ArticlePDF Available

Drought-induced shift in tree response to climate in floodplain forests of Southeastern Europe


Abstract and Figures

Floodplain forests are the most rapidly disappearing ecosystem in the world, especially in temperate regions of Europe where anthropogenic influence has been pronounced throughout history. Research on primeval forests is crucial to further our understanding of their natural dynamics and interaction with climate but is limited by the lack of such preserved forests. The aim of this study was to investigate how a primeval floodplain forest in Southeastern Europe has responded to climate variability during the last 250 years through comparison of tree growth and climate, canopy disturbance and recruitment dynamic of two dominant tree species with different tolerances to flooding/drought. Our analysis revealed induced stress caused by several consecutive severe drought events in the 1940s, which led to a significant increase in sensitivity to increasing temperatures and decreasing river water levels. This trend is particularly pronounced in pedunculate oak. Age structure analysis revealed one larger episode of oak regeneration culminating after periods of intense growth release. Such period co-occurs with summer drought, which is part of a complex system of natural disturbances and a significant natural driver of the cyclical regeneration of primeval oak ecosystems.
Content may be subject to copyright.
SCientiFiC REPORTS | (2018) 8:16495 | DOI:10.1038/s41598-018-34875-w
Drought-induced shift in tree
response to climate in oodplain
forests of Southeastern Europe
Stjepan Mikac
1,2, Anja Žmegač1,2, Domagoj Trlin1, Vinko Paulić1, Milan Oršanić1 &
Igor Anić1,3
Floodplain forests are the most rapidly disappearing ecosystem in the world, especially in temperate
regions of Europe where anthropogenic inuence has been pronounced throughout history. Research
on primeval forests is crucial to further our understanding of their natural dynamics and interaction
with climate but is limited by the lack of such preserved forests. The aim of this study was to investigate
how a primeval oodplain forest in Southeastern Europe has responded to climate variability during
the last 250 years through comparison of tree growth and climate, canopy disturbance and recruitment
dynamic of two dominant tree species with dierent tolerances to ooding/drought. Our analysis
revealed induced stress caused by several consecutive severe drought events in the 1940s, which led to
a signicant increase in sensitivity to increasing temperatures and decreasing river water levels. This
trend is particularly pronounced in pedunculate oak. Age structure analysis revealed one larger episode
of oak regeneration culminating after periods of intense growth release. Such period co-occurs with
summer drought, which is part of a complex system of natural disturbances and a signicant natural
driver of the cyclical regeneration of primeval oak ecosystems.
Global forest decline caused by drought has been recorded worldwide and has signicantly increased since
19701,2. Recent changes in climate are associated with increased temperatures and changes in precipitation pat-
terns, with more frequent, prolonged and intense episodes of drought as a consequence. Such events result in
long-lasting changes in ecosystem function, community composition and structure, especially in water sensitive
ecosystems such as oodplain forests3. Lowland oodplain forest ecosystems are characterized by high produc-
tivity, diverse microhabitat conditions and considerable biodiversity4. ey are widespread in all biogeographic
regions of the world on alluvial deposits of large rivers with which they have a constant hydrologic interaction5.
According to a study by Tockner et al. in 20026, the world’s remaining oodplain forests cover an area of approx-
imately 2.24 × 106 km2.
e continuous expansion of settlements and infrastructure, as well as exploitation of natural resources, has
ultimately resulted in the widespread disappearance of primeval lowland oodplain ecosystems7,8. In Europe, nat-
ural lowland oodplain forests have all but vanished, and with them, a very important research reference point for
forestry and ecology. In the last century deforestation due to agriculture has wiped out 90% of Europe’s oodplain
forests9,10. e remnants of relatively natural forest occur mostly in Eastern and Southeastern Europe3,11. Apart from
deforestation, oodplain forests have been impacted by numerous activities, particularly river regulation (con-
struction of dams, dykes, drainage systems, etc.). ese interventions have disrupted the sensitive ood patterns
and assisted the progression of mesohydric species1214. Regional episode of pedunculate oak (Quercus robur L.)
decline were recorded during the 20th century in oodplain forests in almost all of Europe15. As oak and other
species die out, another problem in lowland oodplain ecosystems is the spread of mesohydric species, such as
hornbeam (Carpinus betulus L.), which are becoming increasingly dominant, especially in drier, oak dominated
habitats. In the last 20 years signicant decline in narrow-leaved ash (Fraxinus angustifolia Vahl) was observed
through the whole landscape. e greatest threat to the stability of forest ecosystems of narrow-leaved ash is cur-
rently posed by the phytopathogen Hymenoscyphus fraxineus T. Kowalski16 but also constant increase of temper-
ature and environment dryness. e continuation of oak and ash decline could have long-lasting consequences
1University of Zagreb, Faculty of Forestry, Department of Forest Ecology and Silviculture, Svetošimunska 25, 10002,
Zagreb, Croatia. 2University of Zagreb, Faculty of Forestry, Croatian Dendroecology Laboratory, Svetošimunska
25, 10002, Zagreb, Croatia. 3Croatian Academy of Sciences and Arts, Zrinski trg 11, 10000, Zagreb, Croatia.
Correspondence and requests for materials should be addressed to S.M. (email:
Received: 25 July 2018
Accepted: 28 October 2018
Published: xx xx xxxx
Content courtesy of Springer Nature, terms of use apply. Rights reserved
SCientiFiC REPORTS | (2018) 8:16495 | DOI:10.1038/s41598-018-34875-w
for biodiversity as well as for the European forestry sector and is a huge challenge for nature conservation. Forest
management in Europe strives to implement a close-to-nature approach based on mimicking natural stand
dynamics17. In order to better understand the dynamics of natural lowland oodplain forests and their interaction
with climate, primeval natural ecosystems need to be studied. However, such studies are lacking in Europe since
it is almost impossible to nd a primeval lowland oodplain area with an exclusively natural composition and
structure. erefore, we carried out this study in an area with one of the best preserved oodplain forest complex
of pedunculate oak and narrow-leaved ash in Europe – Lonjsko Polje Nature park (LPNP)13.
e aims of this study were to: (1) analyze the long-term growth sensitivity of oak and ash to climate variabil-
ity and changes in river water regime; (2) determine if climate is the driver of growth releases in the study area;
(3) reconstruct disturbance history through analysis of the relationship between canopy disturbance, tree growth
and recruitment.
We expect that: (1) oak growing on drier sites is more sensitive to precipitation, PDSI and river water regime
than ash; (2) extreme climatic events drive the growth releases and the establishment of recruitment in these
Materials and Methods
Research area. e oodplain forests in the lowland part of Croatia occupy an area of about 1,980 km2,
mainly along the Sava, Drava and Danube rivers. One of the best preserved natural oodplain regions in Europe
is the Sava River basin, with an area of 97,713 km2. Geographically, it lies between 13.67E–20.58E longitude
and 42.43N–46.52N latitude. It represents 12% of the Danube River basin, making it the second largest Danube
sub-basin. Its preserved naturalness is the result of historical circumstances. Up to the 19th century the Sava River
was a natural barrier between the Austro-Hungarian and the Ottoman empires. In that period, the forests were
under strict control of the military authorities and were protected. e largest and best preserved oodplains and
forest ecosystems in Europe can be found in the central part of the Sava River basin and make up what is called
“e Blue Heart of Europe” (Lonjsko polje Nature Park, LPNP). e area of approximately 511 km2 hosts a mosaic
of preserved wild lowland oodplain forests and alluvial swamps. e dominant tree species are narrow-leaved
ash (Fraxinus angustifolia Vahl) and pedunculate oak (Quercus robur L.). Ash is found on wet and heavy gley soil
and forms the swamp (“wet”) forest edge which is regularly ooded during the year (spring & autumn), while oak
is found on drier terrain, oen out of the reach of regular annual oods, and it represents the upper (“dry”) edge
of these forests. Oak is the main indicator of mesophilic conditions, but common hornbeam (Carpinus betulus L.)
also occurs and is becoming progressively more dominant in oodplain ecosystems. is kind of mosaic alter-
nates throughout the entire area regardless of the distance from the riverbed due to the specic micro-terrain
(micro-rises and micro-depressions) caused by the settling of alluvial deposits.
is study was conducted on the eastern edges of the LPNP area, which is under the direct inuence of ood-
ing from the Sava River and its tributaries. e area is characterized by a humid continental climate, with an aver-
age annual air temperature of 9.5 °C and total precipitation of 870 mm with a maximum in June and a minimum
in February. Evaporation is estimated in the range of 520–600 mm per year. For the study, two areas (Fig.1a,b)
were chosen as representative of extremely wet habitats (swamp edge) and extremely dry habitats. e areas were
required to be (i) absent of direct anthropogenic inuence (cutting, grazing etc.), (ii) representative of marginal
populations with respect to the wetness of the habitat, (iii) exposed to the same climate conditions (precipitation
and temperature) and (iv) similarly distant from the Sava riverbed. e rst area is a mixed stand of pedunculate
oak and common hornbeam in the Prašnik Forest Reserve (dry site, elevation = 95 m, distance to river = 3,2 km)
and the second is a pure narrow-leaved ash stand that forms the border between the forest and permanent wet-
lands (wet site, elevation = 90.5 m, distance to river = 2,9 km). Prašnik, with an area of 50 ha, is the last representa-
tive example of the primeval lowland forests that encompassed approximately 400 km2 of the research area before
World War I. e wood volume is approximately 550 m3 ha1 with 24 trees ha1. Pedunculate oak trees reach
impressive dimensions of up to 260 cm in diameter and 45 m in height. A dramatic change in species composition
has been observed in the last ve decades, with common hornbeam having progressed to the point where it is cur-
rently dominating the understory. e second site consists of pure stands of narrow-leaved ash that grow on the
boundaries of constantly wet alluvial swamps. e wood volume is approximately 320 m3 ha1 with 160 trees ha1.
ese narrow-leaved ash stands most probably arose by natural succession during the last 200 years. is was
inferred by studying historical maps from the late 18th century (
Field sampling. In Prašnik (dry site) we established a grid of 10 circular experimental plots, each amounting
to 2500 m2 that were evenly spread through the whole reserve area. Within each plot we positioned and sampled
all trees and dead wood with a diameter over 10 cm (Supplementary Fig.1). At the swamp boundary (wet site)
we established three experimental plots, each amounting to 800 m2, where all of the ash trees were sampled. Two
cores per tree were collected with a Pressler borer at approximately 1.30 m above ground level18. Aer collection,
preparation and drying of the samples, we carried out standard coarse and ne sample processing, incrementally
increasing the sandpaper granulation (granulation of 120 to 600).
