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Climate change has amplified eruptive bark beetle outbreaks over recent decades, including spruce beetle (Dendroctonus rufipennis). However, for projecting future bark beetle dynamics there is a critical lack of evidence to differentiate how outbreaks have been promoted by direct effects of warmer temperatures on beetle life cycles versus indirect effects of drought on host susceptibility. To diagnose whether drought‐induced host‐weakening was important to beetle attack success we used an iso‐demographic approach in Engelmann spruce (Picea engelmannii) forests that experienced widespread mortality caused by spruce beetle outbreaks in the 1990s, during a prolonged drought across the central and southern Rocky Mountain region. We determined tree death date demography during this outbreak to differentiate early‐ and late‐dying trees in stands distributed across a landscape within this larger regional mortality event. To directly test for a role of drought stress during outbreak initiation we determined whether early‐dying trees had greater sensitivity of tree‐ring carbon isotope discrimination (∆13C) to drought compared to late‐dying trees. Rather, evidence indicated the abundance and size of host trees may have modified ∆13C responses to drought. ∆13C sensitivity to drought did not differ among early‐ versus late‐dying trees, which runs contrary to previously proposed links between spruce beetle outbreaks and drought. Overall, our results provide strong support for the view that irruptive spruce beetle outbreaks across North America have primarily been driven by warming‐amplified beetle life cycles whereas drought‐weakened host defenses appear to have been a distant secondary driver of these major disturbance events. [ABSTRACT FROM AUTHOR]

Climate-mediated disturbances outside the range of historical variability can have severe consequences on vital, post-disturbance regeneration processes. High-elevation forests of the Rocky Mountains that are dominated by Engelmann spruce (Picea engelmannii) and subalpine fir (Abies lasiocarpa) are expected to be sensitive to climate change. Additionally, these forests have experienced recent epidemic spruce beetle (Dendroctonus rufipennis) outbreaks that have often resulted in >95% mortality of overstory Engelmann spruce. Therefore, the future distribution of Engelmann spruce forests depends largely on natural regeneration processes. We examined Engelmann spruce seedlings across gradients in soil moisture and stand structural conditions 20 yr post-disturbance on the Markagunt Plateau in southern Utah. All Engelmann spruce seedlings were mapped, measured, and aged, and aspects of stand structure and the microclimate were measured. The goal of our research was to infer processes affecting Engelmann spruce establishment by determining if patterns of advance regeneration that established before the outbreak (~60% of individuals) differed from seedlings that established during and immediately following the outbreak (combined into one group, ~40% of individuals). A generalized linear multi-model approach identified that the density of advance regeneration (seedlings/saplings) was negatively influenced by historical competition with overstory trees. In contrast, post-outbreak regeneration was related to microclimate conditions, including positive relationships with climatic moisture deficit and July soil water content. All seedlings were not significantly clustered around Engelmann spruce snags; however, there was evidence of facilitation of post-outbreak seedlings by pre-outbreak seedlings at higher elevation sites with lower moisture deficit. Together, these findings suggest post-outbreak seedlings were not moisture-limited at lower elevations but instead encouraged by higher evapotranspiration. Moreover, facilitation at higher elevations likely resulted from how pre-outbreak seedlings modify snowpack and associated seedbed environments. Our study provides insight for managing Engelmann spruce after a beetle outbreak. In these forests, pre- and post-outbreak regeneration can increase resilience to climate-disturbance interactions, but are patchy and structured at different scales. Therefore, the presence of advance regeneration and the likelihood of post-outbreak seedlings depend on local environment (soil moisture and stand structure) and could be taken into account to most effectively plan post-disturbance planting activities. [ABSTRACT FROM AUTHOR]