Conclusions
The ubiquity of bark beetle outbreaks across western North America and elsewhere in recent decades provides stark evidence that warming temperatures, drought, and bark beetles are having dramatic impacts on forests (Raffa et al. 2008; Bentz et al. 2010; Fettig et al. 2019). Increased research efforts have been called for to provide greater accuracy in projecting the effects of climate change and pest/pathogen interactions on forests (Anderegg et al. 2015) as well as management actions that may mitigate the impact of bark beetles on forest ecosystem functions (Morris et al. 2017). Our iso-demographic approach from sites near the southwestern range edge for both P. engelmannii (Fig. 3) and D. rufipennis , demonstrate that drought stress played little role in determining the transition between endemic to outbreak beetle populations (Figs. 4 and 5). Therefore, forecasting of D. rufipennis outbreaks and attendant Picea spp. forest dynamics may be considered a rather straightforward response to temperature (DeRose et al. 2013). On the other hand, if our iso-demographic methods were to be widely deployed within a framework that could differentiate among stand structures that are most likely to concentrate or mediate beetle attack pressure on large trees, then perhaps drought stress may still prove to be a significant and complicating factor influencingD. rufipennis outbreaks (Anderegg et al. 2015). Similarly, if it is a goal to preserve P. engelmannii in areas threatened by climate change and increased bark beetles activity, management actions aimed at reducing drought stress are unlikely to directly affectD. rufipennis susceptibility. However, management that maintain a diversity of age and size classes of trees (DeRose and Long 2014), including small regenerating trees that are not susceptible to beetle attack, will be essential to keeping this species present on the landscape (e.g., Pettit et al. 2019), and allowing it additional time to adapt to climate change and/or migrate to nearby refugia.