Contrasting effects of environmental change on the radial growth of
co-occurring beech and ﬁr trees across Europe
, Daniele Castagneri
, Róbert Sedmák
, Peter Biber
, Marco Carrer
, Paola Nola
, Ionel Popa
, Catalin Constantin Roibu
, Volodymyr Trotsiuk
, Ulf Büntgen
Faculty of Forestry, Technical University in Zvolen, Zvolen, Slovakia
Faculty of Forestry and Wood Sciences, Czech University of Life Sciences Prague, Czech Republic
School of Agriculture, Policy and Development, University of Reading, Reading RG6 6AR, UK
Department TeSAF, Università degli Studi di Padova, Padua, Italy
Forest Growth and Yield Science, Technical University of Munich (TUM), Freising, Germany
National Forest Centre, Forest Research Institute, Zvolen, Slovakia
Department of Earth and Environmental Sciences, Università degli Studi di Pavia, Pavia, Italy
Biotechnical Faculty, Universityof Ljubljana, Slovenia
National Research and Development Institute for Silviculture, Forest Research Station for Norway Spruce Silviculture, Campulung Moldovenesc, Romania
INCE - Mountain Economy Center CE-MONT Vatra Dornei, Romania
Forest Biometrics Laboratory, Stefan cel “Mare”University of Suceava, Romania
Department of Geography, University of Cambridge, CB2 3EN, UK
Swiss Federal Research Institute WSL, Zürcherstrasse 111, 8903 Birmensdorf, Switzerland
CzechGlobe, Global Change Research Institute CAS, Brno, Czech Republic
Masaryk University, Kotlářská 2, 61137 Brno, Czech Republic
•European ﬁr and beech growth acceler-
ated during the last century.
•Beech growth declined in northern
Europe since 2000.
•Fir growth rates increased over most of
Europe since 1980.
•Growth-climate responses were similar
for most tree social classes.
•Climate sensitivity of both species was
not affected by forest management.
Received 11 August 2017
Received in revised form 9 September 2017
Accepted 10 September 2017
Available online 19 October 2017
Editor: Elena Paoletti
Under predicted climate change, native silver ﬁr(Abies alba)andEuropeanbeech(Fagus sylvatica) are the most
likely replacement species for the Norway spruce (Picea abies) monocultures planted across large parts of conti-
nental Europe. Our current understanding of the adaptation potential of ﬁr-beech mixed forests to climate
change is limited because long-term responses of the two species to environmental changes have not yet been
comprehensively quantiﬁed. We compiled and analysed tree-ring width (TRW) series from 2855 dominant,
co-dominant, sub-dominant and suppressed ﬁr and beech trees sampled in 17 managed and unmanaged
mixed beech-ﬁr forest sites across Continental Europe, including Bosnia and Herzegovina, Germany, Italy,
Science of the Total Environment 615 (2018) 1460–1469
⁎Corresponding author at: T.G. Masaryka 24, 960 53 Zvolen, Slovakia.
E-mail address: email@example.com (M. Bosela).
0048-9697/© 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Contents lists available at ScienceDirect
Science of the Total Environment
journal homepage: www.elsevier.com/locate/scitotenv
Romania and Slovakia. Dendroecological techniques that combine various detrending methods were used to in-
vestigate variation in radial growth of co-occurring ﬁr and beechtrees. Coincidental with peak SO
growth of silver ﬁr declined between 1950 and 1980 at most sites, whereas beech growth increased during this
period.Correspondent to a signiﬁcant warming trend from 1990–2010, average beech growth declined, but silver
ﬁr growthincreased. Long-term growth patterns andgrowth-climate sensitivityof ﬁr and beech trees did not sig-
niﬁcantly differ between managed and unmanaged forests. Multi-decadal changes in the growth rate of all ver-
tical tree classes were similar. In contrast to previous indications of limited drought susceptibility of beech mixed
stands, this study suggests that the mixture of tree species in forest stands does not necessarily prevent growth
depressions inducedby long-term environmental change. Our results further imply thatforest managementdoes
not necessarily alter their sensitivity to environmental changes.
