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 (∆13 C) to drought compared to late-dying trees. Rather, evidence indicated the abundance and size of host trees may have modified ∆13 C responses to drought. ∆13 C 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.