Climate data. Climate data (mean monthly air temperature, precipitation and standardized Palmer drought
index – scPDSI) were obtained from the gridded CRU TS3.24.01 database (Fig.2) with a spatial resolution of
0.5° × 0.5° for the 1901–2015 period using the KNMI Climate Explorer platform19 ( For
the long-term climate correlations (>100 years), analysis was done using data (mean monthly temperature and
monthly precipitation sums) from the HISTALP database ( e database contains
monthly homogenized precipitation data from 192 weather stations and homogenized air temperature data from
131 weather stations in the broader Alps and Dinarides area (4° to 19°E latitude and 43° to 46°N longitude)20.
We used grid-mode-2 series that represent absolute monthly air temperature and precipitation values in a
Content courtesy of Springer Nature, terms of use apply. Rights reserved
SCientiFiC REPORTS | (2018) 8:16495 | DOI:10.1038/s41598-018-34875-w
5 × 5 minute resolution (4° to 19°E latitude and 43° to 46°N longitude). Hydrological parameters (mean monthly
water level and river discharge values) for the Sava River in the 1926–2014 period were obtained from the
Croatian Meteorological and Hydrological Service ( e distance between dry site, wet site and
nearest river gauges were 12 km and 7 km, respectively.
Chronology development. Tree-ring width was measured with a LINTAB measuring table with 0.01 mm
precision, equipped with OLYMPUS binoculars and a polarized light source. Cross-dating of samples was done
both visually and using the TSAP-Win dendrochronological soware ( Cross-dating
quality was veried using COFECHA program21,22 by checking the consistency of ring width series among trees
from the same site. We averaged two cores for each tree, thereby obtaining one representative series per tree
(Supplementary Fig.2). For each core cambial age was estimated using concentric circles method. Pith estimates
of 10 missing rings or more were not included in the age data set.
Detrending, that is, removing frequency variability as a consequence of the biological age eect as well as
standardization was done on individual tree-ring width series using the “dplR” package in R23. Several methods
(Negative exponential curve, Regional curve standardization - RCS, Signal-Free RCS - RCSsf, C-method, Spline)
were used (Supplementary Fig.3). Following standardization, individual series were calculated using Tukey’s
biweight robust mean24 to obtain a residual chronology25 (Tree-ring width index - TRWI) which was used in all
subsequent analyses (Fig.3). Since the correlation to climate of chronologies obtained through the mentioned
methods showed almost no dierence (Supplementary Fig.4), the Spline method (frequency response of 0.50 cut
o at 0.67 series length) was chosen.
e quality of the obtained chronology was assessed by using several dendrochronological statistics: mean
sensitivity (MS), which is a measure of year-to-year variability in the tree growth series26 calculated as the dier-
ence between each two successive rings divided by their mean27; rst-order autocorrelation of raw data (AC1),
which determines the variance of the current year’s growth that is explained by the previous year’s growth28; the
expressed population signal (EPS), used to assess chronology reliability where EPS value over 0.85 quanties the
degree to which the constructed chronology represent the hypothetical population29 and mean interseries corre-
lation (Rbar) (Supplementary Table.1).
Climate-growth analysis. Climate–growth relationships were assessed by correlation function as well as
response function analysis. In correlation functions, the coecients calculated between the tree-ring chronology
and monthly climatic variables are univariate estimates of Pearson’s product moment correlation. In response
functions, the coecients are obtained through multiple regression using the principal components of monthly
climatic data to estimate ring-width growth indices. ey are interpreted as average eect of the uctuation of
that monthly climatic variables on tree growth. is regression model is used in tree-ring studies to identify
Figure 1. Location of the study sites. Photographs of oak (dry, Site 1) and ash (wet, Site 2) sampled stands
(a). Positions of sampling areas (black square), the location of the hydrologic water level monitoring station
(white dots) and natural, regularly ooded area (shaded polygon) (b).
Content courtesy of Springer Nature, terms of use apply. Rights reserved
SCientiFiC REPORTS | (2018) 8:16495 | DOI:10.1038/s41598-018-34875-w
the climate origin of variability in the chronologies through avoidance of intercorrelations between climatic
Analysis was performed with the “treeclim” package in R30 for a period of 16 months (June of the previous
year to September of the current year) between climate and hydrologic data and residual chronology (Fig.4).
e signicance (p-value < 0.05) of each coecient was evaluated using 1000 bootstrap replications mimicking
DENDROCLIM2002 soware31. Analysis was performed for the period of 1950–2014 (determined by the greatest
frequency and quality of meteorological data). Seasonal correlation analysis was done using “treeclim” package30
for a 95% signicance level for dierent season durations from 2 to 6 months in monthly increments. In the
results we show seasonal durations for 2 and 4 months (Fig.4).
Temporal stability of Climate-growth relations. Temporal stability of the climate signal was ana-
lyzed using moving window correlations with a 30-year interval (Fig.5). Analysis was performed with the most
Temperature (°C)
Precipitation (mm)
0 500 1000250 Km.
Sava River
Sava Catchment
0 500 1000250 Km.
Sava River
Sava Catchment
1920 1940 1960 1980 2000
River Water Level (cm)
86.82 m a.s.l.
−1 −0.8 −0.6 −0.4 −0.20 0.20.4 0.60.8
S=2082, Z=5.0305, P<0.001
S=-419, Z=1.0104, P=0.3123
S=-962, Z=2.3229, P=0.0202
S=-1163, Z=4.118, P<0.001
Figure 2. Long-term trend of mean annual climate data (Temperature, Precipitation, scPDSI) and River Water
Level. Spatial eld correlation between the mean 12-monthly Sava River Water level with E-OBS 14.0 current
year Tmax (averaged May–August) (a) and gridded precipitation (b) for the period 1950–2014. e correlation
matrix between average monthly water levels (I–XII), temperature (t) and precipitation (p) for the period
1926–2014 (c).
Content courtesy of Springer Nature, terms of use apply. Rights reserved
SCientiFiC REPORTS | (2018) 8:16495 | DOI:10.1038/s41598-018-34875-w
signicant monthly and seasonal variables for the 1901–2014 period using CRU TS3.24.01 climate data, with
additional analysis of pedunculate oak correlations for a longer period (1800–2014) using HISTALP climate data
Disturbance analysis. We collected documented and archived records of salvage logging (drought induced)
and disturbances for the entire lowland region in Croatia for the past century. We extracted information on the
agent of past disturbances and quantity of salvaged wood (m3 ha1) (Fig.6).
is data provides information from a large area giving us a useful insight into past disturbance regimes and
general trends in decline, but it should be kept in mind that it is obtained from managed and seminatural forest
which can dier in their resistance to disturbances32.
For disturbance analysis in our study, series from trees older than 150 years were used (80 trees). For each tree,
the percentage change in growth was calculated according to the method proposed by Nowacki and Abrams33.
is method uses a percent growth change equation:
=− %GC ((M2M1)/M1)) 100,
where % GC is percent growth change from preceding to superseding 10-yr radial average, M1 is the preceding
10-yr mean radial growth (exclusive of the current year) and M2 is the superseding 10-yr mean radial growth
(inclusive of the current year). e minimum threshold for release is 25% growth change for moderate and >50%
for major release. e percentage of trees showing releases was plotted against time (Fig.7).
River water level. e analysis of monthly water level data showed a signicant decrease in the Sava River
water level from 1926 to 2014, with the decrease being especially pronounced aer 1980 (Fig.2). is is caused
mostly by recent increases in temperature, especially in the summer period from May to August (r = 0.79,
p-value < 0.001). Individual monthly and average seasonal air temperature and river water level correlations in
July and August exhibit the largest values (R = 0.72, p-value < 0.001) (Fig.2.). Total annual precipitation for the
LPNP area is 870 mm. Of this amount, 450 mm occurs in the vegetation period (May to September), nearly the
total amount of actual evapotranspiration. e total variability of the water level explained through the negative
inuence of summer air temperatures (May–August) and the positive inuence of precipitation (April–August)
is 68% (p-value < 0.001).
Tree-ring statistics. For the climate-growth analysis a total of 215 samples of pedunculate oak and 85 sam-
ples of narrow-leaved ash were dated. Since some of the oak samples were in very poor condition upon extraction,
with visible rot and variable wood consistency, only 208 samples of oak were used for the climate-growth anal-
ysis (Supplementary Fig.2d). In some cases, the oak tree dimensions exceed 2 m in diameter, making sampling
particularly dicult. Bearing in mind that this area is protected, we limited the size of the sample to the smallest
possible. e chronology range is 1732–2017 for oak and 1885–2015 for ash. e oldest cambial age of oak is 285
Figure 3. Residual tree-ring index chronologies smoothed with 10 years low pass lter to highlight decadal
high-frequency variability (violet and red). Running EPS and running Rbt statistics (Inter trees correlation) for
Fraxinus angustifolia (a) and Quercus robur (b). EPS and Rbt was calculated using a 50-year moving window.
Content courtesy of Springer Nature, terms of use apply. Rights reserved
SCientiFiC REPORTS | (2018) 8:16495 | DOI:10.1038/s41598-018-34875-w
years old, while the oldest ash is 130 years old. Two generations of oak were observed (young <100 years cambial
age, old >100 years cambial age) and separated in further analysis (Supplementary Fig.2c,d).
