Impact of bark beetle infestations on timber production and carbon sequestration
under scenarios of climate change
R. Seidl, M.J. Lexer, W. Rammer and D. Jäger
Institute of Silviculture, Department of Forest and Soil Sciences, University of Natural
Resources and Applied Life Sciences (BOKU) Vienna, Peter Jordan Straße 82, 1190
Wien, Austria. e-mail: rupert.seidl@boku.ac.at
_______________________________________________________________________
Extended abstract
_______________________________________________________________________
Introduction
Dominating issues in the field of forest protection in Europe over the last 15 years have been
two severe storm events in 1990 and 1999 and an enormous progradation of bark beetles in
the wake of them (Wermelinger 2004). The windthrow disturbances coincided with the
extraordinary warm climate of this period (IPCC 2001) and led to the highest amounts of
bark beetle damage in European forests since at least 150 years (Schelhaas et al. 2003). In
addition, the increasing concern about future global warming and it’s possible feedback on
the development of biotic disturbance agents (e.g., Ayres and Lombardero 2000) revealed
the need of a more process-based understanding of disturbance agents to aid decision
makers with robust risk assessment tools even under changing environmental conditions.
This holds particularly true in secondary Norway spruce forests (Picea abies (L.) karst.) at
low elevations at sites naturally supporting broadlead and mixed species stands. Norway
spruce has been promoted far outside its natural range because of the superior productivity
and relatively ease of silvicultural management. The new paradigm of sustainable forest
management (SFM) has brought about a broader view on forest services and functions
beyond timber production. Additionally, the growing awareness of the climate change
mitigation and adaptation potentials in natural resource management has added substantial
complexity to forest management. However, disturbances in general and bark beetles in
particular are hardly explicitly addressed, neither in forest management planning nor in
studies investigating the climate change mitigation potential of forest management.
The aim of this study is twofold: First, we present an integrated modelling framework
including a thermodynamic model of bark beetle infestations, and test the model against
observed stand data on bark beetle damage. Second, the model is used to assess the likely
impacts of bark beetle infestations under scenarios of climate change to timber production
and carbon sequestration at the forest management unit level.
The PICUS v1.41 model
PICUS v1.41 is a hybrid patch model combining elements of a 3-dimensional patch model
and a process-based forest production model (Seidl et al. 2005). The horizontal resolution is
10 x 10 m and spatial interactions among patches are considered via the canopy light regime
and spatial seed dispersal. The concepts of inter- and intra species competition for
resources follow a modified patch model approach (see Lexer and Hönninger 2001),
whereas net primary production at the stand level is derived from utilizable solar radiation
and canopy quantum use efficiency following the 3-PG model (Landsberg and Waring 1997).
The coupling of both elements is inter alia accomplished via the leaf area of a stand and is
described in detail in Seidl et al. (2005). Below-ground processes are modelled applying a
1
biogeochemical process-model of Carbon and Nitrogen cycling (Currie et al. 1999). The
PICUS modelling framework incorporates a flexible management module allowing for all
sorts of thinning and harvesting operations as well as for planting and is designed for
decision support in forest management.
The sub-module of bark beetle (Ips typographus L.) infestation consists of (i) the
computation of the risk of a infestation for a stand, (ii) the derivation of the damage intensity
in case an infestation is predicted, and (iii) the spatial distribution of the damage in the 3D
model environment. The risk of an infestation depends on stand predisposition and on the
potential number of insect generations within a year. Stand predisposition to Ips typographus
is derived from qualitative empirical relationships incorporating the share of Norway spruce,
stand age, stand basal area and an estimate for drought (Netherer and Nopp-Mayr 2005,
Lexer 1995, Lexer and Hönninger 1998). A thermodynamic model of bark beetle
development (Netherer and Pennerstorfer 2001, Netherer et al. 2004) is applied to calculate
the potential number of bark beetle generations under given climatic conditions. Beetle
development is simulated according to a temperature sum required for full beetle
development assuming an optimal bark temperature of 30.4°C. Sister broods are explicitly
simulated under favourable thermal conditions (Netherer et al. 2004). Damage intensity is
related to an index of drought stress and the share of Norway spruce in a stand (Lexer
1995). The spatial distribution of the damage within the 3D patch model framework is
accomplished via the selection of the patch with the highest share of Norway spruce
biomass and the estimated number of infested trees are randomly killed within a radius of
12m from the patch center.
Study layout
For the purpose of evaluating the model with regard to bark beetle infestations a data set of
28 stands (location: central Carinthia, Austria) over a 4-year period (1990-1993) is used. Soil
samples were taken in all stands and climate data are derived from MTClim simulations
(Lexer 1995). The evaluation aims at comparing the observed damage in the period 1990-93
to the predictions of the model in three predisposition classes as proposed by Netherer and
Nopp-Mayr (2005).
