Wood substitution potential in greenhouse gas emission reduction-review on current state and application of displacement factors

Background Replacing non-renewable materials and energy with wood offers a potential strategy to mitigate climate change if the net emissions of ecosystem and technosystem are reduced in a considered time period. Displacement factors (DFs) describe an emission reduction for a wood-based product or fuel which is used in place of a non-wood alternative. The aims of this review were to map and assess DFs from scientific literature and to provide findings on how to harmonise practices behind them and to support coherent application. Results Most of the reviewed DFs were positive, implying decreasing fossil GHG emissions in the technosystem. The vast majority of the reviewed DFs describe avoided fossil emissions either both in processing and use of wood or only in the latter when wood processing emissions were considered separately. Some of the reviewed DFs included emissions avoided in post-use of harvested wood products (HWPs). Changes in forest and product carbon stocks were not included in DFs except in a few single cases. However, in most of the reviewed studies they were considered separately in a consistent way along with DFs. DFs for wood energy, construction and material substitution were widely available, whereas DFs for packaging products, chemicals and textiles were scarce. More than half of DFs were calculated by the authors of the reviewed articles while the rest of them were adopted from other articles. Conclusions Most of the reviewed DFs describe the avoided fossil GHG emissions. These DFs may provide insights on the wood-based products with a potential to replace emissions intensive alternatives but they do not reveal the actual climate change mitigation effects of wood use. The way DFs should be applied and interpreted depends on what has been included in them. If the aim of DFs is to describe the overall climate effects of wood use, DFs should include all the relevant GHG flows, including changes in forest and HWP carbon stock and post-use of HWPs, however, based on this literature review this is not a common practice. DFs including only fossil emissions should be applied together with a coherent assessment of changes in forest and HWP carbon stocks, as was the case in most of the reviewed studies. To increase robustness and transparency and to decrease misuse, we recommend that system boundaries and other assumptions behind DFs should be clearly documented.