A high autocorrelation of raw series was found in old pedunculate oak (0.74) in comparison to ash (0.45),
which points to the signicant accumulated inuence of climate conditions in the previous years. Comparison of
standardized chronologies shows that narrow-leaved ash has higher mean sensitivity than old pedunculate oak
(MSAsh = 0.36 vs MSOak_O = 0.21). e interseries correlation (Rbar) is also higher for narrow-leaved ash (0.51)
than for pedunculate oak (0.36). EPS value > 0.85 quanties the degree to which the constructed chronology
represents the hypothetical population (EPSAsh from 1897, EPSOak from 1760).
Climate-growth correlations. Simple linear correlation between residual chronologies of tree ring width
index (TRWI) for ash and oak and monthly climate data showed that narrow-leaved ash was signicantly more
sensitive to the hydrologic component, especially precipitation, than pedunculate oak. A statistically positive
correlation (p-value < 0.05) was determined for precipitation (Prec), river water level (R), river discharge (Q) and
drought index (scPDSI) in individual monthly as well as individual seasonal values for 2- and 4-months intervals
in the 1950–2014 period (Fig.4).
e highest positive correlations were found for river discharge (r = 0.61) and water level (r = 0.52) in May of
the current year as well as seasonal correlations from May to August (rQ = 0.68, rR = 0.52). Precipitation in April
Figure 4. Bootstrapped mean monthly and seasonal correlation between TRWI and climate. F. angustifolia
(blue) and Q. robur (old – red, young - gray) for selected climate factors (Temperature, Precipitation and
scPDSI) and hydrological parameters (Sava River water level and river discharge) for the 1950–2015 period.
Statistically signicant values are marked with a dot (p-value < 0.05) and signicant response coecient with a
black bordered dot. Shaded area highlights the correlation values with months of previous year.
Content courtesy of Springer Nature, terms of use apply. Rights reserved
SCientiFiC REPORTS | (2018) 8:16495 | DOI:10.1038/s41598-018-34875-w
and July showed a signicant positive correlation (r = 0.38). Highest correlation values were found from April to
July (r = 0.63) for the seasonal values.
Signicant positive correlations between the residual chronology and the drought index (scPDSI) for the
individual month values were determined from February to September of the current year with the highest value
in August (r = 0.61). Due to the high autocorrelation structure of scPDSI monthly values, a response function
analysis that reduces individual month intercorrelation was performed. e results point to a signicant positive
sensitivity of ash to scPDSI values in July of the current year (Fig.4). e seasonal values show the highest corre-
lation from June to September (r = 0.60). Negative correlations for temperature were determined for individual
months in May and June (r = 0.28) and for the seasonal values from May to June (r = 0.35).
Unlike narrow-leaved ash, radial growth in old pedunculate oaks showed signicant sensitivity to scPDSI
and temperature. Positive correlations with scPDSI were determined in July of the current year (r = 0.45). In the
period from June to July, a peak in positive impact of scPDSI was recorded (r = 0.43). In contrast, a signicant
negative air temperature impact was determined for April (r = 0.35) and July (r = 0.39). e negative temper-
ature impact becomes more pronounced looking at the seasonal values from April to July (r = 0.43).
Inuence of the precipitation on radial growth is low but statistically signicant. e highest value was deter-
mined in February of the current year (r = 0.26) and for seasonal values from February to May (r = 0.35). e
same pattern was also observed for river discharge and water level. Both show the highest correlation values for
individual months in May (rQ = 0.31, rR = 0.33) and for the seasonal values from April to July (rQ = 0.35, rR = 0.35)
Young oak trees showed lower climate sensitivity than old oaks, with the most signicant positive correla-
tion for temperature in June of previous year (r = 0.39) and for precipitation in June of current year (r = 0.27)
(Fig.4). We also found signicant raw ring width decrease during drought events in the juvenile growth phase
(Supplementary Fig.5). Interestingly, in 2003 the driest year recorded did not aect the growth rate.
Figure 5. Moving correlation between TRWI and most signicant monthly (a,b) and average seasonal climate
factors from CRU TS4. 01 and HISTALP and hydrological data (river water level - RIVER, and river discharge - Q)
(c,d) using 30 years moving window for F. angustifolia and Q. robur for the period 1901–2014 and longer
(1801–2014) period (e).
Content courtesy of Springer Nature, terms of use apply. Rights reserved
SCientiFiC REPORTS | (2018) 8:16495 | DOI:10.1038/s41598-018-34875-w
e results indicate signicant dierence in climate response between both species. Narrow-leaved ash exhib-
its a more stable signal for the studied period in comparison to oak. e correlation with the precipitation average
for April–July showed a quite stable signal during the studied period. However, on the individual monthly level
(May), signicant increase in correlation with precipitation, river water level and river discharge were observed
(Fig.5a,c). Sensitivity to precipitation in April and May became more pronounced aer 1950. A noticeable
increase in response to the Sava River water level and river discharge in May was determined aer the 1970s, and
reciprocally there was an increase in the negative signal air temperature values in May (Fig.5a).
Oak had a negative but not statistically signicant signal for temperature until the 1950s, aer which the signal
becomes statistically signicant, especially for April and July (Fig.5b,d). At the same time, there were increases
in the positive response to the precipitation, Sava River water level and river discharge in April, May and July.
is pattern was also observed for the 1801–2014 period using HISTALP climate data. e negative tempera-
ture impact was present during the entire studied period, with an increasing trend that becomes especially pro-
nounced aer 1950 for all months between April and July as well as the seasonal average (April–July) (Fig.5e).
Historical evidence of disturbances and salvage logging data. According to the collected historical
records for the period 1900–2010, 7.8 million m3 of dead oak trees have been salvaged. e rst large dieback
occurred from 1910–1925 (1.73 million m3) with three pronounced peaks in 1911, 1916 and 1924. From the
1950s until 1990s salvaging was low compared to the whole period. Still, two large individual events that were
preceded by long-lasting oods (1965 and 1985) occurred in that period.
Aer the 1990s there is evidence of noticeable increase in salvage logging. During the last twenty years total
salvage logging because of individual tree mortality was signicantly higher than all the salvaged wood from the
disturbance events put together. An increase in windthrow events has also been recorded (430153 m3 in the last
two decades) as well as an increase in mortality for ash from 2010 onwards (Fig.6).
e natural disturbance chronology analysis determined four pronounced periods of growth release in the
pedunculate oak primeval forest (1780s, 1850s, 1890s and 1940s). Decadal peaks were determined for the fol-
lowing years: (I) 1788, (II) 1855, (III and IV) 1891 and 1893, and (V and VI) 1942, 1946 and 1950 (Fig.7f). With
narrow-leaved ash, three decades of growth release were determined (1940s, 1960s and 1980s), with peaks in
1946, 1950, 1966 and 1983 (Fig.7h). ese disturbance chronologies show a co-occurrence of growth release with
summer droughts (Fig.7a), high temperatures (Fig.7b), low water level (Fig.7c) and precipitation decit (Fig.7d)
equally in both species. Co-occurrence of peaks in the chronology for both species was found during the 1940s
when a series of dry years was recorded.
e successional response to natural disturbances in the form of cyclical regeneration was observed only once
in the primeval oak forest, but not in ash stands. Oak age structure analysis showed one large period of recruit-
ment that corresponded to the period of intense growth release in the 1940s (Fig.7e). is age structure partially
explains the sigmoidal shape of the diameter frequency distribution characteristic of primeval forests in the tem-
perate zone. Apart from oak, the primeval forest reserve was found to have a signicant distribution of common
Figure 6. Salvage logging data for the lowland area of Croatia. Black histograms represent sum of drought
induced annual salvage logging of oak (a) and ash (b) and windthrow (blue) with individual high-severity
disturbance events (red). NA- data not available for this period.
Content courtesy of Springer Nature, terms of use apply. Rights reserved
SCientiFiC REPORTS | (2018) 8:16495 | DOI:10.1038/s41598-018-34875-w
hornbeam with an average age of 45 years (20 cm in diameter). Its abundant occurrence coincides with the 1970s,
that is, approximately 20 years aer the recorded period of oak growth release.
Numerous studies in Europe suggest that high precipitation values and lower temperatures in the spring and
summer increase the radial growth of pedunculate oak3442.
e results of our research indicate that old oak trees are positively sensitive to scPDSI and hydrological
parameters and at the same time limited by high air temperatures from April to July, whereas the role of precip-
itation is not as pronounced as that found in the above-mentioned research. Oak’s positive sensitivity to river
water level & discharge in May could be attributed to more intensive new root hairs (<1 mm in diameter) growth
during June and their abrupt dying in the surface layer of the soil during July and at the start of August43,44. Similar
Figure 7. Disturbances chronology and climate variability. PDSI reconstruction for summer (June–August)
droughts severity (Cook et al., 2015) (a). Mean temperature dierence from April to July (b, most severe
droughts marked with plus signs). Mean Sava river water level from May to August (c). Dierence in
precipitation and potential evapotranspiration (PET) for the period (April–July) calculated using long-term
climate data series received from the HISTALP database for sum for the last two years (d). Recruitment of oak
and hornbeam (e) and ash (g) based on cambial age. Disturbance chronology with moderate (black) and major
releases (red bar) displayed in annual interval for pedunculate oak (f) and narrow-leaved ash (h).
Content courtesy of Springer Nature, terms of use apply. Rights reserved
SCientiFiC REPORTS | (2018) 8:16495 | DOI:10.1038/s41598-018-34875-w
climate sensitivity was not observed for younger trees (<100 years), which we attribute to high interspecies com-
petition (mainly with hornbeam).
