Interaction between climate change driven bark beetle outbreaks and forest decline and nitrogen deposition driven inertia in ecosystem succession in mountain ecosystems
Lead Author: Carl Beierkuhnlein (UBT)
Contributors: Beierkuhnlein, Schweiger, Hoffmann, Ulloa, Jentsch, Heurich, Irl, Kienle, (UBT)
Protected areas such as the Bavarian Forest National Park (Nationalpark Bayerischer Wald) in Germany are aiming to allow natural dynamics and the development of wilderness. In recent years, increasingly mild winters combined with prolonged summers that are warm and dry, have contributed together with the natural dominance of certain tree species to outbreaks of bark beetles (Ips typographus) that attacked specifically spruce and have caused a large scale breakdown of high altitude forest ecosystems. In altitudes higher than 1000 m a.s.l. there has been no considerable forest management (which is the reason for the protected area) and forest ecosystems have evolved mainly through natural drivers (Fig. 1). It seems a paradox that exactly in the no-management area the outbreak was most prominent. Bark beetle outbreaks, particularly through Ips typographus, are a widespread phenomenon in European Protected Areas. The temporary breakdown of forest structures then are even taken as an argument for intensified economic exploitation counteracting with conservation targets as it is currently the case in the Bialowieza National Park in Poland.
These strong impacts on forest vegetation structure reset the system's state and paved the way for alternative species dominance relations and thus, alternative stable states which diminished forest recovery since then. The process of forest recovery is thereby significantly slowed down by a native grass species (Calamagrostis villosa) which became increasingly competitive due to the sudden, environmental shifts induced by the bark beetle outbreak. This new competitor grows faster than the spruce in conditions of clear cuts, which means higher light and nutrients availability. Furthermore, the productivity of C. villosa is additionally favored by the input of N due to human activity, especially agriculture. Thus, diminished forest recovery after the bark beetle outbreaks can be assumed to be a product of positive feedbacks between several abiotic and biotic ecosystem components adaptively affecting the historic, current and future ecosystem response. The biotic and abiotic (natural and anthropogenic) factors like the natural low diversity of the forest, historical atmospheric acidification and the differences in management synergistically influence not only the sensitivity, likelihood and effect size of bark beetle outbreaks but also subsequent forest regeneration.
In general, collapsing ecosystems are understudied in Europe, because forests have been managed intensely and disturbances and break-down were urgently avoided through anthropogenic interference in most cases. This is fundamentally different in America or Siberia. As such events can be part of long-term natural dynamics we find an excellent opportunity in the Bavarian Forest National Park to improve our understanding of forest ecosystems.
Figure 1: Forest decline in the high altitude of the National Park Bayerischer Wald close to the border between Germany and the Czech Republic in 1200 m a.s.l. close to Mount Rachel.