Life cycle emissions associated with vault storage of wood cleared for fire management in the Western United States

AbstractSection Background

Climate change, fire suppression, and human encroachment contribute to increasingly intense forest fires in the Western United States, releasing hundreds of millions of metric tons (MMT) CO₂/year. Proactive fire-risk reduction treatments coordinated by the US Forest Service (USFS) typically include thinning and burning (or in situ decay) of thinned products and may require thinning on ~ 28 million hectares of public and private land over the next decade. Assuming thinning of only small (~ 30 cm diameter) trees within 0.8 km of existing roads on slopes gentler than a 40% grade, this will produce ~ 1,100 MMT of thinned wood, which, if burned or left to decay, will release ~ 2000 MMT CO₂. Here we evaluate the life cycle emissions of an alternative fate, burial in anoxic wood vaults. We performed a life cycle analysis (LCA) to assess potential net emissions reductions, considering site clearing, transport, site preparation and post-burial decay. We used Monte-Carlo simulations to estimate emissions uncertainty and identify key parameters influencing carbon removal efficiency.

AbstractSection Results

We find wood vaults will decrease emissions relative to current practice by a mean of 66% if wood is transported 100 km, and by 38% at a transport distance of 500 km. If the USFS is able to implement the proposed Wildfire Crisis Strategy, and all of the wood from thinning were buried in wood vaults within 100–500 km of the thinning sites, our results suggest these vaults would thus sequester between ~ 40–140 MMT CO₂/yr over a decade. This annual figure represents ~ 6–12% of 2021 energy-related emissions in the contiguous Western United States. Harvesting thinned products only from gentler (< 20%) slopes within shorter distances from roads (304 m) would result in a greenhouse gas savings equivalent to 3–6% of 2021 Western State emissions. However, these results depend heavily on parameters related to wood decay and post-decay methane emissions that are relatively poorly constrained.

AbstractSection Conclusions

These results suggest wood vaults are a promising emissions-reduction strategy, but challenges remain. It is not clear that the USFS has the resources to manage the additional ~ 20 million hectares targeted for forest thinning. Biogeochemically, the importance of rates of wood decay within the vault, and the fraction of methane generated that escapes the vault, are poorly constrained parameters. Their estimation will be important for narrowing uncertainty in estimates of life cycle emissions. Nevertheless, our analysis suggests wood vaults are a promising, low-tech, ready-to-deploy emissions reduction strategy in places where forest management includes mechanical thinning and burning of wood waste residues. Wood vaults can be particularly impactful in locations that facilitate short transport distances and where biogeochemical conditions at the vault site minimize wood decay.