Methods for quantifying and incorporating biomass carbon sequestration by trees in life cycle assessments

Purpose
Global temperatures are expected to surpass the critical threshold of 1.5 degrees C above pre-industrial levels by 2040, necessitating the urgent need for large-scale and sustained carbon dioxide (CO₂) removal. Tree-based systems offer a promising solution for carbon (C) sequestration and contribute to climate change mitigation. However, there is no consensus on accounting for biomass C sequestration in greenhouse gas (GHG) inventories, particularly in life cycle assessments (LCA). Although LCAs assess GHG emissions from production systems, integrating temporal changes in biomass C stocks remains a significant challenge.

Methods
This study conducted a review to identify different methods for quantifying C sequestration and storage by trees in their biomass and to quantify the climate impact of this sequestered C for incorporation into LCA. Further, a case study on poplar trees was conducted to discuss these methods.

Results and discussion
LCA practitioners can use several modeling approaches to quantify tree biomass C sequestration and storage, each with distinct strengths and limitations. These approaches include allometric, process-based, C-budget, and parametric models. This study found significant variability in the estimated biomass C sequestration and storage among these approaches, primarily due to the underlying methodological differences. Additionally, the variability in C sequestration and storage estimates increased with longer assessment durations. The results indicated that general allometric models may overestimate biomass C compared with species, climate, or site-specific models. However, when general models are adjusted for site-specific conditions and tree species, they provide more comparable estimates. This review identified nine impact assessment methods to quantify the climate change impacts of tree biomass C sequestration. The results showed that these impact assessment methods are time-sensitive, and the results may vary depending on the specific method and assessment duration chosen.

Conclusions
This study concludes that, while simplified approaches to estimate biomass C sequestration and storage as well as impact assessment methods are useful, more detailed approaches may offer greater accuracy when detailed data are available. Therefore, in the future, methods for estimating biomass C sequestration and storage and its climatic impacts must strike a balance between complexity, simplification, and accuracy to improve their applicability and reduce uncertainties.