Forest margins provide favourable microclimatic niches to swarming bark beetles, but Norway spruce trees were not attacked by Ips typographus shortly after edge creation in a field experiment

Abstract

Mass outbreaks of the European spruce bark beetle, Ips typographus, have caused devastating damage to Norway spruce-dominated forests in Central Europe. Biotic and abiotic natural disturbances as well as logging activities promote the fragmentation of forest stands, further increasing their susceptibility to damaging events. The creation of forest edges abruptly alters microclimatic conditions, such as trees suddenly being exposed to the sun. A common hypothesis is that trees remaining at the margins of clear cuts are initiation spots for spruce bark beetle infestation. This study comprehensively investigates the direct effects of forest edge establishment in spring on air and bark temperatures, soil water potential, tree sap flow, resin flow, bark defence compounds, and olfactometric cues for bark beetles provided by volatile organic compounds (VOCs) in proximity to trees. Ips typographus host acceptance was tested in limited-choice (attack box) and non-choice (Eppendorf tubes) field bioassays.Bark surface temperatures and resin flow clearly increased in trees of the freshly cut forest edges in spring, while sap flow rates did not differ from those of shaded trees in the inner forest. Soil water potential was high throughout the study season owing to sufficient precipitation and decreased only in August. Contrary to similar amounts of monoterpenes in the phloem of inner forest and edge trees, VOC concentrations were enhanced in May and June at the forest margin, also due to the presence of fresh logging residuals. The increased bark surface temperatures and VOC concentrations near forest edge trees were observed simultaneously with enhanced moving activity of I. typographus in the attack box bioassays soon after edge establishment in May. The effectiveness of increased resin flow in spring as a contributor to defence of edge trees could not be evaluated in the attack box bioassays, as boring holes at both edge and inner forest trees were observed, mainly later in the season. We conclude that fresh forest edges are not more susceptible to I. typographus attack shortly after establishment than closed forest stands but that microclimatic conditions of forest margins and olfactory signals from standing edge trees and logging residuals favour beetle swarming. Spruce bark beetle infestation patterns in fragmented forests are likely dependent on population densities and timing of predisposing abiotic disturbance and harvesting events.