Abstract
Drought is considered to enhance susceptibility of Norway spruce (Picea abies) to infestations by the Eurasian spruce bark beetle (Ips typographus, Coleoptera: Curculionidae), although empirical evidence is scarce. We studied the impact of experimentally induced drought on tree water status and constitutive resin flow, and how physiological stress affects host acceptance and resistance.We established rain-out shelters to induce both severe (two full-cover plots) and moderate (two semi-cover plots) drought stress. In total, 18 sample trees, which were divided equally between the above treatment plots and two control plots, were investigated. Infestation was controlled experimentally using a novel ‘attack box’ method.Treatments influenced the ratios of successful and defended attacks, but predisposition of trees to infestation appeared to be mainly driven by variations in stress status of the individual trees over time. With increasingly negative twig water potentials and decreasing resin exudation, the defence capability of the spruce trees decreased.We provide empirical evidence that water-limiting conditions impair Norway spruce resistance to bark beetle attack. Yet, at the same time our data point to reduced host acceptance byI. typographus with more extreme drought stress, indicated by strongly negative pre-dawn twig water potentials.
Highlights
Drought has been identified as one of the main triggers of bark beetle (Coleoptera: Curculionidae, Scolytinae) outbreaks in conifer stands (Rouault et al, 2006; Dobbertin et al, 2007; Seidl et al, 2011; Kaiser et al, 2013; Williams et al, 2013)
This study addresses the following research questions: How does the physiological state of the sample trees, indicated by the pre-dawn twig water potential, osmotic potential of the secondary phloem and resin flow (RF), vary seasonally? Is attack activity of I. typographus subject to seasonal changes and are ratios of successful and defended attacks related to the underlying drought treatment? Which tree physiological variables most significantly explain successful and defended attacks?
Soil volumetric water content (SWC) conditions were similar for 0–30 cm and 0–60 cm soil depth; we only present the data relating to the upper, main rooting horizon (Fig. 3d)
Summary
Drought has been identified as one of the main triggers of bark beetle (Coleoptera: Curculionidae, Scolytinae) outbreaks in conifer stands (Rouault et al, 2006; Dobbertin et al, 2007; Seidl et al, 2011; Kaiser et al, 2013; Williams et al, 2013). Infestation of Norway spruce (Picea abies) forests by the Eurasian spruce bark beetle (Ips typographus), a major biotic forest disturbance agent in Europe, was found to be significantly correlated with summer precipitation deficits and increased temperature (Baier et al, 2007; Faccoli, 2009; Marini et al, 2012, 2013). Various studies have demonstrated the relationship between bark beetle attack and pre-dawn leaf/twig water potential, a proxy for drought stress status (Sellin, 1997). This study is the first to investigate how drought stress of mature Norway spruce trees influences both their acceptance as a host by I. typographus and their ability to defend against this pest
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