The second part of the study dealing with impacts of bark beetle infestation on timber
production and carbon sequestration focuses on a forest enterprise (250 ha) in southern
Austria, featuring mainly secondary Norway spruce forests. For a detailed description of the
forest unit we refer to Unegg (1998). One baseline climate (C1, mean annual temperature =
7.6°C, annual precipitation= 1013 mm) and two climate change scenarios (C2, C3) derived
from the IPCC IS92a emission scenario are applied. Three management strategies are
investigated in this study: The first strategy (MS1) represents the current management
practice which is Norway spruce age-class forestry with several thinning interventions from
above and clear cut after a rotation period of 90 years. Within this “business as usual”
strategy the stands are naturally regenerated by means of a shelterwood system. The
second strategy (MS3) aims at a species change towards a higher share of deciduous
species. This conversion is mainly accomplished by underplanting of European beech
(Fagus sylvatica L.) below the canopy of Norway spruce or by afforestation of beech and
pedunculate oak (Quercus robur L.) after clear cutting. The prescribed management
schedule in this strategy includes several thinning interventions and a final clear-felling. The
third strategy (MS4) serves as a reference mimicking “natural” forest development without
any human interference, i.e., no management interventions are carried out.
For all strategies, a current inventory of the forest enterprise serves as initial state for the
simulations over a 100 year period. For an economic evaluation constant prices and costs
are assumed.
2
Results and discussion
Evaluation of the bark beetle infestation model
The comparison of bark beetle damage over a four year period revealed good agreement
between observed and predicted values within the three predisposition classes (Figure 1).
No statistical difference could be found between mean number of killed trees (p= 0.430;
0.704 and 0.496 for low, medium and high predisposition, respectively), but the variation is
considerably higher in the observation. However, considering the stochastic components of
the bark beetle module and the short time frame of four years this difference is likely to origin
from the study layout. Moreover, in the simulation mean predicted values for insect damage
were used neglecting the error around the mean estimate.
observed
predicted
n=5
100
80
60
stems/ha
0
20
40
80
60
stems/ha
0
20
40
80
60
40
0
20
stems/ha
high predisposition
100
medium predisposition
100
low predisposition
observed
predicted
n=17
observed
predicted
n=6
Figure 1 Observed vs. predicted bark beetle damage over a four year period in three predisposition classes.
Impact of bark beetle infestation on timber production and Carbon sequestration
Under current climate (scenario C1) the accumulated bark beetle damage over the 100 year
simulation was highest in MS4 (125.9 m³/ha) and distinctly lower in the intensively managed
strategies (MS1: 89.0 m³/ha or 7.9% of the harvested volume, MS3: 73.5 m³/ha or 6.7% of
the harvested volume). Under climate change conditions bark beetle damage rises distinctly:
In MS1 under the warmer and drier climate scenario C3 21.8% of the total harvest is from
salvage cuttings whereas it is 17.7% under the conversion scenario MS3. For the economic
situation of the enterprise this implies an increasing harvest level and thus increased
contribution margins on a per hectare basis. Under MS1 the harvest level is increased for
10.2% relative to current climate due to increased salvage cutting under climate change (C3)
whereas the contribution margin increases only by 4.8%. Furthermore, the “benefit” of higher
contribution margins is to the cost of reduced standing stock and value in the forest. After the
100 year simulation the standing stock in MS1 under climate change conditions (C3) is
43.1% lower than estimated under current climate (C1) due to increased bark beetle damage
(23.2% lower under MS3).
With the strategies MS1 and MS3, mean Carbon stock in the forest ecosystem is lower with
the bark beetle module applied in comparison to simulations without biotic disturbances,
especially under conditions where disturbance frequency and severity are increasing. The
difference in mean total C-stock between simulations with and without bark beetle
infestations is in the range of -0.5 to -5.4 t C/ha for the climate change scenarios. This loss is
mainly due to reduced aboveground C stocks, whereas soil C is slightly increasing. The
unmanaged strategy (MS4) shows a different pattern: Including the biotic disturbances in the
simulations increased the mean C-stock in the ecosystem due to the deadwood on the site
and faster re-growth of natural regeneration (range over all climate scenarios: +20.0 to +26.1
t C/ha).
