHG reduction of at least 70% compared to the fossile alternative.
Example 2: Stemwood used for construction wood
Since we assess GHG emissions of wood used as material, the CO₂ storage balance is converted from t CO₂/m³ into kg CO₂/kg product. The conversion is based on data provided by Fehrenbach et al. (2022 [1]).
| Element of GHG balance | Low estimate of CO₂ storage balance (kg CO₂/kg product) |
High estimate of CO₂ storage balance (kg CO₂/kg product) |
|---|---|---|
| CO₂ storage balance | 1.28 (equivalent to 0.62 t CO₂/m³) | 2.37 (equivalent to 1.15 t CO₂/m³) |
| CO₂ storage in wood product | -0.97 | -0.97 |
| GHG emissions along production chain (harvest, transport, processing) | 0.18 | 0.18 |
| GHG balance of stemwood used for construction wood | 0.49 | 1.58 |
| GHG emissions from alternative products to replace wood | -2.02 | -2.02 |
| Total GHG mitigation | 76% | 22% |
The GHG balance of this example results in 0.49 kg CO₂e/kg product if a low CO₂ storage balance is assumed. A high CO₂ storage balance results in a GHG balance of 1.58 kg CO₂e/kg product. This is offset by avoided emissions of 2.02 kg CO₂e/kg product through the substitution of fossil or mineral products. In this example, the use of wood as a material achieves a greenhouse gas reduction of 22% to 76% compared to fossil or mineral substitutes.