© 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license
Beech (Fagus sylvatica L.) is Europe's most abundant forest species
(Ellenberg, 1996). Naturally distributed across most of continental
Europe, it either grows in pure broad leaved forests or in mixtures
with conifer species. Beech has been increasingly used to convert coni-
fer monocultures to mixed stands, reﬂecting the current transition of
forest management strategies to accommodate non-productive forest
functions and adapt to climate change (Knoke et al., 2008; Tarp et al.,
2000). Silver ﬁr(Abies alba Mill.) is a coniferous species native to
Europe, with a geographical distribution similar to that of beech, but
largely limited to the Alpine and the Carpathian arcs. There are indica-
tions that silver ﬁr might be more suitable to future European climate
as it grew well under warmer-than-present conditions during the
mid-Holocene (Tinner et al., 2013; Ruosch et al., 2016). Today, mixed
ﬁr-beech forests represent an important forest ecosystem forming an
essential part of central and south-eastern European landscapes (EEA,
Since the mid-19th century or even earlier, a large proportion of
European beech-ﬁr forests was converted to conifer monocultures of
Norway spruce (Picea abies L. Karst.) (Spiecker et al., 2004). However,
considerable areas of Norway spruce forests in central Europe suffered
from acid deposition during the second half of the 20th century
(Ulrich, 1995). Today, in many locations it is clear that Norway spruce
is becoming increasingly susceptible to the more frequent summer
droughts induced by climate change (Lévesque et al., 2013; Zang et al.,
2014), as well as the devastating effects of severe windstorms and sub-
sequent bark beetle outbreaks (Hlásny and Turčáni, 2013; Jönsson et al.,
2009). Therefore, the introduction of appropriate replacement species,
such as beech and ﬁr, has become a key task (IPCC, 2014).
Despite recent publications describing the growth of silver ﬁr and
European beech (Bosela et al., 2016b; Büntgen et al., 2014; Cavin and
Jump, 2016; Dittmar et al., 2003; Gazol et al., 2015; Pretzsch et al.,
2014), relatively little is known about how these species react to envi-
ronmental changes when growing in mixed stands (Vitali et al., 2017).
Recent evidence shows that beech (Bosela et al., 2015), but also ﬁrto
some extent (Toïgo et al., 2015), may reach higher stem growth produc-
tivity in mixed stands. There are suggestions that growth sensitivity of
silver ﬁr and European beech to summer droughts decreases when
growing in mixed stands (Lebourgeois et al., 2013; Metz et al., 2016;
Vitali et al., 2017), but this has neither been tested nor conﬁrmed
under varying natural conditions across different parts of Europe. A
higher diversity of tree species has been shown to potentially stimulate
radial stem growth by better niche utilisation, but also via improved re-
sistance and resilience at a forest ecosystem level (Gazol et al., 2016;
Jucker et al., 2014; Metz et al., 2016; Paquette and Messier, 2011;
Ruiz-Benito et al., 2014; Vitali et al., 2017). Nevertheless, we still lack
unequivocal evidence supporting the notion that growing in mixed
stands mitigates some of the negative long-term effects of rising tem-
peratures, often associated with increased frequency and/or severity
of droughts. A detailed understanding of the relationship between
diversity and ecosystem productivity and stability is indeed crucial for
advising the policy-forming processes at national and international
To further complicate the picture, most dendroecological data that
describe growth-climate relationships in European forests originate
from western Europe and tend to consider only dominant and co-
dominant trees (Büntgen et al., 2014; Cavin and Jump, 2016;
Nehrbass-Ahles et al., 2014; Pretzsch et al., 2014). Yet, it is possible
that populations of the same species inhabiting the eastern part of
their distributional range possess different sensitivity levels to environ-
mental factors due to genetic variation (Bosela et al., 2016a).
A Europe-wide investigation of species-speciﬁcgrowthdynamicsin
mixed forest stands affected by contrasting environmental factors is so
far lacking. To ﬁll this gap, we compiled a database of tree-ring width
(TRW) samples from managed and unmanaged European mixed
beech-ﬁr forests covering most of the regions where these two species
co-occur. Our aim was to test the following hypotheses: (H1) do radial
growth patterns of beech and ﬁr in mixed forests vary geographically,
(H2) does tree status within the canopy or (H3) forest management in-
terventions affect long-term variation in annual growth, and ﬁnally
(H4) was the growth behaviour of co-occurring beech and ﬁrsimilarly
affected by the 1970–1990 period of heavy pollution and the warming
during recent decades?
2. Material and methods
2.1. Tree-ring sampling
A network of 17 beech-ﬁr mixed forest stands located in ﬁve
European countries was used to compile 2855 core samples (Fig. 1).