In old oaks (older than 150 years) climate sensitivity is not stable over the analyzed period. e negative
response to temperature (April–July) has become signicantly pronounced in roughly the last seven decades
(Figs5 and 6), while at the same time the positive response to precipitation and hydrological parameters has been
growing. is kind of change in oak’s response can be explained by extraordinary warm and drought years in the
1940s. (Fig.2). Similar temporal instability was observed in central-west Germany where oaks changed from a
precipitation to temperature sensitivity aer a severe drought in the 1940s38.
Unlike oak, ash exhibited a more pronounced sensitivity, especially to precipitation (April, May and July) and
to river water level (May and August) while high temperatures in May and June reduced the radial growth. e
aforementioned climatic signal in ash has been conrmed in rare researches in Europe45,46.
We attribute this pronounced precipitation signal to the extreme conditions of ash distribution in relation to
oak. Ash occurs on heavy clay soil at the edge of alluvial swamps, which is also lower, concave terrain that retains
rainfall longer and consequently increases the possibility of greater soil inltration47. In such extreme conditions
ash develops a specic shallow root system exposed to seasonal oscillations of extremely wet (spring, autumn)
and dry periods (summer). Like other oodplain species, it is well adjusted to high levels of groundwater and
limited by humidity shortage during drought periods48. Ash has a high tolerance for such extreme conditions,
and it has no competition from any other tree species, which gives it the opportunity to dominate at the edges of
wet lowland swamps49,50.
Climate sensitivity of ash is stable for seasonal correlations over the analyzed period but for monthly correla-
tions (May) there is a constant increase of sensitivity to all variables especially hydrological parameters (Fig.5a).
We explain this through increasing temperature and decreasing in moisture (decrease in river water level - declin-
ing trend 2.24 cm*year1 in May for the 1926–2014 period).
The natural dynamics and structure of lowland floodplain primeval forests is hard to reconstruct due
to the lack of preserved reserves in Europe, most of them are deforested during the end of the 19th century
(Supplementary Fig.6). According to historical data four severe individual events were evident during the past
century (Fig.6). Such events occurred on 5.5% of the whole lowland area. Intensity of this recorded events was
from 46–139 m3 ha1 (mean 70 m3 ha1). e most severe dieback was in 1985 with intensity of 139 m3 ha1.
e main cause for such events was a combination of drought follow by oods of long duration. We also found
an increase of windthrow events especially during last past decades which can be attributed to the dierence in
resistance to natural disturbances of managed and semi-natural forests in contrast to primeval ones32. In our
research three pronounced periods of growth release correspond to droughts. Age structure of these forests shows
only two generations of oak trees revealing the severe drought of 1940s as the only drought that triggered a regen-
eration. Origin of rst generation of oak also corresponds to drought during the 1740s (Fig.7).
is implies the complexity of natural regeneration dynamic in primeval forests and also the signicant role
of drought in development of uniformed age structure of European lowland primeval forests. e lack of regen-
eration aer the disturbances in the 1850s and 1890s (Fig.7e) can be attributed to the fact that oak regenerates
in a small area, is especially dependent on light conditions and is spatially limited by the heavy seed dispersion
mechanism. Another possible factor is high competition with eld elm (Ulmus minor L.) which has been widely
distributed until 1970s in oodplain forests. Also unavoidable is the possible inuence of herbivores (primarily
wild pigs) and birds, which can signicantly decrease abundant seed crops51. Observed growth release of ash trees
did not result in regeneration. Being a pioneer tree species, ash originated through succession on the wetlands
and the structure of these forests are very dense which hinders successful regeneration. erefore, it requires large
scale disturbances for adequate regeneration in contrast to oak.
Rising temperatures and anthropogenic inuence have signicantly changed the hydrologic conditions in
both the river basin area and the river itself52. is study, which was conducted in the still preserved natural reten-
tion area of the Sava River, also found signicant changes. ey manifest in the decline of average, and especially
minimal annual and individual monthly water levels, with particular intensity aer 1980 (Fig.2). Such negative
trends can be explained by increased evaporation due to increases in temperature and evapotranspiration and
have been recorded in most European rivers52. From an ecological standpoint, river water level is an important
indicator of groundwater levels that recharges by inltration from the Sava riverbed during high water levels
(spring and autumn) even up to 5 km from the riverbed53.
e proportion of transpiration in oodplain forests is a high 80% of the total evapotranspiration, and most
of the transpired water originates from underground sources54. In such conditions, climate change together with
the fall of river water levels, increasingly expose oak to unfavorable conditions37,5558.
e results of this research conrm recent trends that higher air temperatures hasten the long-term decline
of trees and that in the dry conditions of this area, oak might be greatly endangered by changes in climate5963.
Although some research suggests an increase in oak radial growth due to higher temperatures56,64, CO2 fertili-
zation and increases in N deposition65, we believe this might be possible only in normal hydrologic conditions.
We conclude that pedunculate oak and narrow-leaved ash dier in their sensitivity to climate and hydrological
parameters to which ash is more sensitive. Constant increase of sensitivity to precipitation and river water level of
ash has become more pronounced during the climate warming period. Our results suggest that extreme climatic
events, especially drought are a signicant driver behind growth release but isn’t always followed by successful
recruitment in the studied forests. We also found that oak has an evident shi in sensitivity triggered by severe
droughts in the 1940s. Such shi may have been due to its adaptation strategy to increasing temperature and drier
environment conditions.
Content courtesy of Springer Nature, terms of use apply. Rights reserved
SCientiFiC REPORTS | (2018) 8:16495 | DOI:10.1038/s41598-018-34875-w
Narrow-leaved ash, as the colonizer of wet swamp edges, could also face the rising pressure of water decit. Its
climate signal is slightly more stable, without the dramatic changes exhibited by oak, but considering the distur-
bance dynamic reconstruction results, it shows greater sensitivity to droughts.
e physiological exibility of oak’s adaptation to drought events was shown to be an optimal survival mecha-
nism in the past. However, it could be expected that in disturbed conditions with declining levels of groundwater,
oak might not be able to utilize this alternative in the future.
1. Allen, C. D. et al. A global overview of drought and heat-induced tree mortality reveals emerging climate change riss for forests.
Forest Ecology and Management 259, 660–684 (2010).
2. Mantgem, P. J. V. et al. Widespread Increase of Tree Mortality ates in the Western United States. Science 323, 521–524 (2009).
3. limo, E. Floodplain forests of the temperate zone of Europe. (Lesnicá práce, 2008).
4. Schnitzler, A., Hale, B. W. & Alsum, E. Biodiversity of oodplain forests in Europe and eastern North America: A comparative study
of the hine and Mississippi Valleys. Biodiversity and Conservation 14, 97–117 (2005).
5. Machar, I. Floodplain forests of Litovelsé Pomoraví and their management. Journal of Forest Science 54(8), 355–369 (2008).
6. Tocner, . & Stanford, J. iverine ood plains: present state and future trends. Environmental Conservation 29(3), 308–330 (2002).
7. Gilg, O. Forêts à caractère naturel: caracte ristiques, conservation et suivi. (LAtelier technique des espaces naturels, 2004).
8. üther, C. & Walentowsi, H. Tree species composition and historic changes of the Central European oa/beech region. In: Floren,
A. & Schmidl, J. (Eds): Canopy arthropod research in Europe (pp. 61–88) (Bioform, 2008).
9. Yon, D. Alluvial Forests of Europe. (Council ofEurope, 1981).
10. Hughes, F. & ichards, . S. e ooded forest: guidance for policy maers and river managers in Europe on the restoration of oodplain
forests. (e FLOBA2 Project, 2003).
11. Schneider-Jacoby, M. reatened Ecosystems of International Importance. Journal of Forestry Society of Croatia 130(5-6), 193–217
12. Stallins, J., Nesius, M., Smith, M. & Watson, . Biogeomorphic characterization of oodplain forest change in response to reduced
ows along the Apalachicola iver, Florida. iver esearch and Applications 26, 242–260 (2009).
13. limo, E. et al. O. Functioning of South Moravian Floodplain Forests (Czech epublic) in Forest Environment Subject to Natural
and Anthropogenic Change. International Journal of Forestry esearch 248749, 1–8 (2013).
14. Janí, D. et al. Tree layer dynamics of the Cahnov-Souto near-natural oodplain forest aer 33 years (1973–2006). European
Journal of Forest esearch 127(4), 337–345 (2008).
15. Delatour, C. Les deperissements de chenes en Europe. evue forestière française 35, 265–282 (1983).
16. owalsi, T. Chalara fraxinea sp. nov. associated with diebac of ash (Fraxinus excelsior) in Poland. Forest Pathology 36, 264–270
17. O’Hara, . What is close-to-nature silviculture in a changing world? Forestry 89, 1–6 (2016).
18. Phipps, . L. Collecting, preparing, crossdating, and measuring tree increment cores. (U.S. Dept. of the Interior, Geological Survey,
19. Trouet, V. & Van Oldenborgh, G. J. NMI Climate Explorer: A web-based research tool for high-resolution paleoclimatology. Tree -
ing esearch 69(1), 3–13 (2013).
20. Auer, I. et al. HISTALP—historical instrumental climatological surface time series of the Greater Alpine egion. International
Journal of Climatology 27, 17–46 (2006).
21. Holmes, . L. Computer assisted quality control in tree-ring dating and measurement. Tree-ring bulletin 43, 69–78 (1983).
22. Grissino-Mayer, H. D. Evaluating Crossdating Accuracy: A Manual and Tutorial for the Computer Program COFECHA. Tree-ing
esearch 57, 205–221 (2001).
