Drought induces spruce beetle (Dendroctonus rufipennis) outbreaks across northwestern Colorado

Abstract

This study examines influences of climate variability on spruce beetle (Dendroctonus rufipennis) outbreak across northwestern Colorado during the period 1650 2011 CE. Periods of broad-scale outbreak reconstructed using documentary records and tree rings were dated to 1843-1860, 1882-1889, 1931-1957, and 2004-2010. Periods of outbreak were compared with seasonal temperature, precipitation, vapor pressure deficit (VPD), the Palmer Drought Severity Index (PDSI), and indices of ocean-atmosphere oscillation that include the El Niño Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), and Atlantic Multidecadal Oscillation (AMO). Classification trees showed that outbreaks can be predicted most successfully from above average annual AMO values and above average summer VPD values, indicators of drought across Colorado. Notably, we find that spruce beetle outbreaks appear to be predicted best by interannual to multidecadal variability in drought, not by temperature alone. This finding may imply that spruce beetle outbreaks are triggered by decreases in host tree defenses, which are hypothesized to occur with drought stress. Given the persistence of the AMO, the shift to a positive AMO phase in the late 1990s is likely to promote continued spruce beetle disturbance.

If Long-Term Resistance to a Spruce Beetle Epidemic is Futile, Can Silvicultural Treatments Increase Resilience in Spruce-Fir Forests in the Central Rocky Mountains?

“…Large diameter Engelmann spruce would need to be removed in order to decrease the QMD and proportion of live spruce [36]. It is important to note, however, that during an epidemic, even in low risk stands, Engelmann spruce greater than 4 cm in dbh can be attacked by spruce beetles [18,20]. Once at epidemic levels, it is no longer just the high-risk stands that are impacted but the entire landscape.…”

“…However, composition may shift with a spruce beetle outbreak because resilience (i.e., adequate regeneration) would be limited in the short term. Given the increasing likelihood of stressed spruce trees due to increasing summer drought, the group selection method would not treat a large enough area of the stand quickly enough to provide adequate short-term resistance and long-term resilience at either the stand or landscape scale [20,30,65].…”

“…Of the 3.8 million hectares in Colorado, Utah, and Wyoming in the spruce and fir forest types, FIA (Forest Inventory and Analysis) data suggest nearly half of the area has experienced some level of mortality due to insects [19]. Recent spruce beetle epidemics have probably been driven by a combination of factors including susceptible stand structure and composition coupled with widespread drought [20]. Spruce beetles favor large diameter (>25 cm diameter at breast height (dbh)) Engelmann spruce for both protection and nutrition [21,22].…”

Monitoring Changes in Water Use Efficiency to Understand Drought Induced Tree Mortality

“…The self-calibrated Palmer drought severity index (scPDSI; Table 1; Figure 2) was used to quantify drought severity and to calculate the frequency of severe drought (2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015). The scPDSI is an improved version of the Palmer Drought Severity Index, which has been widely used to monitor drought conditions [4,[36][37][38][39][40][41][42][43]. Monthly scPDSI data was available through the Western Regional Climate Center (https://www.wrcc.dri.edu/).…”

Individual-Based Modeling: Mountain Pine Beetle Seasonal Biology in Response to Climate

“…Many insect species, including those deemed pests due to their significant ecological and economic impact, have been influenced by a changing climate (Bale et al 2002). Prime examples are bark beetles in the genus Dendroctonus for which a clear connection between weather and population irruptions and subsequent landscape-scale tree mortality has been shown (Hansen et al 2001;Berg et al 2006;Aukema et al 2008;Chapman et al 2012;Preisler et al 2012;Hart et al 2014). Changing climatic conditions are also responsible for a range shift in at least one species, Dendroctonus ponderosae, the mountain pine beetle (MPB).…”