Taking in Slovakia, Romania, Bosnia & Herzegovina, Italy and
Germany, the network covers a large part of Europe's natural distribu-
tion range of mixed beech-ﬁr forests. The sites were allocated to 8 re-
gional groups, reﬂecting differences in post-glacial recolonization from
different forest refugia, but also current climatic conditions (Tables 1
and S1). The followingselection criteria were followed atall sites to en-
sure reasonable comparability of observations: 1) growing conditions
were characterised by mesic soils and mean annual precipitation in ex-
cess of 800 mm year
, 2) no forest management interventions were
carried out for at least 30 years prior to sampling, 3) specimens of
both species were present in all four social classes, as described by
Oliver & Larson (1996; dominant, co-dominant, sub-dominant and sup-
pressed), 4) all trees above a registration threshold within a ﬁxed-area
plot were marked and then either all or a randomly selected subset
were cored. A single location in Slovakia where managed beech-ﬁr
stands are found in close vicinity to unmanaged stands was used to
investigate the impact of forest management on growth-climate
1461M. Bosela et al. / Science of the Total Environment 615 (2018) 1460–1469
2.2. Tree-ring standardisation
Four standardisation techniques were applied toremove age-related
trends from raw TRW measurements, all aim to preserve the effects of
environmental signals at inter-annual to multi-decadal timescales. The
following detrending methods were used and compared in this study;
a) Modiﬁed Exponential Function (MEF, Fritts, 2001). The MEF method
was ﬁrst applied to individual tree TRW series, followed by the cal-
culation of ring-width indices (RWI) deﬁned as the ratio between
raw measurements and corresponding MEF function values. Mean
RWI chronologies for each site were calculated as bi-weight robust
means of all individual tree series (Cook and Kairiukstis, 2013)
b) cubic Smoothing Spline with 50% frequency response cut-off at
100 years (SS; Cook, 1985). This setting was used to preserve the
inter-annual to multi-decadal growth ﬂuctuations (Büntgen et al.,
2008). The SS standardisation was applied in the same manner as
the MEF approach, the only variation was the use of a different
c) Regional Curve Standardisation (RCS; Briffa and Melvin, 2011). RCS
detrending was applied to individual tree data, but only after the ex-
clusion of all partial-length TRW series where it was not possible to
estimate the number of rings from the beginning of the core to the
pith (Table 1). For example, this includes data series where the be-
ginning of the core was too far from the pith and no ring arc was vis-
ible (Briffa and Melvin, 2011). Raw TRW series were aligned by
cambial age (ring number from bark to pith), followed by mean
TRW calculation for each series. A smoothing spline with a 50% fre-
quency response cut-off at 10% maximum cambial age curve wave-
length (i.e. the Regional Curve (RC)) was then ﬁtted to mean TRW
series. RWIs were calculated as ratios between individual series
and the smoothed RC. Individual RWI series were re-aligned by
calendar dates. The ﬁnal site chronology was then developed by
using bi-weight robust means.
d) Korf growth function (Korf; Korf, 1939). Multi-decadal growth
changes may be underestimated by common detrending methods
in even-aged forests (Briffa and Melvin, 2011). To counter this ten-
dency we applied a growth function developed by Korf (1939).
Raw TRW series were aligned by age andthe mean curve was calcu-
lated. Then, in contrast to the RCS method, regression parameters of
the Korf function were estimated from the ﬁrst 50 years of the mean
curve only. An extrapolation to the full length of the curve was then
carried out, assuming that the ﬁrst 50 years indicate both growth
culmination and decline. RWI series at each site were then calculat-
ed as the ratio between raw TRW and estimated Korf's function.
Bi-weight robust mean was used to develop a mean site chronology
as in previous methods, and Korf standardisation was used only on
sites where growth culmination was clearly visible.
Most dendrochronological studies report site chronologies devel-
oped from data describing the dominant trees only (Nehrbass-Ahles
et al., 2014), an approach which may not capture the growth history
of the whole stand. To shed light on this issue, at each site we developed
a mean site chronology for both species from a sample of the 15 largest
trees only (hereafter “dominant trees”) and compared it to that cover-
ing all measured individuals.
2.3. Growth variability and sensitivity assessments
Growth trends describing several predeﬁned periods were
compared to identify effects of known environmental factors. The
predeﬁned periods, such as the decades characterised by heavy air
Fig. 1. a) Sampling sites used in the study, with corre sponding Climate Research Unit (CRU) climate data points; b) current distribution of Abies alba and c) current distribution of Fagus sylvatica.