23. Bunn, A. G. A dendrochronology program library in  (dpl). Dendrochronologia 26(2), 115–124 (2008).
24. Mosteller, F. & Tuey, J. W. Data analysis and regression: a second course in statistics. (Addison-Wesley, 1977).
25. Coo, E. . A time series analysis approach to tree ring standardization. (University Microlms International, 1985).
26. Fritts, H. C. Tree rings and climate. (e Blacburn Press, 2014).
27. Douglass, A. E. Evidence of Climatic Eects in the Annual ings of Trees. Ecology 1, 24–32 (1920).
28. Speer, J. H. Fundamentals of tree-ring research. (e University of Arizona Press, 2013).
29. Wigley, T. M. L., Bria, . . & Jones, P. D. On the Average Value of Correlated Time Series, with Applications in Dendroclimatology
and Hydrometeorology. Journal of Climate and Applied Meteorology 23, 201–213 (1984).
30. Zang, C. & Biondi, F. treeclim: an  pacage for the numerical calibration of proxy-climate relationships. Ecography 38, 431–436
31. Biondi, F. & Waiul, . DENDOCLIM2002: A C program for statistical calibration of climate signals in tree-ring chronologies.
Computers & Geosciences 30(3), 303–311 (2004).
32. Nagel, T. A. et al. e natural disturbance regime in forests of the Dinaric Mountains: A synthesis of evidence. Forest Ecology and
Management 388, 29–42 (2017).
33. Nowaci, G. J. & Abrams, M. D. adial-Growth Averaging Criteria For econstructing Disturbance Histories From Presettlement-
Origin Oas. Ecological Monographs 67(2), 225–249 (1997).
34. Čufar, . et al. Tree-ing Chronology of Pedunculate Oa (Quercus robur) and its Potential for Development of Dendrochronological
esearch in Croatia. Drvna industr ija 65(2), 129–137 (2014).
35. ern, Z. C. A. et al. Multiple tree-ring proxies (earlywood width, latewood width and δ13C) from pedunculate oa (Quercus robur L.),
Hungary. Quater nary Internati onal 293, 257–267 (2013).
36. Nechita, C., Popa, I. & Eggertsson, Ó. Climate response of oa (Quercus spp.), an evidence of a bioclimatic boundary induced by the
Carpathians. Science of e Total Environment 599-600, 1598–1607 (2017).
37. Netsvetov, M., Sergeyev, M., Niulina, V., orniyeno, V. & Proopu, Y. e climate to growth relationships of pedunculate oa in
steppe. Dendrochronologia 44, 31–38 (2017).
38. Friedrichs, D. A. et al. Complex climate controls on 20th century oa growth in Central-West Germany. Tree Physiology 29(1), 39–51
39. Tessier, L., Nola, P. & Serre-Bachet, F. Deciduous Quercus in the Mediterranean region: tree-ring/climate relationships. New
Phytologist 126(2), 355–367 (1994).
40. ozas, V. & Olano, J. M. Dendroclimatic responses of four European broadleaved tree species near their south-western range edges.
Dendrobiology 77, 65–75 (2017).
41. Bednarz, Z. & Pta, J. e inuence of temperature and precipitation on ring widths of oa (Quercus robur L.) in the Niepolomice
forest near Cracow, Southern Poland. Tree-ing Bull etin 50, 1–10 (1990).
42. Stojanović, D., Levanić, T., Matović, B. & Orlović, S. Growth decrease and mortality of oa oodplain forests as a response to change
of water regime and climate. European Journal of Forest esearch 134(3), 555–567 (2015).
43. onôpa, B., Yuste, J. C., Janssens, I. A. & Ceulemans, . Comparison of Fine oot Dynamics in Scots Pine and Pedunculate Oa in
Sandy Soil. Plant and Soil 276, 33–45 (2005).
Content courtesy of Springer Nature, terms of use apply. Rights reserved
SCientiFiC REPORTS | (2018) 8:16495 | DOI:10.1038/s41598-018-34875-w
44. Ponti, F., Minotta, G., Cantoni, L. & Bagnaresi, U. Fine root dynamics of pedunculate oa and narrow-leaved ash in a mixed-
hardwood plantation in clay soils. Plant and Soil 259, 39–49 (2004).
45. Muñoz, N. G., Linares, J. C., Castro-Díez, P. & Sass-laassen, U. Contrasting secondary growth and water use eciency patterns in
native and exotic trees co-occurring in inner Spain riparian forests. Forest Systems 24, 017 (2015).
46. Marques & Gomes, I. C. Fraxinus angustifolia Vahl as a valuable species in riparian rehabilitation projects. From annual growth to
habitat preference of narrow-leaved ash in southern Portugal. Handle Proxy (1970), Available at, http://hdl.handle.
net/10400.5/13381. (Accessed: 26th September 2018).
47. Fuare, P. Narrow-leaved ash and its morphological variability. Fraxinus angustifolia Vahl (=Fr. oxycarpa Willd.). Glasni za
Šumse Pouse 14, 133–258 (1960).
48. Singer, M. B. et al. Contrasting water-uptae and growth responses to drought in co-occurring riparian tree species. Ecohydrology
6(3), 402–412 (2013).
49. Drvodelić, D., Ugarović, D., Oršanić, M. & Paulić, V. e Impact of Drought, Normal Watering and Substrate Saturation on the
Morphological and Physiological Condition of Container Seedlings of Narrow-Leaved Ash (Fraxinus angustifolia Vahl). South-east
European forestry 7(2), 135–142 (2016).
50. Gomes, A. . S. & ozlowsi, T. T. Growth esponses and Adaptations of Fraxinus pennsylvanica Seedlings to Flooding. Plant
Physiology 66, 267–271 (1980).
51. Harmer, . & Morgan, G. Development of Quercus robur advance regeneration following canopy reduction in an oa woodland.
Forestry 80, 137–149 (2007).
52. St ahl, . et al. Streamow trends in Europe: evidence from a dataset of near-natural catchments. Hydrology and Earth System
Sciences Discussions 7, 5769–5804 (2010).
53. Brić, Ž., Briši M. & Marović, T. Use of hydrochemistry and isotopes for improving the nowledge of groundwater ow in a
semiconned aquifer system of the Eastern Slavonia (Croatia). Catena 142, 153–165 (2016).
54. Čermá, J. & Prax, A. Transpiration and soil water supply in oodplain forests. Eologia 28(3), 248–254 (2009).
55. Oosterbaan, A. & Nabuurs, G. J. elationships between oa decline and groundwater class in e Netherlands. Plant and Soil 136(1),
87–93 (1991).
56. Tumajer, J. & Treml, V. esponse of oodplain pedunculate oa (Quercus robur L.) tree-ring width and vessel anatomy to climatic
trends and extreme hydroclimatic events. Forest Ecology and Management 379, 185–194 (2016).
57. Arend, M., Brem, A., uster, T. M. & Günthardt-Goerg, M. S. Seasonal photosynthetic responses of European oas to drought and
elevated daytime temperature. Plant Biology 15(1), 169–176 (2013).
58. Siweci, . & Ufnalsi, . eview of oa stand decline with special reference to the role of drought in Poland. Forest Pathology 28(2),
99–112 (1998).
59. Andersson, M., Milberg, P. & Bergman, -O. Low pre-death growth rates of oa (Quercus robur L.)-Is oa death a long-term
process induced by dry years? Annals of Forest Science 68, 159–168 (2011).
60. Doležal, J. Mazůre, P. & limešová, J. Oa decline in southern Moravia: the association between climate change and early and late
wood formation in oas. Preslia 82, 289–306 (2010).
61. Urli, M. et al. e high vulnerability of Quercus robur to drought at its southern margin paves the way for Quercus ilex. Plant
Ecology 216(2), 177–187 (2014).
62. Bauwe, A., Jurasinsi, G., Scharnweber, T., Schröder, C. & Lennartz, B. Impact of climate change on tree-ring growth of Scots pine,
common beech and pedunculate oa in northeastern Germany. iForest - Biogeosciences and Forestry 9, 1–11 (2015).
63. Helama, S. et al. Oa decline analyzed using intraannual radial growth indices, δ13C series and climate data from a rural hemiboreal
landscape in southwesternmost Finland. Environmental Monitoring and Assessment 186(8), 4697–4708 (2014).
64. int, V. et al. adial growth change of temperate tree species in response to altered regional climate and air quality in the period
1901–2008. Climatic Change 115(2), 343–363 (2012).
65. Becer, M., Nieminen, T. & Gérémia, F. Short-term variations and long-term changes in oa productivity in northeastern France.
e role of climate and atmospheric CO2. Annales des Sciences Forestières 51(5), 477–492 (1994).
S.M., A.Ž., D.T., V.P., M.O. and I.A. were supported by the Croatian National Science Foundation under the
project IP-2014-09-1834, A.Ž. received support from Ministry of Agriculture of the Republic of Croatia. Many
thanks to omas Andrew Nagel for his helpful suggestions and comments as well as Jan Nagel for improving the
Author Contributions
S.M. and A.Ž. designed the research and prepared the manuscript, D.T., V.P. analyzed data, M.O. and I.A. helped
supervise the project. All authors together contributed to the interpretation of results.
Additional Information
Supplementary information accompanies this paper at
Competing Interests: e authors declare no competing interests.
Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and
institutional aliations.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International
License, which permits use, sharing, adaptation, distribution and reproduction in any medium or
format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Cre-
ative Commons license, and indicate if changes were made. e images or other third party material in this
article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the
material. If material is not included in the article’s Creative Commons license and your intended use is not per-
mitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the
copyright holder. To view a copy of this license, visit
© e Author(s) 2018
Content courtesy of Springer Nature, terms of use apply. Rights reserved
Terms and Conditions
Springer Nature journal content, brought to you courtesy of Springer Nature Customer Service Center GmbH (“Springer Nature”).
Springer Nature supports a reasonable amount of sharing of research papers by authors, subscribers and authorised users (“Users”), for small-
scale personal, non-commercial use provided that all copyright, trade and service marks and other proprietary notices are maintained. By
accessing, sharing, receiving or otherwise using the Springer Nature journal content you agree to these terms of use (“Terms”). For these
purposes, Springer Nature considers academic use (by researchers and students) to be non-commercial.