3
Bark beetle damage in vulnerable forest ecosystems such as secondary coniferous forests is
found to rise distinctly under a warming climate. The implications for sustainable forest
management are potentially large since the sustainable preservation of all required forest
functions is likely to become more difficult. Potential to mitigate the adverse effects are in
silvicultural measures such as the conversion to tree species compositions better adapted to
the prevailing site conditions. However, the study showed that under a “realistic” (e.g., slow)
conversion scenario the time horizons until positive net effects occur are long and damages
are still to rise. Nevertheless, adaptive silvicultural planning could prepare the ground for
future mitigation strategies. In a bigger context, strategies of climate change mitigation often
incorporate measures of forest management and are even discussed within the frame of the
Kyoto Protocol (UNFCCC 1997). However, with this study we could show that assessing the
Carbon sequestration potential of forest ecosystems and not accounting for the main
disturbance agents is likely to introduce a considerable bias. Above- and below-ground C
dynamics are considerably altered through disturbances and the need for sanitary fellings
and adaptive management measures potentially interferes with the goals of C sequestration.
Especially in unmanaged forest development, often serving as “baseline” scenario,
disturbances are major constituents of the systems dynamic and contribute largely to
processes of Carbon cycling.
References
Ayres, M.P. and Lombardero, M.J. (2000) Assessing the consequences of global change for forest
disturbance from herbivores and pathogens. The Science of the Total Environment 262: 263-286.
Currie, W.S.; Nadelhoffer, K.J. and Aber, J.D. (1999) Soil detrital processes controlling the movement of 15N
tracers to forest vegetation. Ecological Applications 9: 87-102.
IPCC
(2001)
Intergovernmental
Panel
on
Climate
Change:
The
scientific
basis.
http://www.grida.no/climate/ipcc_tar/wg1/index.htm
Landsberg, J.J. and Waring, R.H. (1997) A generalized model of forest productivity using simplified concepts
of radiation-use efficiency, carbon balance and partitioning. Forest Ecology and Management 95: 209-228.
Lexer, M.J. (1995) Beziehungen zwischen der Anfälligkeit von Fichtenbeständen (Picea abies (L.) Karst.) für
Borkenkäferschäden und Standorts- und Bestandesmerkmalen unter besonderer Berücksichtigung der
Wasserversorgung. Dissertation, University of Natural Resource Management and Applied Life Sciences (BOKU)
Vienna. 210 p. (in German)
Lexer, M.J. and Hönninger, K. (1998) Simulated effects of bark beetle infestations on stand dynamics in Picea
abies stands: coupling a patch model and a stand risk model. Beniston, M. and Innes, J.L. (Eds.). The impacts of
climate variability on forests. Springer, Berlin. pp. 288-308.
Lexer, M.J. and Hönninger, K. (2001) A modified 3D-patch model for spatially explicit simulations of vegetation
composition in heterogeneous landscapes. Forest Ecology and Management 144: 43-65.
Netherer, S. and Nopp-Mayr, U. (2005) Predisposition assessment systems (PAS) as supportive tools in forest
management – rating of site and stand-related hazards of bark beetle infestation in the High Tatra Mountains as
an example for system application and verification. Forest Ecology and Management 207: 99-107.
Netherer, S. and Pennerstorfer, J. (2001) Parameters relevant for modelling the potential development of Ips
typographus (Coleoptera: Scolytidae). Integrated Pest Management Reviews 6: 177-184.
Netherer, S.; Pennerstorfer, J.; Baier, P.; Schopf, A. and Führer, E. (2004) Modellierung der Entwicklung
des Fichtenborkenkäfers, Ips typographus L. als Grundlage einer umfasenden Risikoanalyse. Mitteilungen der
Deutschen Gesellschaft für allgemeine und angewandte Entomologie 14: 277-282. (in German)
Schelhaas, M.J.; Nabuurs, G.J. and Schuck, A. (2003) Natural disturbances in European forests in the 19th
and 20th centuries. Global Change Biology 9: 1620-1633.
Seidl, R.; Lexer, M.J.; Jäger, D. and Hönninger, K. (2005) Evaluating the accuracy and generality of a hybrid
forest patch model. Tree Physiology, in press.
Unegg, F. (1998) Erstellung und Anwendung eines Regelwerks zur waldbaulichen Entscheidungsfindung in
schneebruchgeschädigten, sekundären Fichtenbeständen. Dargestellt am Beispiel des Forstbetriebes Kleinszig
unter Anwendung von GIS. Diploma Thesis, University of Natural Resource Management and Applied Life
Sciences (BOKU) Vienna. 113 p. (in German)
UNFCCC (1997) Kyoto Protocol to the United Nations Framework Convention on Climate Change.
http://unfccc.int/essential_background/kyoto_protocol/background/items/1351.php
Wermelinger, B. (2004) Ecology and management of the spruce bark beetle Ips typographus – a review of
recent research. Forest Ecology and Management 202: 67-82.
4