Spatiotemporal characteristics of tree mortality from bark beetle outbreaks vary within and among bark beetle-host tree associations in the western United States

Abstract

Eruptive outbreaks of bark beetles have caused extensive and often severe tree mortality across tens of thousands to millions of hectares in temperate forests since the late 1990s. Many individual bark beetle-host tree (BB-host) associations have experienced outbreaks in recent decades, and associations may exhibit unique spatiotemporal characteristics of tree mortality. We investigated the spatiotemporal characteristics of tree mortality from the nine most damaging BB-host associations in the western US. We first extended a dataset of gridded mortality area (MA; annual area of killed trees within a 1-km grid cell) from aerial surveys to 1997–2023. Total MA was highest for the mountain pine beetle (Dendroctonus ponderosae)-lodgepole pine (Pinus contorta) association. We delineated the spatial extent and period of 49 outbreaks (4–7 per BB-host association). Multiple characteristics of tree mortality were compared within and among BB-host associations at two spatial scales: (1) the outbreak scale (all grid cells aggregated) and (2) stand scale (individual grid cells). Our results highlight notable differences in maximum annual and cumulative MA, duration of mortality, and the average rate of MA among associations, with considerable variability among outbreaks within some associations. Associations exhibited all combinations of shorter to longer duration and lower to higher severity (cumulative MA) and results were similar at the stand and outbreak scale. Pinyon ips (Ips confusus) outbreaks in pinyon pine forests were the shortest duration and higher severity, resulting in significantly more rapid rates of MA than other associations at both spatial scales. Outbreaks of western balsam bark beetle (Dryocoetes confusus) in subalpine fir (Abies lasiocarpa) and Douglas-fir beetle (Dendroctonus pseudotsugae) in Douglas-fir (Pseudotsuga menziesii) produced very low MA rates due to low cumulative MA and longer or shorter outbreaks, respectively. Outbreaks resulted in >75 % forest MA (cumulative MA divided by forest cover) from bark beetles across 19,892 km² of forest, with the amount and percent of forest loss varying by BB-host association. Although subject to some uncertainty, our analysis of aerial surveys incorporates large areas and multiple decades, thereby providing results complementary to field studies that are limited in spatial and/or temporal extent. By synthesizing the spatiotemporal characteristics of tree mortality from outbreaks by BB-host associations, our results inform land managers’ understanding of impacts of past outbreaks and the potential outcomes from future outbreaks for forests during a period of rapid changes to forest structure and composition from bark beetles.