These Terms are supplementary and will apply in addition to any applicable website terms and conditions, a relevant site licence or a personal
subscription. These Terms will prevail over any conflict or ambiguity with regards to the relevant terms, a site licence or a personal subscription
(to the extent of the conflict or ambiguity only). For Creative Commons-licensed articles, the terms of the Creative Commons license used will
We collect and use personal data to provide access to the Springer Nature journal content. We may also use these personal data internally within
ResearchGate and Springer Nature and as agreed share it, in an anonymised way, for purposes of tracking, analysis and reporting. We will not
otherwise disclose your personal data outside the ResearchGate or the Springer Nature group of companies unless we have your permission as
detailed in the Privacy Policy.
While Users may use the Springer Nature journal content for small scale, personal non-commercial use, it is important to note that Users may
use such content for the purpose of providing other users with access on a regular or large scale basis or as a means to circumvent access
use such content where to do so would be considered a criminal or statutory offence in any jurisdiction, or gives rise to civil liability, or is
otherwise unlawful;
falsely or misleadingly imply or suggest endorsement, approval , sponsorship, or association unless explicitly agreed to by Springer Nature in
use bots or other automated methods to access the content or redirect messages
override any security feature or exclusionary protocol; or
share the content in order to create substitute for Springer Nature products or services or a systematic database of Springer Nature journal
In line with the restriction against commercial use, Springer Nature does not permit the creation of a product or service that creates revenue,
royalties, rent or income from our content or its inclusion as part of a paid for service or for other commercial gain. Springer Nature journal
content cannot be used for inter-library loans and librarians may not upload Springer Nature journal content on a large scale into their, or any
other, institutional repository.
These terms of use are reviewed regularly and may be amended at any time. Springer Nature is not obligated to publish any information or
content on this website and may remove it or features or functionality at our sole discretion, at any time with or without notice. Springer Nature
may revoke this licence to you at any time and remove access to any copies of the Springer Nature journal content which have been saved.
To the fullest extent permitted by law, Springer Nature makes no warranties, representations or guarantees to Users, either express or implied
with respect to the Springer nature journal content and all parties disclaim and waive any implied warranties or warranties imposed by law,
including merchantability or fitness for any particular purpose.
Please note that these rights do not automatically extend to content, data or other material published by Springer Nature that may be licensed
from third parties.
If you would like to use or distribute our Springer Nature journal content to a wider audience or on a regular basis or in any other manner not
expressly permitted by these Terms, please contact Springer Nature at
... Although it demonstrates narrowed intrapopulation variance both at SSR and non-outlier SNP loci, it harbors wider than average intrapopulation variance at outlier loci, which are affected by selection [10]. The reasons for increased F IS in P10 (Stara Gradiška) could be similar to those of P16 but could also have been partially caused by a great dieback of oak in this area, recorded between 1910 and 1925 [88], around the time of the P10 establishment. ...
... Climate extremes, changes in groundwater level, regulation of rivers and pests are putting pressure on the populations, affecting their fitness and regeneration, therefore directly calling the future structure of these populations into question. Pronounced oak dieback has been present in this area throughout the last century and has particularly quickened its pace since the 1980s [88]. The latest extreme event, the supercell storm in July 2023, caused major damage to the most valuable stands in Spačva basin, destroying a number of trees and causing unforeseeable problems with the natural regeneration of these stands. ...
Full-text available
Croatian pedunculate oak (Quercus robur L.) populations represent southern range peripheral populations, often viewed as sources of valuable diversity and drought-resistant ecotypes. At the same time, they endure stronger selection pressures as a result of climate change. The leaves of 20 individuals per population (17) were sampled in a field trial and analyzed using 10 nuclear and 9 chloroplast SSRs to determine the level of intrapopulation genetic variability and genetic differen-tiation. Analysis with nSSRs revealed deviation from HWE in seven populations. AMOVA showed a high intra-population diversity (98.53%) and a small but statistically significant inter-population differentiation. Isolation by distance explained 19.6% of differentiation. Average FST between pop-ulations was low (0.013) compared with usual values for peripheral populations. Populations were rich in cpSSR haplotypes, confirming the hotspot of diversity caused by the encounter of recolo-nization routes. Unbiased haplotype diversity (HE) from 9 chloroplast SSRs and 325 individuals was (HE = 0.440). Sixty-six different haplotypes were grouped in three maternal lineages by both a me-dian-joining network and a neighbor-joining algorithm. AMOVA for cpSSRs showed statistically significant diversity among populations (70.23%), suggesting genetic differentiation, but also a probable anthropogenic effect. AMOVA of nSSRs within and between lineages showed that original recolonization patterns of nuclear diversity were subsequently erased by gene flow.
... Pedunculate oak (Quercus robur L.) is the dominant tree species in many natural and planted European lowland and riparian forests excluding large parts of Spain, Portugal and North Scandinavia (Sergeant et al., 2011). Among the deciduous, ring-porous oak species, pedunculate oak Q. robur is one of the most abundant species in Europe (Haneca et species due to their long history of exploitation for wood production and construction (Mikac et al., 2018). ...
... Climate change, as cumulative challenging factor, is a major driver of drought, and, therefore, influencing Q. robur gene pools in Serbia with dry seasons and shortage of water availability in soil and air (Kostić et al., 2019;2021a, 2021bStojanović et al.,2021). Although a number of studies conducted in the Sava river basin (Medarević et al., 2009;Bauer et al., 2013;Stojanović et al., 2013;Stojnić et al., 2014;Stojanović et al., 2014;Kostić, 2019Kostić, , 2021aKostić, , 2022 have been focused on different causes of oak mortality (attacks of pests and diseases, climate fluctuations, water level change, and inappropriate tending measures), there is still no consensus within the scientific community about the causes of oak dieback in south-east Europe (Stojanović et al., 2015, Mikac et al., 2018. Dependence of this species on soil and water properties plays an uncertain role in its survival and stress response (Kostić et al., 2021c) and, therefore, a focus on ecophysiology of this valuable broadleaved species should be a priority in the research, breeding and forest management (Pilipović et al., 2020). ...
Full-text available
Although being present much widely in terms of its geographical distribution, pedunculate oak ( Quercus robur L.) represents one of the crucial species in forest ecosystems in South Eastern Europe. We wanted to contribute to existing efforts in designing genetic monitoring methodology for this noble broadleaved species, through the screening of genotypes with different health status and belonging to two phenological varieties [early (var. praecox ) vs late (var. tardissima )] using seven oxidative stress parameters (total protein content, total phenolic compounds content, total flavonoids content, DPPH, ABTS, NO radical scavenger activities and Ferric reducing antioxidant power essay - FRAP). Results of this study provided significant biological message, contributing to better understanding of existing biodiversity of pedunculate oak in Serbia. Four out of seven oxidative stress screening tests (total phenolic compounds content, total flavonoids content, RSC NO and FRAP) clearly discriminated late vs early individuals sampled from selected pedunculate oak gene pool. Although presented results did not show significancy in using mentioned parameters to distinct damaged from vital individuals, nevertheless, knowledge gained through discussion and comparison with previous studies contributed to precise determination of biochemical tests which might be used in future efforts of genetic monitoring of this species.
... Moreover, the frequency of extreme climate events such as severe drought and heat waves are expected to increase globally in the future due to the current climate change, thereby challenging the adaptive capacity of urban forest trees (yin et al. 2023). Some indigenous species floodplain forests are particularly sensitive to climate warming and increased drought, as shown by recent oak (Quercus robur) dieback and mortality episodes (Mikac et al. 2018;Pericolo et al. 2023). However, even drought-tolerant tree species like Pinus sylvestris have been recently proven to be affected by prolonged and intensified drought in midsummer and autumn (Hunziker et al. 2022). ...
... Liebl.), while DE-Hai has 28% of ash (Fraxinus excelsior). Some studies attribute higher sensitivity of ash to environmental changes than oak 70,71 , but whether this explains the differences in lagged responses remains unclear and requires further research. Parametric models with splines can account for non-linear responses. ...
Full-text available
Forests play a major role in the global carbon cycle, and droughts have been shown to explain much of the interannual variability in the terrestrial carbon sink capacity. The quantification of drought legacy effects on ecosystem carbon fluxes is a challenging task, and research on the ecosystem scale remains sparse. In this study we investigate the delayed response of an extreme drought event on the carbon cycle in the mixed deciduous forest site 'Hohes Holz' (DE-HoH) located in Central Germany, using the measurements taken between 2015 and 2020. Our analysis demonstrates that the extreme drought and heat event in 2018 had strong legacy effects on the carbon cycle in 2019, but not in 2020. On an annual basis, net ecosystem productivity was [Formula: see text] higher in 2018 ([Formula: see text]) and [Formula: see text] lower in 2019 ([Formula: see text]) compared to pre-drought years ([Formula: see text]). Using spline regression, we show that while current hydrometeorological conditions can explain forest productivity in 2020, they do not fully explain the decrease in productivity in 2019. Including long-term drought information in the statistical model reduces overestimation error of productivity in 2019 by nearly [Formula: see text]. We also found that short-term drought events have positive impacts on the carbon cycle at the beginning of the vegetation season, but negative impacts in later summer, while long-term drought events have generally negative impacts throughout the growing season. Overall, our findings highlight the importance of considering the diverse and complex impacts of extreme events on ecosystem fluxes, including the timing, temporal scale, and magnitude of the events, and the need to use consistent definitions of drought to clearly convey immediate and delayed responses.
... For example, Norway spruce (Picea abies (L.) H. Karst) has a more conservative stomatal regulation strategy relative to European beech (Fagus sylvatica L.) (Klein, 2014). Consequently, moisture stress may initially reduce carbon assimilation to a greater extent in spruce (Mikac et al., 2018;Sanginés de Cárcer et al., 2018), but will also protect the integrity and function of water conducting xylem cells in this species (Tomasella, 2018;Vitasse et al., 2019). In broadleaved species such as beech, the entire photosynthetic apparatus is replaced each spring, making any damage to the recently formed canopy leaves resulting from temperature extremes or water shortages highly detrimental to tree growth. ...
