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Climate drivers of bark beetle outbreak dynamics in Norway spruce forests
Lorenzo Marini
1,2
, Bjørn Økland
3
, Anna Maria Jönsson
4
, Barbara Bentz
5
, Allan Carroll
6
, Beat Forster
7
, Jean-Claude
Grégoire
8
, Rainer Hurling
9
, Louis Michel Nageleisen
10
, Sigrid Netherer
11
, Hans Peter Ravn
12
, Aaron Weed
13
, Martin
Schroeder
2
1
DAFNAE-Entomology, University of Padova, viale dell’Università 16, 35020 Legnaro, Padova, Italy
2
Department of Ecology, Swedish University of Agricultural Sciences, Box 7044, SE-750 07, Uppsala, Sweden
3
Norwegian Institute of Bioeconomy Research, Box 115, N-1431 Ås, Norway
4
Department of Physical Geography and Ecosystem Science, Lund University, Sölvegatan 12, SE-223 62 Lund,
Sweden
5
USDA Forest Service, Rocky Mountain Research Station, Logan, Utah 84321, USA
6
Department of Forest & Conservation Sciences, University of British Columbia, 2424 Main Mall, Vancouver, BC,
V6T 1Z4, Canada
7
Swiss Federal Research Institute WSL Zürcherstr. 111, CH-8903 Birmensdorf, Switzerland
8
Biological Control and Spatial Ecology, Université Libre de Bruxelles, CP160/12, 50 av FD Roosevelt, 1050
Brussels, Belgium
9
Department of Forest Protection, Section Beetles and Test Plant Protection, Northwest German Forest Research
Institute (NW-FVA), Grätzelstr. 2, D-37079, Göttingen, Germany
10
Ministère de l'Agriculture, de l'Alimentation et de la Forêt, Département Santé des Forêts, Champenoux, 54280
Seichamps, France
11
Department of Forest and Soil Sciences, BOKU – University of Natural Resources and Life Sciences, Institute of
Forest Entomology, Forest Pathology and Forest Protection, Hasenauerstraße 38, 1190 Wien, Austria
12
Department of Geosciences and Natural Resource Management, University of Copenhagen, Rolighedsvej 23, DK-
1958 Frederiksberg C., Denmark
13
National Park Service, Inventory and Monitoring Program, Fredericksburg, VA 22405, USA
Corresponding author: Lorenzo Marini, DAFNAE-Entomology, University of Padova, viale dell’Università 16,
35020 Legnaro, Padova, Italy. E-mail: lorenzo.marini@unipd.it. Tel.: +39 049 8272807
Decision date: 22-Oct-2016
This article has been accepted for publication and undergone full peer review but has
not been through the copyediting, typesetting, pagination and proofreading process,
which may lead to differences between this version and the Version of Record. Please
cite this article as doi: [10.1111/ecog.02769].
Accepted Article
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ABSTRACT
Bark beetles are among the most devastating biotic agents affecting forests globally and
several species are expected to be favored by climate change. Given the potential
interactions of insect outbreaks with other biotic and abiotic disturbances, and the
potentially strong impact of changing disturbance regimes on forest resources,
investigating climatic drivers of destructive bark beetle outbreaks is of paramount
importance. We analyzed 17 time-series of the amount of wood damaged by Ips
typographus (L.), the most destructive pest of Norway spruce forests, collected across 8
European countries in the last three decades. We aimed to quantify the relative
importance of key climate drivers in explaining timber loss dynamics, also testing for
possible synergistic effects. Local outbreaks shared the same drivers, including
increasing summer rainfall deficit and warm temperatures. Large availability of storm-
felled trees in the previous year was also strongly related to an increase in timber loss,
likely by providing an alternative source of breeding material. We did not find any
positive synergy among outbreak drivers. On the contrary, the occurrence of large
storms reduced the positive effect of warming temperatures and rainfall deficit. The
large surplus of breeding material likely boosted I. typographus population size above
the density threshold required to colonize and kill healthy trees irrespective of other
climate triggers. Importantly, we found strong negative density dependence in I.
typographus that may provide a mechanism for population decline after population
eruptions. Generality in the effects of complex climatic events across different
geographical areas suggests that the large-scale drivers can be used as early warning
indicators of increasing local outbreak probability.
Accepted Article
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INTRODUCTION
Climate is currently changing at an unprecedented rate with potentially profound effects
on disturbance regimes in forest ecosystems (Ayres and Lombardero 2000; Weed et al.
2013). Bark beetles are amongst the most devastating biotic agents affecting forests
globally (Anderegg et al. 2015) and several species are expected to be favored by
climate change (Worrell 1983; Jönsson et al. 2009; Bentz et al. 2010). In particular, the
combination of increasing frequency of drought events and warmer temperatures are
considered important predisposing factors triggering bark beetle outbreaks directly by
affecting insect population dynamics, and indirectly through alteration of host plant
growth and defense (Jactel et al. 2012; Weed et al. 2013; Hart et al. 2013; Raffa et al.
