Characterizing Recent Bark Beetle-Caused Tree Mortality in the Western United States from Aerial Surveys

Abstract:

Forest insects are major disturbance agents in western North America, killing billions of trees in recent decades. Monitoring and assessments of tree mortality are therefore critical for quantifying patterns and increasing understanding of drivers and impacts. Tree mortality from aerial surveys is the only long-term, spatially extensive data set that documents tree and insect species and severity. Here we discuss updates to a gridded, annual product of bark beetle-caused tree mortality using aerial surveys and describe the spatial and temporal patterns of recent outbreaks. The USDA Forest Service recently changed survey approaches, necessitating methods that harmonize older and newer results. We estimated tree mortality in the western United State during 1997-2018 with four methods. We found substantial variability among the methods, suggesting some uncertainty in the findings. Based on limited analysis and a desire to be conservative, we recommend one method that accounts for observer underestimation and minimizes overestimation within a grid cell. Using this method, we estimate that bark beetles have killed 3.8 billion trees, accounting for 4.3 Mha of mortality area (4.7% of the forested area in the western US). Recent outbreaks (since 2012) have continued killing trees in the region, with extensive tree mortality in California and Colorado. Our tree mortality product is available for download to interested persons.