1462 M. Bosela et al. / Science of the Total Environment 615 (2018) 1460–1469
pollution in Europe (1950–1990) and the unprecedented climate
warming (1980–2000, resp. 1990–2010), were selected on the basis of
published descriptions of major environmental issues and trends
(Bosela et al., 2016a, 2016b; Büntgen et al., 2014; Gazol et al., 2015;
Jump et al., 2006; Linares and Camarero, 2012). Simple linear regression
was used to describe growth trends within selected periods; regression
coefﬁcients denoting the slope were then used to compare tree growth
between the periods and between regional groups of sites. In this com-
parison,we applied generalized additive models using “gamm”function
in “mgcv”R package (Wood, 2011)toﬁlter out inter-annual high
frequency variation and preserve multi-decadal growth trends. The
“gamm”function used Generalized Cross Validation (GCV) to estimate
the smoothing parameter. Populations of regression parameters de-
scribing individual site RWI series were assessed for differences be-
tween species, regional groups, time periods and detrending methods
by ANOVA. All populations were tested for normality of distribution
and equality of variance; no conversion of data was necessary.
Bonferroni correction was used in pair-wise comparisons, and differ-
ences were considered signiﬁcant at p b0.05.
Monthly temperature means, precipitation totals and drought indi-
ces (scPDSI –self-calibrated Palmer Drought Severity Index) were ob-
tained from gridded CRU TS 3.10 database (http://www.cru.uea.ac.uk/
data/) via Climate Explorer (http://climexp.knmi.nl/)(Dai, 2011;
Harris et al., 2014; Mitchell and Jones, 2005). Data from half-degree
lat/lon grid points nearest to each study site were used to analyse the re-
lationship betweengrowth of both species and climate variation at each
site (Fig. 1). Radial growth responses to climate (standardised mean
chronologies) were quantiﬁed by Pearson's correlation coefﬁcients
computed over 31-yr moving window segments to investigate tempo-
ral changes in the climate-growth relationships (Büntgen et al., 2010;
Wilson and Elling, 2004). Bootstrapping was applied to calculate 95%
conﬁdence intervals of the correlation coefﬁcients using the ‘bootRes’
R package (Zang and Biondi, 2013). The signiﬁcance of the correlation
was then tested using 95% percentile range method (Dixon, 2002).
Correlation coefﬁcients describing the relationship between site
chronologies and climate variables were analysed by principal compo-
nent analysis (PCA). PCA was performed with the “hclust”function in
the R Stats package (R Development Core Team, 2008). The distance
matrix was computed using the Euclidian measure. The “dendextent”
package (Galili, 2015) was used to visualise PCA results. Ward Hierar-
chical Clustering was used to interpret growth trend responses to cli-
mate. Findings from the Ward clustering were then compared with
PCA grouping in a 2-dimmensional space.
Regional growth trends identiﬁed by the four detrending methods
applied in this study show a wide variation of growth between sites,
species and time periods across Europe (Fig. 2), yet a certain amount
of generalisation is possible (Table 2). Beech growth accelerated be-
tween the 1950s and 1980s in the more ‘northern’forests, loosely de-
ﬁned as those above 47°N parallel. Growth acceleration in the north
was followed by a slow decline in annual ring width, with the exception
of the Bavarian forest where beech growth accelerated continuously
since about the 1940s (Fig. 2,Table 2).
At the ‘southern’sites, mid-century acceleration has slowed down or
even decreased. At the Cansiglio site in northern Italy, we observed the
earliest onset of this decline starting in the 1950s. While forming the
same forest stands in a mixture with beech, ﬁr has shown very different
growth patterns during the last century (H1). A decline of ﬁr growth
rate was observed between the1950s and 1980s in most locations
apart from the populations in the southern Carpathians, which
Brief description of site location, total number of tree-ring width series used per site (N1) and the number of tree-ring width series that intercepted the pith or were sufﬁciently close to
estimatethe number of ringsto the ptith (N2). Timespan is the minimumand maximum calendaryear of the site chronology. B&Hdenotes Bosnia andHerzegovina, Altdenotes altitudein
Country Species Locality (abr.) N1 N2 Time span Mountain range Long Lat Alt
B & H Abies Lom 158 158 1583–2005 Western Dinaric Mts 16.47 44.45 1350
