Sub-fossil bark beetles as indicators of past disturbance events in temperate Picea abies mountain forests
Abstract
Temperate mountain forests have experienced an increase in frequency and severity of natural disturbances (e.g., droughts, fires, windstorms and insect outbreaks) in recent decades due to climate and environmental change. Outbreaks of bark beetles have caused significant dieback of conifer forests in Central Europe and it is essential to model and predict the potential severity of future bark beetle outbreaks.
However, to predict future bark beetle activity, historical baseline information is required to contextualize the magnitude of current and potential future outbreaks. A fossil beetle record from a forest hollow in the Tatra Mountains, Slovakia; one of the best-preserved national parks in Central Europe, was produced to identify insect outbreaks during the last millennia.
Sub-fossil bark beetle remains were compared with parallel pollen and charcoal to assess whether peaks in conifer bark beetle remains correspond with indications of disturbance documented in historical or sedimentary fossil records. Three peaks in bark beetle remains were detected (1) post-2004, (2) AD 1140-1440, and (3) AD 930-1030.
The abundance of species Pityogenes chalcographus and Pityophthorus pityographus in the two top samples can be linked directly to large bark beetle outbreaks in the High Tatra Mountains after 2004. P. chalcographus and P. pityographus are also the abundant species in the second peak (AD 1140-1440) while the third peak (AD 930-1030) consists of the species Polygraphus poligraphus.
The most prominent conifer bark beetle in Central Europe, Ips typographus, was found to be present in most of the samples but always at very low numbers. It is plausible that P. chalcographus and P. pityographus fossils might be useful proxies for past conifer bark beetle outbreaks in Central Europe, as they occur together with fossils of I. typographus but appear to be well-preserved.
A significant correlation was found between primary bark beetles and macroscopic charcoal densities in the sediment, highlighting the complex interactions between disturbance agents, bark beetles and fire, in this long-term regime of natural disturbances. Our 1400-year disturbance record shows how bark beetle outbreaks have been an important component of the regional natural disturbance regime for over a millennium and have intensified with increasing anthropogenic activity.
Bark beetle outbreaks are likely one of the drivers promoting the future ecological stability of the temperate conifer ecosystem over decades to centuries.