Forest Carbon Storage in the Western United States:Distribution, Drivers, and Trends

Forests are a large carbon sink and could serve as natural climate solutions that help moderatefuture warming. Thus, establishing forest carbon baselines is essential for tracking climate‐mitigation targets.Western US forests are natural climate solution hotspots but are profoundly threatened by drought and altereddisturbance regimes. How these factors shape spatial patterns of carbon storage and carbon change over time ispoorly resolved. Here, we estimate live and dead forest carbon density in 19 forested western US ecoregionswith national inventory data (2005–2019) to determine: (a) current carbon distributions, (b) underpinningdrivers, and (c) recent trends. Potential drivers of current carbon included harvest, wildfire, insect and disease,topography, and climate. Using random forests, we evaluated driver importance and relationships with currentlive and dead carbon within ecoregions. We assessed trends using linear models. Pacific Northwest (PNW) andSouthwest (SW) ecoregions were most and least carbon dense, respectively. Climate was an important carbondriver in the SW and Lower Rockies. Fire reduced live and increased dead carbon, and was most important in theUpper Rockies and California. No ecoregion was unaffected by fire. Harvest and private ownership reducedcarbon, particularly in the PNW. Since 2005, live carbon declined across much of the western US, likely fromdrought and fire. Carbon has increased in PNW ecoregions, likely recovering from past harvest, but recentrecord fire years may alter trajectories. Our results provide insight into western US forest carbon function andfuture vulnerabilities, which is vital for effective climate change mitigation strategies.