Interaction between climate change driven bark beetle outbreaks, forest decline and nitrogen
deposition driven inertia in ecosystem succession in mountain ecosystems
Lead Author: Carl Beierkuhnlein (UBT)
Contributors: David Kienle (UBT), Andreas Schweiger (UBT), Frank Weiser (UBT), Marco Heurich
(Bavarian Forest National Park)
Focal Ecosystems: Mountain Ecosystems (Forests, Grasslands), Freshwater Ecosystems (catchments,
Focal Ecosystem Functions: Plant pests and pathogens, tree cover, water cycles and discharge
Focal Ecosystem Services: Tourism, Forestry, Carbon Sequestration
PAs involved in the ECOPOTENTIAL: Bavarian Forest National Park
Forests within Central Europes mountain areas are species-poor by nature. Only few tree species
contribute to the structure and main biomass of these forests such as beech (Fagus sylvatica) and
Norway Spruce (Picea abies). One reason for this simplicity is the extinction of many European tree
species during the Pleistocene and another one is the late post-glacial recolonization. However, such
ecosystems can be rich in structure and host a large number of other species. In addition, Central
Europe has been widely deforested during the middle ages and remnant forests were exploited for
early industrial resource use mainly related to glass production, ore melting, and salt production.
Only very marginal ecosystems were not impacted.
Today, mountain ecosystems and species can be attributed to climate sensitive zones (Bässler et al.
2010). Extinction risks for local populations and for a few endemic species can be assessed (Bässler
et al. 2009). In times of changing environmental conditions, however, such ecosystems whose func-
tioning is controlled by a small number of dominating trees can be seen as sensitive and more likely
to be at risk. The reason for this is that decreasing performance of certain species cannot be com-
pensated by other species that are better adapted (Yachi & Loreau 1999).
In the 1970s, Central European mountain forest ecosystems, particularly those that occur on sili-
ceous bedrock have been impacted by acid rain. Siliceous-originated soils are especially vulnerable
to acid rain because of their lower buffering capacity. Furthermore, the naturally strongly nitrogen-
limited areas experienced a high amount of atmospheric deposition of nitrogen. The resulting forest
decline happened at landscape scales, causing strong emotional and political responses, which trig-
gered the development of technologies, such as catalysts, in vehicles and industry. The breakdown
of Eastern European political and economic regimes lacking modern technologies additionally
caused a substantial reduction of pollutants. Consequently, the Waldsterben (forest die-back) syn-
drome disappeared, a sign of success of environmental policy. However, there is still a legacy of past
pollution and regardless of reduced sulphur emission, and subsequent deposition of nitrogen from
agricultural and traffic sources still remains a load. Today, and increasingly in the near future, climat-
ic changes (modified extremes, shifted seasonality, warming, modified precipitation regimes) are
posing new pressures to these ecosystems. Responses of perennial species with low capacity to
adapt to novel conditions are uncertain and may lead to novel ecosystems. However, the species
pool that could contribute to these future systems is very limited.
The legacy of the former acidification processes and the critical loads of pollutants, as well as the
spatial pattern of nitrogen deposition and discharge, can be well investigated in forest springs that
serve as sensitive indicator systems for the state of forest catchments. These rather punctual ecosys-
tems are still very natural in terms of human disturbances, which enables profit from these points of
aquifer discharge. Further, this allows to examine how the species in spring ecosystems respond to
geochemical and thermal conditions in their catchments.
Protected areas such as the Bavarian Forest National Park in Germany are aiming to allow natural
dynamics and the development of wilderness. In recent years, winters have been increasingly mild,
alongside prolonged summer that were warm and dry. Together with the natural dominance of cer-
tain tree species, this has led to outbreaks of bark beetles (Ips typographus), which specifically attack
spruce, causing a large scale breakdown of high altitude forest ecosystems (Zeppenfeld et al. 2015,
Seidl et al. 2016). In altitudes higher than 1000 m a.s.l., there has been no considerable forest man-
agement (which is the reason for the protected area) and forest ecosystems have evolved mainly
through natural drivers (Fig. 1). Paradoxically the outbreak was most prominent exactly in this area.
Bark beetle outbreaks, specifically of Ips typographus, are a widespread phenomenon in European
Protected Areas. The temporary breakdown of forest structures are taken as an argument for inten-
sified economic exploitation counteracted with conservation targets, as is currently the case in the
Bia?owie?a National Park in Poland (Schiermeier, 2016).
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. near the top of Großer Rachel
(Picture: David Kienle).
In the Bavarian Forest National Park, an extension of the area towards the north was implemented
some years ago, with salvage logging within the extension as a political compromise following public
participation. This ignores natural dynamics and the role of bark beetles to shape long-term trajecto-
ries of mountain forest ecosystems (Beudert et al. 2015). The highest values of species richness were
found in early successional stages after bark beetle attacks (Lehnert et al. 2013).
Bark beetle outbreaks can now be well understood and modelled (e.g. Fahse & Heurich 2011), but
these models now need to be linked with earth observation. Supressing outbreaks through logging is
hardly possible if a critical development of Norway spruce biomass has been reached. Particularly in
old growth forests that are still to be found in remote places of the park, clearance and logging must
be avoided (Mehr et al. 2012, Chylarecki & Selva 2016).
Now, open space could be reclaimed by tree regeneration and even adaptation to climate change via
upward shift of beech. This does not seem to be happening due to a native grass species (Cala-
magrostis villosa) that is profiting strongly from the high availability of light after forest dieback (Fig.
