Small beetles, large-scale drivers: How regional and landscape factors affect local bark beetle outbreaks
Bark beetles are among the most important biotic disturbance agents in forests globally. Damage from bark beetles has increased in many ecosystems recently, and is expected to further intensify as a consequence of climate change. While there is growing knowledge on bark beetle biology and local predisposing factors, the contingency of outbreak dynamics on large-scale drivers remains poorly understood. Here, our aim was to investigate the contribution of regional and landscape factors to local bark beetle infestation risk. Focusing on Central Europe centered around the Bavarian Forest National Park (Germany) we analyzed drivers across eight orders of magnitude in scale (from 102 to 1010 m²) with regard to their influence on Ips typographus infestation over a 23-year period. Time-discrete hazard modeling was used to control for local factors and temporal dependencies. Subsequently, beta regression was applied to determine the influence of regional (i.e., variation in climate and bark beetle population pressure) and landscape (i.e., spatial structure of the host and beetle populations, characterized by graph theory) factors.
We found that – in addition to local predisposition – large-scale drivers strongly influenced bark beetle infestation risk. Outbreak waves were closely related to spatial connectedness on the landscape as well as to regional bark beetle population pressure. Furthermore, regional summer drought was identified as an important trigger for local infestation pulses. Large-scale synchrony and connectivity were thus key drivers of the unprecedented bark beetle outbreaks recently observed in Central Europe. Our multi-scale analysis provides strong evidence that the risk for biotic disturbances is highly dependent on drivers beyond the influence of traditional stand-scale management. Consequently, the response of ecosystem management to changing disturbance regimes should focus on fostering resilience, and requires the consideration of landscape and regional processes. In this regard our finding of strong top-down controls also suggests their utility as early warning indicators, and calls for regional, cross-border disturbance monitoring and analysis schemes.