B & H Fagus (bh_lom) 440 440 1625–2005
B & H Abies Perucica b1 44 43 1786–2006 Eastern Dinaric Mts 18.71 43.3 1200
B & H Fagus (bh_perb1) 81 75 1595–2006
B & H Abies Perucica b2 76 69 1663–2007
B & H Fagus (bh_perb2) 70 63 1509–2007
B & H Abies Perucica f1 118 109 1686–2006
B & H Fagus (bh_perf1) 35 27 1614–2006
B & H Abies Perucica f2 82 71 1702–2007
B & H Fagus (bh_perf2) 64 60 1703–2007
Germany Abies Bodenmais 28 –1820–1995 Bavarian Forest 13.1 49.09 800
Germany Fagus (de) 21 –1821–1995
Italy Abies Cansiglio 140 140 1931–2012 Southern Alps 12.42 46.1 1100
Italy Fagus (it) 205 184 1856–2012
Romania Abies Botiza 54 25 1774–2013 Eastern Carpathians 24.09 47.61 1050
Romania Fagus (ro_bo) 67 23 1614–2013
Romania Abies Sinca 281 184 1665–2013 Southern Carpathians 25.17 45.67 1140
Romania Fagus (ro_si) 163 101 1556–2013
Slovakia Abies Polana 22 22 1860–2010 Western Carpathians 19.57 48.62 760
Slovakia Fagus (sk_p) 58 58 1867–2010
Slovakia Abies Spis S1 20 20 1896–2010 20.73 48.77 760
Slovakia Fagus (sk_s1) 19 19 1881–2010
Slovakia Abies Spis S2 36 36 1794–2010 20.73 48.77 760
Slovakia Fagus (sk_s2) 33 33 1811–2010
Slovakia Abies Spis S3 59 59 1820–2011 20.67 48.79 760
Slovakia Fagus (sk_s3) 14 14 1821–2011
Slovakia Abies Spis S4 24 24 1848–2010 20.72 48.76 830
Slovakia Fagus (sk_s4) 26 26 1898–2010
Slovakia Abies OBR 25 21 1805–2013 Western Carpathians 19.47 48.88 887
Slovakia Fagus (sk_obr) 146 120 1740–2014
Slovakia Abies SRA 57 57 1783–2013 19.11 49.19 1048
Slovakia Fagus (sk_sra) 133 133 1717–2013
Slovakia Abies SUT 29 28 1814–2013 19.09 49.18 1029
Slovakia Fagus (sk_sut) 27 24 1761–2013
1463M. Bosela et al. / Science of the Total Environment 615 (2018) 1460–1469
exhibited a slight acceleration of growth during this period (Fig. 2). This
period of ﬁr decline was followed by a rapid acceleration of growth in
the ‘north’, and a steep decline in the ‘south’, again with the exception
of the southern Carpathians.
A comparison of growth trends created from dominant trees only or
all trees above a DBH threshold (H2) did not show any effect of canopy
position (Fig. S5). Regression parameters denoting the slope of the ﬁt
were not affected by tree social status in any of the time periods
under consideration, nor for beech (p = 0.128 to 0.516) or ﬁr(p=
0.336 to 0.990). Similarly, we did not ﬁnd any difference in annual
growth between managed stands and old-growth unmanaged forests
in the western Carpathians when comparing mean RWI growth trends
within each of the time periods under consideration (H3, p = 0.063 to
0.441), indicating that factors other than forest management affect
long-term growth trends in beech and ﬁr. We made use of the pre-
deﬁned periods of environmental stress to explore whether beech and
ﬁr respond differentially to acid deposition (1950–1980) and accelerat-
ing climate change (1990–2010). We found a strong interaction be-
tween species and time period (H4; p b0.001) when comparing mean
growth rates in these two periods. Considering all sites used in this
study (Fig. 3a), ﬁrRWIy
was smaller than thatof beech in the period
1950–1980 but the growth trends of these two species reversed by
1990–2010. An interesting observation emerged when considering the
more northern and southern sites separately; the relationship between
Fig. 2. Mean ring-width index (RWI) chronologies of silver ﬁr and European beech after modiﬁed exponential (exp) and Regional Curve Standardisation (RCS) techniques for aggregate
sites (west_carp_man: managed stands in western Carpathians, west_carp_unm –unmanaged forests in western Carpathians, east_carp: eastern Carpathians, south_carp: southern
Carpathians, east_dinaric:eastern Dinaric,west_dinaric,south_alps: southernAlps, BavarianForest). Generalized Additive Model (GAM)was applied to ﬁlter outthe inter-annual variation
and preserve multi-decadal changes. The shaded bands denote 95% conﬁdence intervals.
A comparison of detrending methodsapplied to tree-ringwidth data describing radialgrowth of Europeanbeech (Fagus sylvatica) and Silverﬁr(Abies alba)intwotime
periods characterisedby different environmental conditions. Linear regressionwas ﬁtted to data detrended by Modiﬁed exponential function(MEF), smoothing spline
(SS), regional curvestandardisation (RCS) andKorf growth function(Korf). Colouredcell backgrounds denotea negative regression trend.Stars denote signiﬁcance level
of regression ﬁtat:***-b0.001, ** - b0.01, * - b0.05. Empty cells represent a non-signiﬁcant ﬁt, and a dash is used in cases where regression was not possible to ﬁt.