Global change outcomes for forests will be strongly influenced by the demography of juvenile trees. We used data from an extensive network of forest inventory plots in Europe to quantify relationships between climate factors and growth rates in sapling trees for two ecologically dominant species, Norway spruce and European beech. We fitted nonlinear regression models with annual radial growth measurements from ~17,500 trees in primary forests to investigate the sensitivity of individuals to temperature and measures of water supply. We controlled for multiple, potentially confounding factors, including ontogeny, resource competition and the deposition of anthropogenic nitrogen and sulphur. The growth potential of spruce was markedly elevated relative to beech, reflecting species-specific relationships with environmental drivers. Declining water availability more strongly limited productivity in spruce, while beech was notably tolerant of observed levels of moisture limitation. Warming promoted growth in both species, but growing season temperatures that exceeded thermally optimum conditions constrained wood production. We identified long-term positive trends in reconstructed annual rates of juvenile tree growth since the early 19 th century, likely driven by industrial-era warming. However, our findings suggest that sustained warming and more prevalent future drought may ultimately inhibit growth due to thermal thresholds and a differential tolerance of water stress. Consequently, global change factors may be expected to affect future species abundance patterns, biomass production, and the carbon sink capacity of forests in Europe.
Full-text available
Comparative studies of river flows increase our understanding of the environmental effects of rivers on the vegetation along its borders. The riverine areas are particularly sensitive to the hydrological cycle, which is used as a suitable indicator to show environmental changes. Considering that the plant elements of the riparian forests are affected by river fluctuations and climatic variables. The current research focused on to investigate the effect of the Climate factors and river discharge rate on the growth of white poplar (Populus alba), Russian olive (Elaeagnus angustifolia), and tamarisks (Tamarix sp).
Full-text available
Aim. The aim of this study was to conduct an analysis of long‐term changes in the number of sturgeon populations and its relationship with the fishing regime and the current state of the Ural River basin, as well as develop measures for the conservation and restoration of their resources. Discussion. Six sturgeon species inhabit the waters of the Ural River basin: Huso huso Linnaeus, 1758; Acipenser gueldenstaedtii Brandt & Ratzeburg, 1833; Acipenser persicus Borodin, 1897; Acipenser stellatus Pallas, 1771; Acipenser nudiventris Lovetsky, 1828 and Acipenser ruthenus, 1758. The degradation of the geosystems of the Ural River basin and the Caspian Sea due to changes in the hydrological regime, increased exploitation of oil and gas fields, as well as poaching has led to a decrease in the number of sturgeons. Moreover, the areas used for the development of petroleum hydrocarbons and the development of related infrastructure coincide with the main areas for feeding and migration of sturgeon and other commercial fish species. The Ural River is the only one in Europe with unregulated lower and middle reaches, thanks to which, until the 90s of the last century, favourable conditions were maintained for the natural reproduction of sturgeon and other fish species. In world practice, the development of commercial sturgeon breeding and reproduction of livestock in regional natural reservoirs provide a promising direction for the preservation of the unique sturgeon gene pool. At the same time, the principal role in the formation of the number and species composition of sturgeons, is given to natural reproduction. In the Ural River Basin, there are opportunities to restore the sturgeon population through a combination of natural reproduction and artificial farming. Conclusions. Despite a slight increase in the occurrence of sturgeons in recent decades, the state of their populations requires special conservation measures. It is necessary to develop joint measures for the protection and reproduction of fish stocks at the interstate level.
Floodplain forests are sensitive to climate warming and increased drought, as showed by recent oak (Quercus robur) dieback and mortality episodes. However, a comprehensive comparison of coexisting tree species under different climate settings or biomes are lacking. Herein, we compared growth rates, growth responses to climate and drought severity, and modeled climate mediated growth of oak and three coexisting tree species (ash, Fraxinus angustifolia; alder, Alnus glutinosa; elm, Ulmus minor). Two floodplain forests subjected to cooler (temperate climate, Ticino) and warmer (Mediterranean climate, Bosco Pantano) conditions in northern and southern Italy, respectively, were analyzed. Ash seemed to be the most sensitive to drought, particularly at the Mediterranean site where oak and elm growth were also negatively affected by water shortages. Alder appeared to be the least sensitive species in terms of growth variability to drought under both temperate and Mediterranean climate conditions. Furthermore, the growth model revealed the influence of soil moisture in spring and summer on the constrained growth of ash and oak and illustrated how oak growth could be severely reduced during drastic hotter droughts. Alder seemed to be the most drought-resistant species under both environmental conditions. These results could represent the first attempts in documenting the ecological consequences of drought in terms of projected climate trends in less investigated Mediterranean floodplain forests. Furthermore, these results highlight how climate and tree-ring data combined with growth models could be useful tools to detect early warning signals of growth decline and impending dieback in floodplain forests in response to dry spells.
Tree species inhabiting riparian forests under Mediterranean climate have evolved to face summer water shortage but may fail to cope with future increases in drought severity. Thus, understanding tree growth phenological variations in response to environmental conditions is necessary to assess the impact of seasonal drought in riparian forests. In this study, we investigated the response of stem radial growth to climate in the narrow-leaved ash (Fraxinus angustifolia) over its distribution in southern Europe. We simulated intra- and inter-annual growth patterns using the Vaganov-Shashkin (VS) model considering five sites subjected to summer drought but showing different climate conditions. The growth pattern in this species varied from unimodal in cool-wet sites to facultative bimodal in warm-dry sites. Bimodal patterns were characterized by two growth peaks coinciding with favourable climate conditions in spring and autumn. The spring growth peak occurs earlier (May) in warm-dry sites than in wet-cool sites (June to July). The variation in the season growth length and growth timing suggests different strategies adopted by this species to cope with summer drought. The VS model revealed different growth patterns across which would be relevant in predicting the response of this and other riparian tree species to climate warming and aridification. Differences in the length of the growing season, timings of growth peaks and the shift from unimodal to bimodal growth patterns should be considered when assessing growth adjustments to future climate scenarios.
Full-text available
Trees can contribute to the reduction of atmospheric CO2 concentrations, but they do so at varying rates. The carbon sequestration rate (CSR) of trees is influenced by many factors including tree age, forest density, site conditions, and extreme events such as droughts and floods. This research reconstructs past carbon stocks of Pedunculate oak (Quercus robur) trees in floodplains with different hydrological conditions (low and high active floodplain, the seepage water zone of the former floodplain, and the tributary floodplain). We used increment tree cores and allometric equations and compared the annual changes in stocks as a way of assessing CSR. Furthermore, we used time series data on floods and droughts to analyze possible effects of extreme events on CSR. The aims of this study are to (a) compare the CSR of dominant Q. robur in young plantations to dominant trees in old forest stands, (b) compare the CSR of old Q. robur trees under different hydrological conditions, and (c) analyze how drought and flood events influence the CSR of Q. robur in different hydrological conditions. From 2009 to 2018, old Q. robur trees on the low active floodplain had an average CSR of 18.4 ± 1.1 (SE) Kg tree⁻¹ year⁻¹, while young trees had an average CSR of 8.7 ± 0.6 (SE) Kg tree⁻¹ year⁻¹. From 1976 to 2018, the overall CSR of Q. robur was highest on the high active floodplain (18.6 ± 1.7 (SE) Kg tree⁻¹ year⁻¹) and lowest on the seepage water zone of the former floodplain (13.1 ± 1.1 (SE) Kg tree⁻¹ year⁻¹). CSR was higher during flood years in all hydrological conditions, but was significantly reduced by drought only on active floodplains with a comparatively high elevation. Floodplains are well-suited areas for reforestation as natural climate solutions because the dominant trees in these areas have a high CSR even under severe conditions which are predicted to become more common with climate change in the future.
Full-text available
Streamflow observations from near-natural catchments are of paramount importance for detection and attribution studies, evaluation of large-scale model simulations, and assessment of water management, adaptation and policy options. This study investigates streamflow trends in a newly-assembled, consolidated dataset of near-natural streamflow records from 441 small catchments in 15 countries across Europe. The period 1962–2004 provided the best spatial coverage, but analyses were also carried out for longer time periods (with fewer stations), starting in 1932, 1942 and 1952. Trends were calculated by the slopes of the Kendall-Theil robust line for standardized annual and monthly streamflow, as well as for summer low flow and low flow timing. A regionally coherent picture of annual streamflow trends emerged, with negative trends in southern and eastern regions, and generally positive trends elsewhere (especially in northern latitudes). Trends in monthly streamflow for 1962–2004 elucidated potential causes for these changes, as well as other changes observed in hydrological regimes across Europe. Positive trends were found in the winter months in most catchments. A marked shift towards negative trends was observed in April, gradually spreading across Europe to reach a maximum extent in August. Low flows have decreased in most regions where the lowest mean monthly flow occurs in summer, but vary for catchments which have flow minima in winter and secondary low flows in summer. The study largely confirms findings from national and regional scale trend analyses, but clearly adds to these by confirming that these tendencies are part of coherent patterns of change, which cover a much larger region. The broad, continental-scale patterns of change are congruent with the hydrological responses expected from future climatic changes, as projected by climate models. The patterns observed could hence provide a valuable benchmark for a number of different studies and model simulations.
