COS 129-5 - Live tree forest carbon storage in the Western United States: distribution, trends, and drivers

Abstract: Forests in the western United States (US) are contending with several threats from natural and human sources. Concurrently, there is consensus that forest-based natural climate solutions are a small but critical component of pathways to curb warming. Thus, information on how threats to western US forests are influencing recent trends and current baselines of carbon storage is essential. Here, we leverage Forest Inventory and Analysis (FIA) data to estimate live tree carbon storage in forests of the western U.S. and determine: 1) current carbon distributions 2) recent trends in carbon storage 3) drivers underpinning the spatial distribution of current carbon. We collected forest survey data (2005-2019) within 20 ecoregions with ≥ 20% forest cover. We compared carbon between ecoregions, and assessed ecoregion-level recent carbon trends using linear models and a shape selection algorithm. Finally, we assembled a suite of potential carbon drivers, including human removals (North American Forest Dynamics forest loss attribution), fire history and severity (Western US MTBS-Interagency, Landsat-derived composite burn index), insect and disease presence (Aerial Detection Survey), and climate indices (PRISM) of temperature and vapor pressure deficit. We evaluated drivers of current carbon for each ecoregion by comparing the variance explained using separate random forest models.

Forests in the Pacific Northwest (PNW) ecoregions were most carbon dense, with the highest plot-level mean carbon densities in the Coast Range (175.92 Mg ha^-1). Southwest ecoregions contained the lowest mean carbon densities (Madrean Archipelago, 11.36 Mg ha^-1). Live tree carbon density generally increased in the PNW and decreased in the Upper Rockies (UR) and Southwest ecoregions between 2005 and 2019, with the greatest estimated increases and decreases relative to current carbon averages occurring in the Willamette Valley (42%) and Southern Rockies (-30%), respectively. Random forest models accounted for 5% to 34% of the variance in live tree carbon density. Removals were the dominant driver of carbon in five PNW ecoregions, fire in four ecoregions in the PNW, California, and UR, insects and disease in the Blue Mountains and Southern Rockies, and drought-related climate variables in the remaining ecoregions. Our results indicate that the high productivity PNW ecoregions are likely increasing carbon stocks due to recovery from harvest but rapid recent increases in fire portend growing threats to these productive forests. Declines in low to moderate productivity ecoregions are due to fire, insects, and climate change.