European beech Silver fir
MEF SS RCS Korf MEF SS RCS Korf
de *** - - - - - - *-*** - - - - -
sk_p *** ** *** *** *** *** ** * *** ***
sk_s1 ** ** * **** *** *** *** ** *** ***
sk_s2 *** *** *** *** *** *** *** *** *** *** *** *** *** ** ***
sk_s3 *** *** * *** *** * *** *** *** *** *** ***
sk_s4 *** ** ** ** *** *** *** *** *** *** ***
sk_obr *** ** ** *** ** *** *** ***
sk_sra *** * *** - - ** *** *** *** ** *** ** ***
sk_sut * ***** - - *** *** *** *** ** *** *** ***
ro_bo * ** - - *** *** *** *** ***
ro_si * ** * * *** ****** ** ** **
1464 M. Bosela et al. / Science of the Total Environment 615 (2018) 1460–1469
environmental factors and the rate of radial growth was not affected by
species in thesouth (p = 0.359; Fig. 3c), but there was a strong effect in
the north (p b0.001, Fig. 3b). There was no difference between
the trends identiﬁed by the four standardisation methods in 1950–80
(p = 0.249) and only the Korf standardisation method differed from
the other three in 1990–2010 (p = 0.004).
Differential response of ﬁr and beech growth to environmental
change was conﬁrmed by a PCA testing as well as by a cluster analysis
for the strength of relationship between previous- and current-year
temperature, precipitation and PDSI and tree growth (Fig. 4). Fir growth
at the southern sites was mainly inﬂuenced by drought (scPDSI),
whereas temperature was the most dominant driver of tree ring
width at the northern sites. There was considerable variation between
individual sites in observed effects of climate on ﬁr and beech growth,
however a very similar grouping of tree populations and sites was
achieved by cluster analysis (Fig. 5).
In a further attempt to contrast the growth behaviour of the
two species within the sampled range of sites, we calculated correlation
coefﬁcients between RWI and current-year summer temperature and
drought over the century-long time period (Fig. 6). Fig. S6 shows a
large temporal variation of correlation coefﬁcients over the past centu-
ry. On average, a negative correlation between RWI and drought domi-
nates in the south, while a positive correlation with temperature is
present in the north.
4.1. European beech
Published literature describing the dendroecology of European
beech at European and regional scales does not paint a clear picture;
there is evidence of (i) either increased or decreased growth rates of
beech in the last two decades in Central Europe (Dittmar et al., 2003;
Pretzsch et al., 2014), and (ii) either increased or decreased radial
growth of beech at the southern edge of distribution (Jump et al.,
2006; Tegel et al., 2014). Our study conﬁrms that beech growth rates
have increased during the period between 1950 and 1980 across
Europe, an observation which is in line with published measurements
(Bosela et al., 2016b; Hlásny et al., 2011; Pretzsch et al., 2014)and
model simulations (Hlásny et al., 2011). However, our results contradict
those of Dittmar et al. (2003), who in a Europe-wide study found no de-
tectable increase of beech growth in Europe, but documented a decline
of the rate of growth at high altitudes in central Europe. Interestingly,
the authors found that high summer temperatures favoured radial
growth at the expense of vertical growth. Existing studies and observa-
tions presented in this paper suggest a positive effect of increasing sum-
mer temperature at higher latitudes or altitudes on beechgrowth. Thus,
given the summer period warming observed in the last century
(Büntgen et al., 2011; Luterbacher, 2004) and predicted warming across
Europe (IPCC), it seems reasonable to expect further acceleration of
beech growth at the northern edge of its distribution. A recent
Europe-wide study documents an increased basal area increment in
the last decades in beech forests in temperate and continental core re-
gions of the species distribution range (Cavin and Jump, 2016). Our
study adds evidence suggesting that the growth decline in the southern
localities started in mid-20th century and continues until today. How-
ever, a recent increase in beech radial increments in some Mediterra-
Tegel et al., 2014) suggests strong regional
differences, probably related to regional climate or site productivity
(Aertsenet al., 2014; Bosela et al., 2016b), which limit any broad extrap-
olation of our results.
Hacket-Pain et al. (2016) found no clear spatial pattern in the
drought sensitivity of European beech, indicating that the populations
from the southern and northern range edges respond to summer
drought equally. In contrast, we found a strong spatial pattern in the
growth responses to summer temperature and to drought. While radial
growth of the species generally did not respond to summer drought in
central Europe (Germany, Slovakia and Romania), it became highly re-
sponsive in the Balkan Peninsula (Bosnia & Herzegovina, except for
the Lom site). Our study thus supports existing observations showing
that some southern European beech populations are increasingly suffer-
ing from summer drought (Linares and Camarero, 2012; Piovesan et al.,
2008). Not only can we conﬁrm the same trends, but we are also able to
pinpoint the onset and the severity of the decline allowing for investiga-
tion of site-speciﬁc reasons for growth modiﬁcation. For example,
unlike beech trees in Perucica, which were sensitive to drought, the
population in the Lom site in the same climatic zone was non-
responsive to sc-PDSI. Trees at the Lom site were substantially younger
than those at Perucica (Fig. S1), growth plasticity of a younger popula-
tion of beech might thus explain the difference –as outlined by the
age-related climate response hypothesis (Carrer and Urbinati, 2004;
Primicia et al., 2015).