2). This dominance of the grass exhibiting leaf litter that is hardly decomposed and accumulates at
the soil surface creates inertia for tree establishment because tree seedlings cannot survive dry pe-
riods in these close mats of grass litter. The high amounts of grass biomass are very likely promoted
by the anthropogenic nitrogen deposition that reaches more than 20 kg N per year resulting in air-
borne fertilization. The role and impact of the inertia caused by the grass layer on the dynamics of
forest regeneration is likely to be relevant at various scales. Thus, this is also a topic for cross-scale
Figure 2: Dominance of Calamagrostis villosa is suppressing the establishment of juvenile trees
after forest dieback. The litter of this grass species is piling up to tens of centimetres.
There is uncertainty about the question of how important the contribution of Calamagrostis villosa
was before times of elevated nitrogen depositions. This may be proven by pollen records that could
be taken out of Rachel Lake, which is a natural high mountain lake with a comparable history to the
alpine glacial lakes (Fig. 3). This lake is also an important habitat for many species (Seibold et al.
2013). It can be hypothesized that the current nitrogen input plays an important role for the devel-
opment of mountain ecosystems, and this cannot be ignored in management and practice.
Figure 3: Rachel lake, a natural limnic system that is extremely isolated and offers options for
studies on historical developments based on pollen in the lake sediments.
The challenge for conservation management and for local policy makers is the fact that the outbreak
was most severe in natural ecosystems of the core area of the national park. When the park was
extended to the north, compromises were made. For instance, in the additional park area, dead
wood is removed, which is against ecological understanding and just a visual improvement. Howev-
er, environmental education has changed our perception of the dead forest ecosystem.
There is a fascinating example of added effects by subsequent environmental changes (historically
acidified soils, current nitrogen deposition and future climate change). Such impacts may create
inertia that is suppressing the natural regeneration of forest ecosystems, especially within this core
area of the national park with no active management (e.g. planting of trees) is applied that could
assist or stimulate ecosystem recovery.
Other groups of species also contribute to forest dynamics, for example red deer (Cervus elaphus).
Red deer move seasonally within their range and reach high densities at high elevations during
summer. Linking animal movement with remotely sensed data on ecosystem phenology would could
provide valuable insights and improve our understanding of red deer. Generally, interactions be-
tween disturbances, climate change, environmental pollution, animal movement, successional tra-
jectories of vegetation and herbivory are challenges for ecological research. Disentangling these
processes is necessary for improved and adaptive management of the park.
The Bavarian Forest National Park has extended knowledge and data in various fields of earth obser-
vation and remote sensing, ranging from airborne hyperspectral missions to LIDAR data (Bässler et
al. 2011, Leutner et al. 2012, Müller et al. 2014). Together with a well-equipped scientific depart-
ment of the park, these are excellent conditions for research and implementation of research out-
put. Further, the communication of knowledge to the public is enhanced by a series of facilities, in-
cluding the largest canopy trail on earth and two modern information centers.
Table 1: List of Critical Variables that describe the state and trends in ecosystem services and main
drivers of change in ECOPOTENTIAL protected areas in Central European forested mountains.
Drivers and Pressures
Method [reference] (type)*
Norway Spruce Die-Off
Tree cover (R)
Inertia for Tree Seedlings
Grass layer area
Grass cover (R)
Inertia for Tree Seedlings
Grass layer thickness
Field surveys (I)
Soil Nitrogen content
Dry and wet N-deposition (M)
Spring Water Nitrogen
Water samples (I)
Data loggers on water temperature (I)
* Type: R remote sensed; M Modelling output (based on EO); I in-situ/field collected data
Within simple ecosystems, it is easier to detect and quantify fundamental processes. In the case of
the Central European mountain forest ecosystems, these do cover large surfaces also outside of
In addition, collapsing ecosystems are understudied in Europe, because forests have been managed
intensely and disturbances and breakdown were urgently avoided through anthropogenic interfer-
ence in most cases. This is fundamentally different in America or Siberia. As such, these events can
be part of long-term natural dynamics (Jentsch et al. 2002), we find an excellent opportunity in the
Bavarian Forest National Park to improve our understanding of forest ecosystems.
Three Master theses had been conducted in the Bavarian Forest. One student studied the effects of
microhabitat and space limitation on Norway spruce regeneration in bark beetle affected areas. The
thesis tested whether lying dead wood influences positively Norway spruce regeneration, whereas
the cover of Calamagrostis villosa is antithetic to the frequency of woody microhabitats. The study
showed that positive temporal effects have a stronger impact on Norway spruce cover than spatial
aspects such as the amount of woody microhabitats or grass (graminoid) cover. The dominant grass-
vegetation seems todelay tree regeneration towards a closed canopy stage.
Another study tested the effect of distances to adult grown-up reproductive Norway spruce trees on
forest regeneration, based on the hypothesis that distance to adult trees acting as seed sources is a
major driver in this process. The results indicate that the distance to seed sources does not a major
role in forest regeneration.
Forest springs were another research topic in the national park. A student studied the effects of
different environmental drivers on species richness. There were no significant correlations between
conductivity as indicator for the mineral content of the spring water and vascular plant species rich-
In October 2019, the course Remote Sensing in Biodiversity Research was offered for master stu-
dents from the University of Bayreuth in the Bavarian Forest. The students developed their own
research hypotheses based on remote sensing datasets and existing field data. During the weeklong
stay in the national park, the students were introduced to its unique features, processes and chal-
lenges, while developing further skills within their own research projects.
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Last update: May, 2019