Critical review of drought stress vs temperature in driving bark
beetle outbreaks
Disentangling the effects of temperature from drought stress on bark
beetles has been a difficult task because many investigations into
climatic influences on outbreaks have been retrospective and are lacking
in direct measurements of tree drought stress (Kolb et al. 2016). Of the
many species of bark beetles, only Ips confusus andDendroctonus brevicomis unequivocally influence tree
susceptibility to attack when drought stress is experimentally induced
(Gaylord et al. 2013; Kolb et al. 2019). In contrast to these
multivoltine-capable bark beetle species that occupy and kill their
hosts within forests that occupy consistently dry growing seasons
(Breshears et al. 2005; Raffa et al. 2008; Fettig et al. 2019), D.
rufipennis hosts occupy cold and wet, montane, boreal and subarctic
forests where evidence has clearly demonstrated how temperatures exert
strong and direct effects on beetle life cycles (shift from semivoltine
to univoltine; Hansen et al. 2001; Hansen and Bentz 2003; DeRose et al.
2013).
During the D. rufipennis outbreaks that peaked in the early 1990s
across south-central Alaska, Berg et al. (2006) concluded that decades
of warmer than average temperatures were directly affecting bark beetle
populations, but that the potential for drought to have contributed to
the outbreaks could not be ruled out. The outbreak we investigated was
characterized in a similar manner to Berg et al. (2006), and it was
concluded that periods of regional drought were associated with pastD. rufipennis outbreaks (DeRose and Long 2012a). However, like
most other retrospective studies of bark beetle outbreaks, prior to the
instrumental record, DeRose and Long (2012a), had no means to
differentiate among how precipitation deficits versus warmer
temperatures may have contributed to reconstructions of meteorological
drought severity (i.e., PDSI). Additionally, regional tree-ring
reconstructions of PDSI across this region can be biased toward sites
with greater climate sensitivity (Klesse et al. 2018), which may better
reflect mid- to low-elevation forests where cloud cover and
precipitation from summer convective storms and/or monsoon rains are
lower and less consistent compared to the high elevations that P.
engelmannii occupies.
D. rufipennis outbreaks identified from documentary records
across the southern Rockies of Utah and Colorado have been found to be
driven by three sets of variables, listed in order of importance: winter
temperatures, late summer and fall temperatures, and annual PDSI
(Hebertson and Jenkins 2008). Since PDSI is strongly influenced by
temperature (Sheffield et al. 2012), it can be concluded from the
results of Hebertson and Jenkins (2008) that the primary cause ofD. rufipennis outbreaks has been temperature and that there may
be indirect evidence for drought stress having had a smaller but
detectable influence. It has also been shown that meteorological
drought, often in association with the Atlantic Multi-decadal
Oscillation (AMO) have induced D. rufipennis outbreaks in
Colorado (Hart et al. 2014a). However, the evidence is equivocal whether
drought stress can be differentiated from temperature effects on
outbreaks for that region because precipitation did not show significant
effects for any season (i.e., Hart et al. 2014a, Appendix B). In another
retrospective study, D. rufipennis -killed trees in Alaska had
higher correlations between interannual tree-ring δ13C
and spring and summer temperatures when compared to surviving trees from
the same areas, but no differences in precipitation responses were
detected (Csank et al. 2016). Similar to the conclusions of Hart et al.
(2014a), the results of Csank et al. (2016) fit within the narrative
that bark beetles responded to drought, but only to the extent that
meteorological drought is a function of water balance driven by
precipitation and temperature. However, these results can be viewed
through a lens that distinguishes between meteorological drought (see
Fig. 1). Indeed, in our study, there was a lack of significant
relationships between ring-width growth parameters and various
meteorological drought metrics across all of our sites (Appendix S1:
Table S2). These results underscore that tree- or stand-level drought
stress may be independent or only weakly related to regional
meteorological drought conditions for these high elevation spruce
forests. Overall, these previous studies indicate that beetle-killed
trees were more responsive to temperature than surviving trees, which
agrees with our primary conclusion that drought stress played little
role in determining patterns of tree mortality.
There may be a distinct role for drought stress in the coldest boreal or
montane forests whereby water, still frozen in tree stems or in soils,
can limit water transport and photosynthesis and thereby promote
susceptibility to D. rufipennis attack (Hard 1987; Bowling et al.
2018). However, drought stress caused via this mechanism occurs at local
scales related to topographic variation, and therefore cannot explain
the large D. rufipennis outbreaks that have swept across manyP. engelmannii landscapes of the Rocky Mountains in recent
decades. Overall, the body of literature on climatic drivers of D.
rufipennis outbreaks has found ample evidence that outbreaks were
associated with periods of prolonged warmer air temperatures that may
have been associated with meteorological drought, whereas evidence is
absent or of minor importance that host drought stress increased tree or
forest susceptibility leading up to or during outbreaks.