4.2. Silver ﬁr
Silver ﬁr experienced a severe growth decline in Europe during
1970–1990, driven by sulphur dioxide emissions (Büntgen et al.,
2014), an event often referred to as Europe-wide ﬁr dieback (Cramer,
1984; Larsen, 1986; Meyer, 1957). Our study provides tree-ring width
evidence of this event; however, it is not possible to exclude the possi-
bility that lower summer temperatures during this period might have
contributed to the growth depression (Fig.S2). In contrast to other stud-
ies, our investigation shows that the growth of silver ﬁr did not decline
in the eastern part of its distribution range during this period (southern
Carpathians and partly eastern Dinaric Mts.). It has been suggested that
the greater genetic diversity of the Balkan populations helps the species
mitigate effects of changing environmental conditions (Bosela et al.,
2016a), and this study supports that suggestion. Increased genetic di-
versity, but also a greater functional diversity of forest stands where it
occurs, have been shown to increase the capacity of silver ﬁr to tolerate
drought (Gazol et al., 2016). The latter relationship is indicative of the
need to understand the implications of ecosystem diversity for species
performance and production stability.
Fig. 3. Linear regression trends of silver ﬁr and European beech ring-width indices (RWI)
in two distincttime periods of acid deposition (1950–1980) and climate warming (1990–
2010). Dots indicate average RWI change per annum representing the whole range of
stands considered in this study (A) or in two sub-sets according to geographical location
(B and C). Error bars denote standard deviation.
1465M. Bosela et al. / Science of the Total Environment 615 (2018) 1460–1469
Followingthe period of growth decline, silver ﬁr experienced a rapid
recovery reaching unprecedented levels across most of its distributional
range, coincidental with successful pan-European effort to limit acid de-
position. Even at the Cansiglio site (Italy) characterised by warm and
dry conditions, the growth pattern almost exactly followed that seen
in Slovakia until around 1995–2000, when at Cansiglio it turned to an-
other decline. This ﬁnding brings a new angle to a recent Europe-wide
study of silver ﬁr growth throughout the Holocene. Büntgen et al.
(2014) showed an increasing radial increment of ﬁr trees growing in
Italian Alps and Apennines until 2000, but did not indicate growth
trends after that year. Our observations from the Cansiglio site indicate
that ﬁr populations in the southern parts of the Alps may have recently
experienced a drought-stress related growth decline (Fig. 6a).
4.3. Long-term patterns of radial growth
For the ﬁrst time, to our knowledge, this study compares growth
pattern response to climate between managed and unmanaged forests.
We were able to make this comparison only for a series of sites in the
western Carpathians, however a pattern typical for many other sites
where these two species co-exist emerges. There was no discernible dif-
ference between tree growth in managed and unmanaged forests, but
beech RWIs were positive during 1950–1980 while those of ﬁr were
negative. By 1990–2010, the pattern reversed; RWIs describing ﬁr
growth were positive as the species recovered both in managed and un-
managed forests, but those depicting beech growth declined to negative
values. Again, this observation underlines the effect of long-term
Fig. 4. Principal componentanalysis (PCA) of growth-climate responses of silverﬁr and European beech across Europe: a) dissimilarities between the study localities and species and in-
dication of northern and southern sites in the ordination space; b) ordination of contributing climate variables. The matrix of the correlation coefﬁcients between site chronologies (pro-
duced by smoothing spline with 70% cut-off at 10-year segments) and climate variables (temperature, precipitation and sc-Palmer Drought Index) was used as an input to the PCA.
Abbreviations in labels: bh –Bosnia & Herzegovina, it –Italy, ro –Romania, sk –Slovakia, per –Perucica; a –silver ﬁrandb–European beech; temp –monthly temperature, ptemp –
monthly temperature in the previous year, prec –monthly precipitation, pprec –monthly precipitation in the previous year, pdsi –sc-Palmer drought index, ppdsi –monthly sc-Palmer
drought index in the previous year.
