Mapping disturbance across California’s rapidly changing forests

H. Anu Kramer, Elizabeth Ng, Jason Winiarski, Alexander Koltunov, Michele Slaton, Gavin M Jones, Marcus Peery

Disturbances shape assemblages and spatial patterns of flora and fauna across the globe, and accurate disturbance mapping can aid conservation science and decision-making. However, mapping and differentiating among disturbance types using remote sensing is challenging, especially in forests with hidden subcanopy disturbances. On federal lands in the western US, wildfire, drought, and fuels management are three primary disturbance agents of forest change. The US Forest Service’s (USFS) Forest Activity Tracking System (FACTS) provides nationwide fuels management data on USFS lands, but has not been widely utilized to understand the drivers of forest change, partially due to missing data and spatial and temporal uncertainty. We compared fuels management areas as represented in FACTS with annual, remotely-sensed predictions of canopy loss (Mortality Magnitude Index in the eDaRT system for Landsat processing; MMI) and assessed their spatial and temporal accuracy. We determined that a temporal window spanning two years before to one year after the reported FACTS completion year accounted for 98.5% of high-change fuels management areas delineated using remote sensing and visually confirmed using NAIP imagery. Our approach indicates that FACTS, once buffered temporally (and possibly spatially, depending on the user’s objectives), can provide reliable information on the history of fuels management. We used these data in conjunction with estimates of fire severity (composite burn index) and drought-related tree mortality (MMI) to characterize annual patterns in forest disturbance on USFS lands in the Sierra Nevada and Southern California from 2003 to 2022. We found that 73% and 76% of these regions were disturbed, respectively (25,000 km2 across both regions). Of the 25,000 km2 affected, wildfire was the dominant disturbance agent (17,204 km2; 69%), followed by drought/other mortality (12,813 km2; 51%), and fuels management (3,472 km2; 14%), with some overlap between these categories across the 20-year span. These results underscore recent widespread disturbance agents and the possible transformation of California forests, changes that are having profound effects on biodiversity. The accompanying disturbance dataset and processing code provide new and potentially powerful opportunities for scientists and managers studying and stewarding these rapidly changing ecosystems.