Discussion
Temperature and other factors are known to directly impact D. rufipennis populations, but little is known of whether and how host drought stress may influence the success of beetle attacks, and whether that fosters a transition between endemic and outbreak phases of population dynamics (Fig. 1). To address whether drought stress modified the timing of tree death during a D. rufipennis outbreak, our study specified early- and late-dying trees (Fig. 2) and assessed their drought-sensitivity leading up to the outbreak. Here we documented that the slopes of ∆13C to CMD (i.e., drought sensitivity) did not differ among early- and late-dying trees (Figs. 4 and 5; Appendix S1: Table S3). Therefore, overall, drought stress in P. engelmannii was not a significant factor driving D. rufipennispopulations to transition from endemic levels to outbreak. Conversely, this evidence strongly suggests that the role of warmer temperatures was restricted primarily to direct effects on bark beetle population dynamics that ultimately decimated P. engelmannii across the landscape.