Species climatic suitability explains insect-host dynamics in the Southern Rocky Mountains, USA

Recent extreme events of drought and heat have been associated with insect-driven tree mortality. However, there is substantial uncertainty about the impact of climate variability and extreme climatic episodes on insect–host dynamics, especially over species biogeographical ranges. Here, we use climatic suitability indices derived from species distribution models to analyze the spruce beetle (Dendroctonus rufipennis) outbreak dynamics in spruce-fir forests across the Southern Rocky Mountains (USA) during a warm and dry episode (2000–2013). We estimated the historical climatic suitability of the host tree (1969–1998), its inter-annual variability, and the climatic suitability during the 2000–2013 episode for both beetle and host tree. Overall, outbreak was more likely to occur in host tree populations inhabiting areas with historically suitable climatic conditions that were also characterized by loss of suitability during the episode. Specifically, the outbreak initiation was located in areas with suitable climatic conditions for the beetle and high historical suitability for the host. However, the year-to-year analysis revealed that low–moderate amounts of outbreak initiation and spread were also determined by high host historical climatic suitability, with high historical inter-annual variability, and a modest reduction of suitability during the episode. Years with high amounts of outbreak initiation and spread mostly occurred in dense forests with large trees and were promoted by suitable climate conditions for the beetle. This study highlights the importance of considering the climatic suitability of the insect–host system to understand and anticipate outbreak dynamics at different temporal scales.