Spruce beetle outbreak was not driven by drought stress: Evidence from a tree‐ring iso‐demographic approach indicates temperatures were more important

Abstract

Climate change has amplified eruptive bark beetle outbreaks over recent decades, including spruce beetle (Dendroctonus rufipennis). However, for projecting future bark beetle dynamics there is a critical lack of evidence to differentiate how outbreaks have been promoted by direct effects of warmer temperatures on beetle life cycles versus indirect effects of drought on host susceptibility. To diagnose whether drought‐induced host‐weakening was important to beetle attack success we used an iso‐demographic approach in Engelmann spruce (Picea engelmannii) forests that experienced widespread mortality caused by spruce beetle outbreaks in the 1990s, during a prolonged drought across the central and southern Rocky Mountain region. We determined tree death date demography during this outbreak to differentiate early‐ and late‐dying trees in stands distributed across a landscape within this larger regional mortality event. To directly test for a role of drought stress during outbreak initiation we determined whether early‐dying trees had greater sensitivity of tree‐ring carbon isotope discrimination (∆13C) to drought compared to late‐dying trees. Rather, evidence indicated the abundance and size of host trees may have modified ∆13C responses to drought. ∆13C sensitivity to drought did not differ among early‐ versus late‐dying trees, which runs contrary to previously proposed links between spruce beetle outbreaks and drought. Overall, our results provide strong support for the view that irruptive spruce beetle outbreaks across North America have primarily been driven by warming‐amplified beetle life cycles whereas drought‐weakened host defenses appear to have been a distant secondary driver of these major disturbance events.

Spruce Beetle Outbreak Increases Streamflow From Snow‐Dominated Basins in Southwest Colorado, USA

“…Bark beetles, in particular spruce beetle ( Dendroctonus rufipennis ) and mountain pine beetle ( Dendroctonus ponderosae ), have affected approximately 58.8 million acres of coniferous forest in the region since 2000 (USDA Forest Service, 2020). Bark beetle population growth over the last several decades has been attributed to increased reproduction rates (Mitton & Ferrenberg, 2012) and warmer air temperatures (Pettit et al., 2020). Bark beetles and associated secondary fungal infections restrict water uptake and kill host trees over the course of a growing season (Frank et al., 2014; Hubbard et al., 2013).…”

Reference Gene Selection for Normalizing Gene Expression in Ips Sexdentatus (Coleoptera: Curculionidae: Scolytinae) Under Different Experimental Conditions

“…However, favorable conditions such as drought can lead to an outbreak (epidemic phase). Beetles start attacking the healthy standing trees due to weakened defenses caused by warmer temperatures (Bouhot et al, 1988 ; Marini et al, 2017 ; Pettit et al, 2020 ) and larger windthrows (Kausrud et al, 2012 ; Biedermann et al, 2019 ; Sommerfeld et al, 2020 ). Furthermore, conifers have self-defense mechanisms to ward off bark beetle infestations with secondary metabolites such as terpenes (Ferrenberg et al, 2014 ; Denham et al, 2019 ).…”

Do Extreme Climate Events Cause the Degradation of Malus sieversii Forests in China?

“…The negative effects of extreme climate are following expressed by drought, heat waves and wildlife and insect disturbance causing increased tree mortality and dead wood, reduced species diversity and reduced productivity ( Ciais et al, 2005 ; Rammig et al, 2010 ; Birch, 2014 ). Extreme high temperature causes lower water use efficiency by accelerating the transpiration of vegetation, and also exacerbates outbreaks of forest pests and diseases ( Kolb et al, 2016 ; Pettit et al, 2020 ), all of which slow tree growth. Extreme drought results in reduced photosynthesis, which decreases growth and increases mortality of trees ( Feldpausch et al, 2016 ), and also impacts on phenology, such as flowering cycles and end-of-season vegetative growth ( Nagy et al, 2013 ; Cardil et al, 2014 ).…”