Recent Tree Mortality in the Western United States from Bark Beetles and Forest Fires

Forests are substantially influenced by disturbances, and therefore accurate information about the location, timing, and magnitude of disturbances is important for understanding effects. In the western United States, the two major disturbance agents that kill trees are wildfire and bark beetle outbreaks. Our objective was to quantify mortality area (canopy area of killed trees), which better represents impacts than affected area (by beetles) or burn perimeter area, and characterize patterns in space and time. We based our estimates on aerial surveys for bark beetles and the Monitoring Trends in Burn Severity database (from satellite imagery) for fires. We found that during the last three decades, bark beetle-caused mortality area was 6.6 Mha (range of estimates, 0.64-7.8 Mha; 7.1% [0.7-8.4%] of the forested area in the western United States) and fire-caused mortality area was 2.7-5.9 Mha (2.9-6.3%). Mortality area from beetles and fire was similar to recent harvest area from a national report. Although large outbreaks and fires occurred before 2000, substantially more trees were killed since then. In several forest types, mortality area exceeded 20% of the total forest type area. Our mortality area estimates allow for comparisons among disturbance types and improved assessment of the effects of tree mortality.