Epidemic spruce beetle outbreak changes drivers of Engelmann spruce regeneration

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 (similar to 60% of individuals) differed from seedlings that established during and immediately following the outbreak (combined into one group, similar to 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.