Invasion of Norway spruce diversifies the fire regime
in boreal European forests
Mikael Ohlson1*, Kendrick J. Brown2, H. John B. Birks3,4, John-Arvid Grytnes3,
Greger Ho¨rnberg5, Mats Niklasson6, Heikki Seppa¨7 and Richard H. W. Bradshaw8
1Department of Ecology and Natural Resource Management, Norwegian University of Life Sciences, PO Box 5003,
NO-1432 A˚s, Norway; 2Geological Survey of Denmark and Greenland – GEUS, Ø. Voldgade 10, DK-1350 Copenha-
gen K, Denmark and Canadian Forest Service, Northern Forestry Centre, 5320 – 122nd Street, Edmonton AB T6H
3S5, Canada; 3Department of Biology, University of Bergen, PO Box 7803, NO-5020 Bergen, Norway; 4Bjerknes
Centre for Climate Research, University of Bergen, NO-5007 Bergen, Norway and School of Geography and
the Environment, University of Oxford, Oxford OX1 3QY, UK; 5The Institute for Subarctic Landscape Research,
SE-930 90 Arjeplog, Sweden; 6Southern Swedish Forest Research Centre, PO Box 49, SE-230 53 Alnarp, Sweden;
7Department of Geosciences and Geography, University of Helsinki, PO Box 64, FI-00014 Helsinki, Finland; and
8Department of Geography, University of Liverpool, Roxby Building, Liverpool L69 7ZT, UK
Summary
1. Global wildfire activity and biomass burning have varied substantially during the Holocene in
both time and space. At the regional to continental scale, macroclimate is considered to be the pre-
dominant control regulatingwildfire activity. By contrast, the role of forest tree composition is often
considered as a subsidiary factor in studies addressing temporal variation in regional wildfire activ-
ity.
2. Here, we assemble a spatially comprehensive data set of 75 macroscopic charcoal records that
reflect local burning and forest landscapes that are spread over a substantial part of the European
boreal forest, spanning both oceanic and continental climates.
3. We show that the late-Holocene invasion of Norway spruce Picea abies, a new forest dominant
in northern Europe, significantly reduced wildfire activity, thus altering forest disturbance dynamics
at a subcontinental scale.
4. Synthesis. Our findings show that a biotic change in the local forest ecosystem altered the fire
regime largely independent of regional climate change, illustrating that forest composition is an
important parameter that must be considered when modelling future fire risk and carbon dynamics
in boreal forests.
Key-words: charcoal, climate change, forest history, Holocene, palaeoecology and land-use
history, Picea abies, species invasion, spruce forest, wildfire activity
Introduction
Changes in the abundance of a single species can trigger pro-
found alterations in the properties of an ecosystem (Chapin
et al. 2004). Indeed, the invasion of Norway sprucePicea abies
in northern Europe during the late Holocene (Tallantire 1972;
Giesecke &Bennett 2004) transformed forests over a subconti-
nental area, culminating in the emergence of a new boreal for-
est keystone species (Seppa¨ et al. 2009a). Both forest structure
and biodiversity were significantly altered as Norway spruce
replaced the previous dominants, mainly pine and birch,
to become the most abundant tree species in North European
forests (Seppa¨ et al. 2009a). Given that Norway spruce
invaded northern Europe from the east (Tallantire 1972; Gies-
ecke & Bennett 2004), forest transformation reached northern
Sweden about 4000 years ago (Fig. 1). Thereafter, spruce
advanced in a south-westerly direction as an apparent wave of
expanding populations, propelled by a combination of driving
forces that are not yet fully understood (Giesecke & Bennett
2004). Climatic change is postulated as a possible causal forc-
ing mechanism (Tallantire 1972; Bradshaw & Lindbladh
2005), although other possible drivers include rate of local
adaptation (Kullman 2001), competitive suppression (Miller
et al. 2008; Seppa¨ et al. 2009a) and human land use (Bjune
et al. 2009). Today the natural limit of spruce distribution in
northern Europe occurs in westernNorway (Fig. 1).*Correspondence author. E-mail: mikael.ohlson@umb.no
 2011 The Authors. Journal of Ecology  2011 British Ecological Society
Journal of Ecology 2011, 99, 395–403 doi: 10.1111/j.1365-2745.2010.01780.x

One important ecosystem process to be affected by this
late-Holocene ecosystem transformation was fire regime (Try-
terud 2003), which describes the pattern of fire at any given
location through time including the frequency, intensity, sea-
sonality, extent and type of burning. Variations in fire regime
are controlled by a complex interplay of climatic variability,
vegetation and fuel characteristics, sources of ignition and
human activities (Lynch, Hollis & Hu 2004; Colombaroli,
Marchetto & Tinner 2007; Odion, Moritz & DellaSala 2010).
