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.