Chronicle of bark beetle infestations
Forest mortality has been increasing in recent years. This is primarily because bark beetle populations are growing at a rapid rate, decimating sometimes relatively healthy trees. Are we really experiencing unprecedented extremes, or have similar disturbances occurred throughout history? And can past events help us to deal with the current situation? Scientists have been searching for answers to these questions in the pristine region of the High Tatra Mountains. Petr Kuneš and Helena Svitavská Svobodová from the Department of Botany at the Faculty of Science, Charles University contributed significantly to research led by Nick Schafstall from the Czech University of Agriculture.
Temperate coniferous forests are an important ecosystem in the northern hemisphere. However, in recent years they have encountered numerous stressors. As a result of climate change, the frequency and severity of droughts, fires, and windstorms are increasing. This has been accompanied by a rapid growth in the number of insects, including bark beetles. These can attack trees that are already dead and thus have a positive effect on the decomposition of woody matter. In recent years, however, primary infestations, where bark beetles attack trees that are still growing, have become increasingly common. They often decimate large forest stands.
Illustrative photo: K. Fraindová.
There is no doubt that the current situation is critical, but bark beetle and other disturbances have been accompanying forest stands since time immemorial. Importantly, it is the study of past episodes that can help us today. Finding out under what conditions such situations occur can give us a better chance of predicting current potential outbreaks and of putting appropriate measures in place.
A group of experts therefore travelled to the nearby High Tatra Mountains. This environment is ideal for studying the conditions of forest disturbance, as it is the largest non-intervention zone in Central Europe. Yet, in the past few years, there has been strong pressure from human activities and the effects of climate change. The scientists literally delved into the past here, with the help of fossil records of bark beetle remains from the forest hollow peat sequence. Alongside this, they also investigated changes in vegetation using pollen analysis and used sedimentary charcoal to reconstruct incidences of fires. These retrospective reconstructions were used to determine the occurrence of bark beetle calamities in conjunction with other disturbances in the area.
In 2016, a master sediment core, 5 × 50 cm in diameter with a D-shaped cross section, was extracted in an area called Diera Hollow, which is located near Tatranská Lomnica at an altitude of 982 m a.s.l. The area was severely affected by Windstorm Elizabeth in 2004 and subsequently by bark beetle infestations in 2007. To perform a quantitative analysis of the fossil remains, an additional 12 cores of proximal sediment were collected in 2017 using a gridded system (50 cm between cores). The main core was analysed using radiocarbon dating, and its geochemistry (chemical composition of the substrate), fossil beetle abundance, pollen, and charcoal particles were also assessed. The geochemistry and fossil occurrence of beetles were determined for the additional sedimentary cores.
The principal advantage of this research was the long data series, which went back 1400 years. The researchers recorded several significant periods of bark beetle occurrence during this time. They further compared these periods with pollen, charcoal, and geochemical records, which served to evaluate other conditions and disturbances at the time. The researchers identified 3 main peaks of bark beetle remains – after 2004, between 1140 and 1440, and between 930 and 1030.
The samples contained a large number of Pityogenes chalcographus and Pityophthorus pityographus and only a small number of Ips typographus. It is thus likely that fossils of P. chalcographus and P. pityographus could be useful proxies for past conifer bark beetle outbreaks in Central Europe. They occur along with fossils of I. typographus but appear to be well preserved. A significant correlation was found between the representation of macroscopic charcoal densities in the sediment and the occurrence of bark beetles, indicating an interaction between disturbance agents, bark beetles, and fire.
Records of disturbances dating back 1400 years have thus revealed that bark beetle outbreaks have been an important part of the regional regime of natural disturbances in the High Tatra Mountains for more than a millennium. In recent decades, they have intensified with increasing anthropogenic activity. The study thus provided an important basis for research on forest disturbances and bark beetle occurrences in other mountain areas. The authors also suggest that although the bark beetle is currently decimating forest ecosystems, it is not the only cause of tree mortality and is likely to be one of the factors promoting ecological stability and increased resilience of temperate conifer ecosystems over a period ranging from decades to centuries.
Kateřina Fraindová
Published:
May 02, 2022 04:05 PM
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