During the Holocene, global wildfire activity and biomass
burning are known to have varied substantially in both time
and space (Carcaillet et al. 2002; Power et al. 2008), often in
response to changes in the climate system (Carcaillet et al.
2001; Brown et al. 2005; Marlon et al. 2009) or human activ-
ity (Willis & Birks 2006). At the regional to continental scales,
climatic factors are frequently proposed as the predominant
controls regulating fire regime (Carcaillet et al. 2001; Whit-
lock, Shafer & Marlon 2003; Westerling et al. 2006). For
example, recent climate warming coupled with high fuel loads
are proposed as the dominant factors contributing to the cur-
rent increase in wildfire activity in the western USA, Europe
and Australia (Pausas 2004; Westerling et al. 2006; Pitman,
Narisma & McAneney 2007). Moreover, there is also general
agreement that recent climate warming has lengthened the fire
season and increased the burned area across boreal forests
world-wide (Soja et al. 2007). Commensurate with these
changes, it is now predicted that rising summer temperatures
will increase the risk of fire in the circumboreal area by 50%,
significantly increasing the area burned by the end of this cen-
tury (Flannigan et al. 2009). In contrast to climate, forest tree
composition is often considered as a subsidiary factor in stud-
ies addressing temporal variation in wildfire activity at the
regional to continental scales (Marlon, Bartlein & Whitlock
2006; Gavin et al. 2007). It has, however, recently been shown
that interactions between forest tree species composition and
fire have the potential to overshadow direct effects of climate
change on fire regimes in boreal forests of Alaska (Brubaker
et al. 2009; Higuera et al. 2009), revealing that vegetation
composition can be an important driver of wildfire activity.
Consequently, vegetation composition requires much more
consideration than hitherto when considering climate change,
fire risk and carbon transfer between the boreal forest and the
atmosphere.
Here, we assemble an extensive network of peat, humus and
tree-ring records from forest landscapes spanning the longitu-
dinal axis of Scandinavia to analyse late-Holocene stand-scale
forest composition and fire disturbance in the boreal forest of
northern Europe (Fig. 1). Forest peat and humus records are
the main target for our study because they contain strati-
graphic sequences of pollen and macroscopic charred particles
(‡ 0.25 mm) that reveal the history of local forest composition
and stand-scale burning at a high spatial resolution over a mil-
lennial Holocene time-scale (Jacobson & Bradshaw 1981; Ohl-
son & Tryterud 2000). The samples were collected using a
nested sampling strategy with a broad coverage of the Euro-
pean boreal zone, combined with a denser sampling strategy at
eight localities where up to 15 spruce forest sites were investi-
gated within a given forest landscape. Given that spruce
invaded time-transgressively throughout the study region in a
north-east–south-west direction during the last 4000 years
(Fig. 1), this sampling strategy was used to facilitate a compar-
ison of fire history before and after local spruce invasion at
both local and regional spatial scales. Thus, through compari-
son of charcoal and spruce pollen records it is possible to assess
the influence of both climate and vegetation composition on
the fire regime. For example, if regionally synchronous
changes in charcoal content are detected independent of the
presence or absence of spruce, thenmacroscale climatic factors
must be considered as the likely driving mechanism in the
absence of human activity. Alternatively, if there was a signifi-
cant change in the fire regime following the local invasion of
spruce, then forest tree species and vegetation composition
should be considered as an important regulator of boreal wild-
fire. Here, we show that the local invasion of spruce was a key
contributor to the alteration of wildfire activity, suggesting that
vegetation change combined with climate change can produce
ecological changes of much greater magnitude than would be
expected from climate change alone.
Fig. 1. Location of the study sites and the Holocene invasion of the
Norway spruce forest in northern Europe. The study sites are located
in the Scandinavian countries of Norway and Sweden and each dot
represents a landscape location in which one or more peat or humus
profiles were sampled for charcoal content. Dark grey areas are
mountains. The invasion of spruce is generalized from Giesecke &
Bennett (2004). The contours are for calibrated years bp (see Table S1
for further information about the study sites).
396 M. Ohlson et al.
 2011 The Authors. Journal of Ecology  2011 British Ecological Society, Journal of Ecology, 99, 395–403