2015; Bentz and Jönsson 2015; Meddens et al. 2015). Given the potential interactions of
insect outbreaks with other biotic and abiotic disturbances and the potentially strong
impact of changing disturbance regimes on forest resources (Buma 2015), investigating
the climate drivers of bark beetle outbreaks is of paramount importance.
In Europe, Ips typographus (L.) is considered one of the most destructive
pests of conifer forests causing significant economic losses on a regular basis (Grégoire
et al. 2015). At low population densities, I. typographus breeds in fresh wind-thrown or
dying spruces and usually does not succeed in colonizing healthy trees that are protected
by effective defense mechanisms (Krokene 2015). Extensive drought or windfall events
can trigger beetle outbreaks by lowering tree resistance or raising the population size
above the density threshold required to colonize and kill healthy trees (Christiansen and
Bakke 1988). Such outbreaks may last several years, can cause important economic
losses (Schelhaas et al. 2003), and usually end when the supply of suitable breeding
material is exhausted or trees recover their resistance (Økland and Bjørnstad 2006).
Accepted Article
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At the regional scale, I. typographus population dynamics is expected to
be strongly affected by climatic factors (Kausrud et al. 2011). Warmer spring and
summer temperatures generally accelerate bark beetle development, thus increasing
attack probabilities for standing forests (Jönsson et al. 2009; Marini et al. 2012).
Moreover, severe drought events combined with warm temperatures can impair tree
water supply and enhance tree susceptibility to I. typographus attacks (Wermelinger
2004; Netherer et al. 2015). Storm disturbance is another key driver of population
outbreaks as shown by several studies analyzing long term regional fluctuations of bark
beetle damage (Økland and Berryman 2004; Økland and Bjørnstad 2006; Marini et al.
2013). Storms provide a surplus of suitable host material in the form of wind-thrown
trees devoid of vital defenses. Mass propagation of bark beetles even in small storm
gaps potentially results in gradations strongly impacting the surrounding, still standing
forests (Schroeder 2010; Kärvemo et al. 2014). Despite ample investigation of these
factors regionally, a large-scale synthesis of outbreak data is still missing.
Potential positive synergies between density-independent processes such
as climate drivers may also affect outbreak propensity. A positive synergy implies
interactions among drivers whereby the total effect on a population outbreak is greater
than the sum of each individual driver contribution. For instance, standing trees may be
at higher risk of mortality when drought and high temperatures coincide (e.g. ‘hotter
drought’ Millar & Stephenson 2015), or when the effects of storm and drought co-occur
(Worrell 1983). In previous studies analyzing time-series of I. typographus-caused tree
mortality (e.g. Økland and Berryman 2004; Marini et al. 2013), the low frequency of
storms, the short length of the time-series and the relatively small spatial extent of the
regions examined have constrained the exploration of these synergies. Although these
interactions have often been hypothesized based on observations of single regional
Accepted Article
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outbreaks (reviewed in Wermelinger 2004; Kausrud et al. 2011) no large-scale
empirical test has been possible to date. This significant knowledge gap is currently
preventing our ability to predict the potential destructive impact of I. typographus under
a global change scenario.
Although the role of top-down biotic factors is considered secondary
compared to the abiotic triggers explained above (Marini et al. 2013), a strong
indication for a negative density feedback regulating I. typographus populations was
shown at the regional scale (Økland and Berryman 2004; Marini et al. 2012, 2013).
First, availability of breeding material can strongly regulate I. typographus populations
given, for instance, the lack of suitable stressed trees after large demographic
explosions. Second, at high population density when offspring are forced to attack
unsuitable healthy trees, many individuals are exposed to the risk of colonization failure
or at least of lower performance (Komonen et al. 2011). Understanding how density-
dependent factors affect attack dynamics may be critical for predicting the future
trajectory of outbreaks.
Distribution of I. typographus in Western Europe covers a wide climatic
range from Scandinavia to the Southern Alps. Despite proven local correlations
(reviewed in Wermelinger 2004; Kausrud et al. 2011; Grégoire et al. 2015; Seidl et al.
2015), it is unknown to what extent the drivers described above control outbreak
dynamics along this broad latitudinal gradient. Furthermore, potential synergies among
drivers have been largely overlooked, although we expect that several variables
associated with climate change may interact to influence outbreak dynamics. Here, we
examined 17 time-series collected across Europe documenting timber volume loss due
to I. typographus. When attacking standing trees, I. typographus is obliged to kill the
plant to reproduce. Hence, tree mortality is expected to be positively related to
Accepted Article
