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Doctoral thesis2024Open access

Carbon castles vs. beetle-fungus armies : phenology of the spruce bark beetle and Norway spruce resistance in times of climate change

Öhrn, Petter


In recent years, carbon sequestration in European forests has decreased due to extensive droughts and bark beetle outbreaks brought on by climate change. The Eurasian spruce bark beetle (Ips typographus (L.)) is a natural disturbance agent in forests where Norway spruce (Picea abies (Karst)) is dominant. Through mass attacks the beetle can overwhelm defences of healthy trees. Drought stress can further reduce tree resistance and predispose the trees to bark beetle attack.

The aim of this thesis was to generate knowledge and investigate both the region-specific seasonal phenology of the spruce bark beetle in southern Sweden and the resistance of its host, the Norway spruce. Pheromone traps and felled trees that were colonised were used to determine flight activity and required thermal sums (degree-days, dd >5°C) for flight start, reemergence of parental beetles, and the emergence of filial beetles. The influence of weather, phenology, and soil moisture conditions, and lag effects following a severe drought event in 2018, on spruce tree resistance was quantified by inoculating a blue stain fungus associated with bark beetles on three sites in southern Sweden on four occasions during the 2019 and 2021 growing seasons. Fungal growth was expected to show susceptibility to bark beetle attack.

Flight activity started at the end of April (47 dd after 1 January). Re-emergence of parental beetles from the first brood started at the end of May (122 dd after colonisation), and continued flight showed that sister broods were frequent. The new generation started to emerge at the end of June (437 dd after colonisation) to initiate a second generation. Tree resistance was lower in the early season (June) and differed with both precipitation levels and local soil moisture conditions. Resistance was higher in 2021 than 2019, indicating a recovery. In both years, there was a significant correlation between lesion size (tree resistance) and water availability in the autumn of the previous year. In the current climate in southern Sweden, tree resistance is lowest during tree attacks from sister brood flights (June). Understanding thresholds in both bark beetle development and tree drought stress variables is crucial for predicting the impact of future bark beetle outbreaks in a climate change context.


Picea abies; Grosmannia europhioides; blue stain fungi; Ips typographus; flight activity; drought; soil moisture

Published in

Acta Universitatis Agriculturae Sueciae
2024, number: 2024:35ISBN: 978-91-8046-334-8, eISBN: 978-91-8046-335-5Publisher: Swedish University of Agricultural Sciences