Fig. 5. Dissimilaritiesin climate responses of silver ﬁr and European beech across Europeansites using hierarchical clustering. The analysis showsa differentiation between northern (red
and dashed-line rectangle) andsouthern (blueand solid-line rectangle) populationsof ﬁr and beech. The southernsites that were clusteredwithin the northerncluster are highlighted by a
blue rectangle around the labels. Abbreviations in the x-axislabel: bh –Bosnia & Herzegovina, it –Italy, ro –Romania, sk –Slovakia, per –Perucica; a –silver ﬁrandb–European beech.
1466 M. Bosela et al. / Science of the Total Environment 615 (2018) 1460–1469
environmental conditions on tree-ring width, which seems to override
even the effect of forest management designed to stimulate bole wood
While our observations of European beech show a wide variation of
growth patterns driven by severalfactors, the resultsfor European silver
ﬁr are quite consistent. A number of recent studies provides evidence
that diversity, whether species or functional, has a positive effect on
tree growth (Toïgo et al., 2015; Zhang et al., 2012). Higher diversity is
also believed to mitigate the negative impacts of extreme climate events
through higher growth resistance and resilience (e.g. Jucker et al., 2014;
Gazol et al., 2016; Metz et al., 2016). Although our study cannot directly
estimate the beneﬁts of growing ina diverse stand, theresults clearly in-
dicate that growing in a mixture does not shield the two species from
impacts of long-term changes in environmental conditions. For exam-
ple, we show that beech growth has been declining over the last two de-
cades in both managed and unmanaged forests and across a range of
conditions in Europe, regardless of the species composition and forest
structure. The same holds true for ﬁr's unusual radial increment pat-
terns, whereby long-term changes of environmental conditions seem
to prevailover local ecology.In this context, anycalculation of resistance
and resilience indices based on RWI must take into account multi-
decadal trends as these form the ‘background’against which tree
growth must be considered.
Our study shows that state-of-the-art dendroecologicial techniques
can unravel complex environmental factors that inﬂuence species-
speciﬁc tree growth trends. Although growing under the same condi-
tions, European beech and silver ﬁr exhibited remarkably different
growth patterns over the last half a century. While ﬁr responded posi-
tively to the recent warming, beech growth has declined across our
range of sites, suggesting that ﬁr is less susceptible to warmer and
drier conditions than beech. A comparison of growth patterns between
managed and unmanaged mixed beech-ﬁr forests revealed that the
long-term growth patterns were the same, suggesting only a limited
scope for tree growth stimulation by active forest management. There
is some support for the use of mixed forests as an adaptation strategy
to climate change. We show that a higher tree species diversity might
help mitigate the effects of short-term climatic events such as drought
and acidiﬁcation, but may not prevent mixed forests from the long-
term consequences of climate change. Thus, any effort to convert
Norway spruce monocultures to preserve long-term growth at the for-
est ecosystem level should consider a purposeful decision to utilise
both beech and ﬁr as replacement species. Further scientiﬁceffort
should be directed towards investigating effects of various management
interventions designed to aid the adaptation of beech-ﬁr ecosystems to
future climate change.
The work was fully supported by the Slovak Research and Develop-
ment Agency (SRDA) under the contract no. APVV-15-0265. BK was also
supported by the SRDA under the contract no. APVV-14-0086. MS and
VT were supported by the projects CIGA no. 20154316 and COST CZ
LD14074 (The Ministry of Education, Youth and Sports of the Czech
Republic). UB received funding from the Ministry of Education, Youth
and Sports of Czech Republic within the National Sustainability Program
I (NPU I; grant number LO1415).
MB conceived the ideasand MB, ML, DC and RS designed methodol-
ogy. MB andML analysed data. MB, ML and UB wrote the manuscript. All
Fig. 6. Correlation between the sitering-width index(RWI) chronologies and meantemperature (a)and sc-Palmer Drought Index(PDSI) (b) duringthe period of Juneto August in silverﬁr
(green dashed lines) and European beech (red solid lines) in different localities across Europe (west_carp_man: managed stands in western Carpathians, west_carp_unm –unmanaged
forests in western Carpathians, east_carp: easternCarpathians, south_carp: southern Carpathians, east_dinaric: eastern Dinaric,west_dinaric, south_alps: southern Alps, Bavarian Forest).
RWI were obtained after detrending by modiﬁed negative exponential function; bootstrapped correlation was computed over the whole period of about 110 years; values shown repre-
sent mean and 95% conﬁdence intervals.
1467M. Bosela et al. / Science of the Total Environment 615 (2018) 1460–1469
authors collected data, contributed critically to the drafts and gave ﬁnal
approval for publication.
Appendix A. Supplementary data
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