Spatial and temporal characteristics of recent bark beetle outbreaks and wildfire in the forests of the western United States
Forests provide significant ecosystem services, and disturbances are important influences of forest composition, structure, and function. Our objective was to increase the understanding of natural disturbances in the western United States in recent decades by quantifying the spatial and temporal patterns of these natural tree-killing disturbances. We combined existing spatial data sets of forest type, burn severity, and beetle-caused tree mortality to estimate the area of killed trees across the region.
We found that during 1984-2010 cumulative wildfire-caused tree mortality area was 2.4-5.1 Mha and during 1997-2010 beetle-caused mortality area was 0.47-5.4 Mha, with more trees killed since 2000 than in earlier periods. Together these disturbances killed trees representing 16% of the total area in western forests and had an annual rate similar to recent harvesting. Fires killed more trees in lower- and mid-elevation forest types such as ponderosa and lodgepole pine than higher-elevation forest types, whereas bark beetle outbreaks also killed trees in higher-elevation forest types such as Engelmann spruce. Over 15% of the lodgepole pine and 10% of the spruce/fir forest types were killed by beetle outbreaks; other forest types had 5-10% killed trees by both disturbance types. Our results document the importance of these natural disturbances in the forests of the western United States and offer the opportunity for further understanding of the response of these disturbances to significant influences such as climate change and forest management.