Climate drivers of bark beetle outbreak dynamics in Norway spruce forests

Abstract

Bark beetles are among the most devastating biotic agents affecting forests globally and several species are expected to be favored by climate change. Given the potential interactions of insect outbreaks with other biotic and abiotic disturbances, and the potentially strong impact of changing disturbance regimes on forest resources, investigating climatic drivers of destructive bark beetle outbreaks is of paramount importance. We analyzed 17 time‐series of the amount of wood damaged by Ips typographus, the most destructive pest of Norway spruce forests, collected across 8 European countries in the last three decades. We aimed to quantify the relative importance of key climate drivers in explaining timber loss dynamics, also testing for possible synergistic effects. Local outbreaks shared the same drivers, including increasing summer rainfall deficit and warm temperatures. Large availability of storm‐felled trees in the previous year was also strongly related to an increase in timber loss, likely by providing an alternative source of breeding material. We did not find any positive synergy among outbreak drivers. On the contrary, the occurrence of large storms reduced the positive effect of warming temperatures and rainfall deficit. The large surplus of breeding material likely boosted I. typographus population size above the density threshold required to colonize and kill healthy trees irrespective of other climate triggers. Importantly, we found strong negative density dependence in I. typographus that may provide a mechanism for population decline after population eruptions. Generality in the effects of complex climatic events across different geographical areas suggests that the large‐scale drivers can be used as early warning indicators of increasing local outbreak probability.

Natural disturbances are spatially diverse but temporally synchronized across temperate forest landscapes in Europe

“…()). Drought—here defined as the combined effect of low water availability and high temperature—directly influences the temporal dynamics of natural disturbance in Europe through controlling bark beetle population dynamics (Marini et al., ). In particular, hot years improve bark beetle development and survival and lead to a higher probability of the completion of a second or third generational cycle per year.…”

“…Climate variability also influences the multivoltine life cycle of bark beetles (Baier, Pennerstorfer, & Schopf, ) and affects host colonization and defense (Netherer et al., ). Hence, climate is a driving force behind bark beetle population dynamics (Marini et al., ). Climate‐mediated amplifying feedbacks in the insect‐host system are particularly important due to an increasing risk of “hotter droughts” in the future (Allen, Breshears, & McDowell, ; Millar & Stephenson, ).…”

“…In addition to the temporal patterns of disturbance, also their large‐scale spatial patterns remain incompletely understood, as the few existing comparative studies in Europe (Marini et al., ) use widely differing data sources that do not allow a stringent analysis and attribution of differences in spatial patterns. More generally, a significant gap in data that is systematic and comprehensive in both space and time has hitherto limited our understanding of the biome‐scale dynamics of forest disturbance (Kautz et al., ).…”

Assessing the exposure of forest habitat types to projected climate change—Implications for Bavarian protected areas

“…It should be borne in mind that the direct impact of climate change is not the only threat to EU habitat types. Altered forest health conditions and pest outbreaks (e.g., bark beetles, see Marini et al, ) are additional challenges. Of outstanding importance for the here investigated forest habitat types is the ash dieback caused by the fungus Hymenoscyphus pseudoalbidus , which infects F. excelsior (Pautasso, Aas, Queloz, & Holdenrieder, ).…”

Traditional and Novel Indicators of Climate Change Impacts on European Forest Trees

“…In Northern Europe, the areas suitable for forests are supposed to extend northwards [84], but the interactions between trees and the bark beetle due to higher winter temperatures [85] and the lack of a genotype suited to the northernmost conditions (with special reference to the length of the day) [86] may limit the potential benefits.…”