Full-text available
Aim of study: The invasive trees Ailanthus altissima and Robinia pseudoacacia are widely spreading in inner Spain riparian forests, where they co-occur with the natives Fraxinus angustifolia and Ulmus minor. In a climate change context, we aimed to identify some of the species traits that are leading these species to success (Basal Area Increment (BAI) and water-use efficiency (iWUE)). We also aimed to describe the main environmental variables controlling studied species BAI. Area of study: Riparian forests of central Spain Material and Methods: We measured tree-ring width and converted it to basal area increment (BAI); intrinsic water-use efficiency (iWUE) was estimated from tree ring carbon isotopes (δ13C). We compared the BAI and iWUE of the last 20 years between origins (native vs exotic) and among species. For each species, we evaluated iWUE and BAI relationships. Linear mixed-effect models were performed to identify the main environmental variables (temperature, precipitation, river flow) affecting BAI. Main result: Native trees showed higher mean BAI than invaders, mainly due to the rising growth rate of U. minor. Invaders showed higher mean iWUE than natives. We did not find significant correlations between iWUE and BAI in any case. Warm temperatures in autumn positively affected the BAI of the natives, but negatively that of the invaders. Research highlights: The contrasting effect of autumn temperatures on native and invasive species BAI suggests that invaders will be more hampered by the rising temperatures predicted for this century. The higher iWUE found for the invaders did not translate into increased radial growth, suggesting that drought stress may have prevented them of taking advantage of increased atmospheric CO2 for a faster growth. These findings point out that neither climate change nor rising CO2 seem to enhance the success of study invasive species over the natives in riparian forests of central Spain. Furthermore, the low BAI of R. pseudoacacia, and its climate-growth model suggest that climate change may especially hamper the success of this invader.
Full-text available
Background and Purpose: Narrow-leaved ash (Fraxinus angustifolia Vahl) is one of the most important tree species of lowland floodplain forests in the Republic of Croatia. Recent significant climate changes, reflected in extreme temperatures and long dry periods, have had a direct impact on the dynamics and levels of groundwater, as well as on the dynamics of precipitation and floodwater, which are of decisive importance for the success of narrow-leaved ash. Our study aims to determine the morphological and physiological characteristics of seedlings of narrow-leaved ash after 72 days of drought, normal watering and substrate saturation. Materials and Methods: The experiment was established in three treatments (drought, normal watering and saturation), and each treatment included two containers or 24 seedlings. A total of 72 seedlings were replanted. After 72 days, height growth and increment, water potential, photosynthetic efficiency and the total chlorophyll content in the leaf mesophyll were measured. Results: Seedlings from the drought treatment wilted immediately. Seedlings saturated with water up to the root collar manifested better height and diameter growth, high water potential and a lesser physiological stress response as compared to the seedlings that were watered normally. Conclusions: Seedlings of narrow-leaved ash, which is a hygrophytic tree species, do not tolerate dry conditions and no watering. Seedlings saturated with water for 72 days showed a better morphological-physiological status than the seedlings that received normal quantities of water.
Tree-ring information and climate response data were applied to investigate the potential of the Carpathian Mountains to influence tree-growth patterns. Recent studies reveal the importance of constructing a dense spatial network of oak tree-ring chronologies in this area, which may be the key to linking the North Central European and East Mediterranean tree records. We establish sixteen oak (Quercus robur L.) and sessile oak (Quercus petraea (Matt.) Liebl.) site chronologies along a longitudinal gradient (from 22.47 to 26.58 E) in Northern Romania in an attempt to elucidate the impact of climate on oak growth. Even with differences generated by interspecific features, habitats and climatic regimes, a common macroclimatic marker for the NW and NE sites was established by comparing two groups of chronologies separated by the Carpathian chain. We found that precipitation in April (P4) and June (P6) were the primary climate factors that affected tree growth in the NW region. For the NE region, the temperature in January (T1) and March (T3) and precipitation in May (P5) were revealed to be the major limiting climatic factors. The spatial variability of the correlation coefficients indicates a decreasing trend in correlation intensity with precipitation from NW to NE, particularly during the current growing season (March–July). Oak trees from the NW and NE regions have adapted to different local climatic conditions and only respond uniformly to severe climate events (e.g., the 1904 drought). The higher occurrence of extreme years during the 20th century, particularly in the NE region, was in accordance with the rise of precipitation variability in the current growing season. The changes in the tree-growth pattern and climatic response of the chronologies of the studied sites in the NW and NE regions were linked to the local climates induced by the Carpathian Mountains.
Pedunculate oak (Quercus robur L.) is a long-lived species that dominates the extra–zonal natural forests in the steppe landscape of southeastern Ukraine. Although Q. robur is considered to be one of the most important species in European dendrochronology, it has received little attention in the steppe zone because of its scarcity in the often-degraded steppe forests. Nevertheless, a small and unique patch of old-growth oak exists within the boundary of Donetsk, a large industrial center in Eastern Europe. This forest is a remnant of an ancient wood and includes several dozen old-age trees that can contribute to filling some of the spatial gaps in pedunculate oak dendrochronology in Eastern Europe. In this study, we aim to determine the effect of climatic variables on pedunculate oak growth in the steppe zone, and to estimate the longevity of this species in the heterogeneous conditions of an urban forest. A total of 20 trees were cored for this study, varying in age from 55 to 254. The resulting tree-ring chronology correlates strongly with local precipitation in spring and summer, and with local temperature in April, June and July. Moving correlation analysis indicates a shift over the last 80 years in the relationship between oak growth and late winter and early spring temperatures, as well as between oak growth and precipitation in February and August. These findings imply that warming has caused both an advance in oak phenology and changes in the climatic conditions in early spring.
Iberian temperate forests are distributed along the boundary between the Atlantic and the Mediterranean biogeographical regions, and represent the south-western range edges of diverse European broadleaved deciduous tree species. Trees growing at the boundary between Atlantic and Mediterranean biomes suffer from different stresses, including increasing moisture deficit which has been identified as one of the main limitations for growth. In this work, dendrochronological techniques were employed to characterize the radial growth of Acer campestre L., Fagus sylvatica L., Fraxinus excelsior L., and Quercus robur L. in a mixed forest in northern Spain, and examine its relationships with local climate near their south-western range edges. Acer and Fagus tree-ring chronologies showed the highest common signal and the strongest responses to climate. Positive effects of precipitation, especially in the previous December and current summer, were relevant for growth of all species. Only Acer growth showed a detrimental effect of maximum diurnal temperatures in the previous autumn and current summer, while Fraxinus and Quercus growth was benefited by above-average winter temperatures. Cloud cover strongly improved the radial growth of all species, probably because cloudy conditions mitigate the detrimental effects of summer water depletion and low winter temperatures. The beneficial effects of precipitation and cloudiness on tree growth were temporally unstable and have become significant generally since the 1970s, suggesting that rising temperatures and decreasing rainfall shape radial growth-climate relationships of broadleaved deciduous trees near their southern range edges.
Increasing temperatures and recent changes in runoff regimes observed in Central Europe might alter the growth and relative water uptake of floodplain trees. To predict responses of floodplain forests to climate change, it is necessary to determine the climatic controls over tree growth and vessel anatomy. We analysed the responses of tree-ring width and earlywood vessel anatomical parameters (average vessel lumen area, vessel density and total vessel lumen area) of pedunculate oak (Quercus robur L.) growing in a floodplain to hydroclimatic conditions represented by temperature, the drought index (scPDSI), river discharge, groundwater level, and occurrence of floods and drought events. Site chronologies were assembled for floodplain and reference sites and, subsequently, correlated with time series of hydroclimatic conditions. Our results show that radial growth of floodplain trees is particularly positively influenced by temperature during the growing season and during previous year’s summer. By contrast, the growth of reference trees is highly drought-limited. Earlywood average vessel lumen area chronologies from both floodplain and reference sites share a positive temperature signal from January to April. However, the effect of water availability (indicated by the drought index) on vessel size is mostly negative for floodplain trees (with a maximum response to the autumn of the year preceding tree-ring formation) and positive or non-significant for reference trees. Vessel density chronologies contain the inverse environmental information as tree-ring width, however, with amplified negative correlations with current year temperatures at floodplain sites. Total vessel area is associated mostly with temperature in previous May and June. The drought index recorded exactly the same information in tree-rings as did river discharges and groundwater levels. The results of both correlation and trend analysis evidence that tree-ring width of floodplain Q. robur unambiguously increases with increasing temperature; on the other hand, droughts can become a serious problem affecting the productivity of reference trees growing in more distal parts of the lowland. Vessel size of Q. robur growing outside the floodplain recently tends to increase with increasing temperatures, making xylem more effective at water transport but also more vulnerable to cavitation.
Quantitative descriptions of natural disturbance regimes are lacking for temperate forest regions in Europe, primarily because a long history of intensive land-use has been the overriding driver of forest structure and composition across the region. The following contribution is the first attempt to comprehensively describe the natural disturbance regime of the dominant forest communities in the Dinaric Mountain range, with an emphasis on the range of natural variability of regime components for the main disturbance agents. Compared to other forest regions in Europe, the mountain range has a history of less intensive forest exploitation and provides a suitable record of natural disturbance processes. Our synthesis is based on multiple types of evidence, including meteorological information, historical documentation, evidence from old-growth remnants, and salvage logging data from National forest inventories. Taken together, the results show that no single disturbance agent dominates the regime in the dominant forest types (i.e. beech and mixed beech-fir forests), and any given agent exhibits remarkable variation in terms of severity and spatial extent both within and among individual disturbance events. Thunderstorm winds cause the most severe damage (i.e. near stand replacement), but blowdown patches are typically limited to stand-scales (e.g. 10s of ha). Ice storms and heavy snow typically cause intermediate severity damage and affect much larger areas (e.g. 100s of km2). A notable exception was the 2014 ice storm, which was nearly an order of magnitude larger and more severe than any other event recorded in the synthesis. Severe and prolonged periods of drought have occurred several times over the past century, and along with secondary insect damage (e.g. bark beetles), have caused episodes of forest decline. Overall, our synthesis indicates that on top of the background of relatively continuous gap dynamics, stand-scale intermediate severity events are an important part of the regime; these events likely have rotation periods that are less than the lifespan of a tree cohort (e.g. several centuries) and create canopy openings large enough to alter successional trajectories.