The state of California has therefore committed considerable resources to reducing wildfire hazard and improving forest resilience (Forest Management Task Force, 2021), but evaluation of the effectiveness of these investments is hampered by a lack of robust quantification of California's forest resources. For example, in California, climate change is projected to reduce forest cover (Shaw et al., 2011) and carbon stocks (Coffield et al., 2021). Building effective and equitable natural climate solutions will require understanding the distribution and impact of climate-driven risks to forests, such as increasing fire, drought, and insect attack (Anderegg et al., 2020 Hurteau et al., 2008 Underwood et al., 2019). In the United States, natural climate solutions could offset as much as 21% of net annual CO 2-equivalent emissions by 2025, and forests could contribute the vast majority of this climate mitigation potential (Fargione et al., 2018). Land management policies designed to reduce net greenhouse gas emissions can provide much of the climate mitigation needed to stabilize warming below 2☌ (Griscom et al., 2017).
Considering ongoing climate warming and disturbance trends, California's forest resources are likely to continue to decline, limiting the carbon sequestration potential of the state's natural and working lands. Fires and tree cover loss generally occurred in warmer areas. Tree cover area initially increased in the 1990s, but rapidly declined after 2000 from larger and more frequent wildfires, resulting in an expansion of shrub and grass cover area. Over these 37 years, California lost 4,566 km 2 of its tree cover area, equal to 6.7% of its initial tree cover in 1985. In this study, we compiled satellite data and archival records with machine learning to map vegetation type (tree, shrub, and grass) and disturbance agent (fire, harvest, and drought) in California from 1985 to 2021. These threats are especially important in areas experiencing rapid climate change, such as in California, yet it remains challenging to measure their impact because of a lack of digital vegetation maps at high spatial resolution that can accurately measure changes over a period of decades. However, increasing fires and drought threaten the integrity of forests in many parts of the world. Trees provide nature-based solutions to slow climate change. California's vegetation is undergoing rapid transformation, with disturbance rates and climate change posing substantial potential risks to the integrity of California's terrestrial carbon sink. Fires and tree cover area loss generally occurred where summer temperatures were greater than 17.5☌, whereas net tree cover gain often occurred in cooler areas, suggesting that ongoing climate warming is threatening forests in many areas. Tree cover loss occurred in all ecoregions but was most severe in the southern mountains, where losses from wildfire were not compensated by regrowth in undisturbed areas. Substantial gains in tree cover area during the 1990s were more than offset by fire-driven declines since 2000, resulting in greater shrub and herbaceous cover area. Considering both changes in cover fraction and areal extent, California lost 4,566 km 2 of its tree cover area (6.7% relative to initial cover) since 1985. To address this, we combined remote sensing observations with geospatial databases to develop annual maps of vegetation cover (tree, shrub, and herbaceous) and disturbance type (fire, harvest, and forest die-off) in California at 30 m resolution from 1985 to 2021. Understanding these threats requires accurate information on vegetation dynamics and their drivers, which is currently lacking in many regions experiencing rapid climate change such as California. Forests provide natural climate solutions for sequestering carbon and mitigating climate change, yet are increasingly threatened by increasing temperature and disturbance.
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