The effects of forest restoration on ecosystem carbon in western North America: A systematic review.

Graphical abstract Highlights • Restoration treatments decrease forest C pools, with limited evidence of recovery. • Long-term carbon loss may be reduced when post-treatment wildfire is present. • Few empirical studies exist at large spatial scales or over time periods >25 years. • The response of deeper mineral soil to treatments remains a major knowledge gap. • Need consistent methodology for measuring and reporting C pools. Abstract Ecological restoration has become an overarching management paradigm for sustaining the health and resilience of forests across western North America. Restoration often involves mechanical thinning to promote development of complex habitats in moist, productive forests and mechanical thinning with prescribed fire to reduce fuels and restore natural disturbance regimes in dry, fire prone forests. This systematic review quantified the impact of restoration treatments on forest ecosystem carbon (C) stocks and identified factors that moderate treatment effects across spatial and temporal scales. Our review process identified 73 studies to be included for analysis, from which we calculated 482 estimates of treatment effect size. We found that restoration treatments significantly reduce C. Prescribed fire had larger impacts on belowground than aboveground carbon pools, while thinning and combined treatments had larger impacts on aboveground pools. The available literature is highly skewed toward shorter timescales (<25 years after treatment), small spatial scales, and is geographically concentrated: 41% of estimated effect sizes came from studies in the Sierra Nevada. Thinning had similar effects on forest carbon in dry forests and moist forests. The relative magnitude of total C losses was significantly less from simulation than empirical studies, although simulations also mostly evaluated long-term impacts (>75 years after treatment) while empirical studies mostly looked at short term (<25 year) effects. Post-treatment wildfire significantly reduced the percentage of carbon lost relative to controls in the aboveground pool. Long-term, treated stands only recovered to control levels of carbon when wildfire was present. Returns on the carbon debt imposed by thinning and prescribed fire depend on the nuances of the treatments themselves but may also depend upon treatment intensity and the frequency and intensity of future wildfire. Ecological restoration in forests across the western US has to carefully balance the budget of ecosystem carbon with competing objectives such as improved wildlife habitat, reduced risk of severe wildfire, and other ecosystem services. [ABSTRACT FROM AUTHOR]