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

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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.
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SCientiFiC REPORTS | (2018) 8:16495 | DOI:10.1038/s41598-018-34875-w
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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: smikac@sumfak.hr)
Received: 25 July 2018
Accepted: 28 October 2018
Published: xx xx xxxx
OPEN
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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
forests.
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 (http://mapire.eu/en/).
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 (http://climexp.knmi.nl). For
the long-term climate correlations (>100 years), analysis was done using data (mean monthly temperature and
monthly precipitation sums) from the HISTALP database (http://www.zamg.ac.at/histalp/). 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
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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 (http://meteo.hr/). 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 (http://www.rinntech.de). 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).
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the climate origin of variability in the chronologies through avoidance of intercorrelations between climatic
predictors26.
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
6
8
10
12
Temperature (°C)
50
100
150
Precipitation (mm)
0 500 1000250 Km.
Legend
Sava River
Sava Catchment
R0.70
-0.58
(b)
0 500 1000250 Km.
(a)
Legend
Sava River
Sava Catchment
R0.29
-0.83
−5
0
5
scPDSI
Years
1920 1940 1960 1980 2000
0
200
400
600
River Water Level (cm)
Max.
Min.
86.82 m a.s.l.
−1 −0.8 −0.6 −0.4 −0.20 0.20.4 0.60.8
1
JANt
FEBt
MARt
APRt
MAYt
JUNt
JULt
AUGt
SEPt
OCTt
NOVt
DECt
JANt
FEBt
MARt
APRt
MAYt
JUNt
JULt
AUGt
SEPt
OCTt
NOVt
DECt
JANp
FEBp
MARp
APRp
MAYp
JUNp
JULp
AUGp
SEPp
OCTp
NOVp
DECp
I
II
III
IV
V
VI
VII
VIII
IX
X
XI
XII
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
(c)
Correlation
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).
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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
(Fig.5e).
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).
Results
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.
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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.
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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).
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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.
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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.
Discussion
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).
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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.
Conclusions
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.
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11
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.
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Acknowledgements
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
translation.
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 https://doi.org/10.1038/s41598-018-34875-w.
Competing Interests: e authors declare no competing interests.
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... The floodplain forests of Central Europe have been severely affected by the accelerating global climate change (GCC) (IPCC, 2021;Schnabel et al., 2021) and human-induced changes in the hydrological regime (Skiadaresis et al., 2019). The longer and more intense drought events caused by increased temperatures and changing precipitation patterns (IPCC, 2021) could be detrimental, especially to water-sensitive ecosystems such as floodplain forests (Cunningham et al., 2011;Capon et al., 2013), which are highly vulnerable to groundwater table declines (Čater, 2015;Stojanović et al., 2015;Mikac et al., 2018;Skiadaresis et al., 2019;Š enfeldr et al., 2021). ...
... Correct estimation of species-specific drought sensitivity under GCC scenarios and groundwater table depletion in floodplain forests is necessary for decision-making processes focused on the mitigation and prevention of these negative effects. The current assessment of floodplain forests' sensitivity was primarily based on tree-ring studies investigating the relationship between monthly climate data and annual ring widths (Stojanović et al., 2015;Mikac et al., 2018;Skiadaresis et al., 2019;Š enfeldr et al., 2021). High-resolution data was used to assess the impact of climate on tree growth (Ježík et al., 2015;Brinkmann et al., 2016;Van Der Maaten et al., 2018) to capture cambial activity that occurred on time scales of hours to days (Michelot et al., 2012). ...
... Unfortunately, these measures have also lowered the groundwater table (Kovář et al., 2016), resulting in much drier conditions, altered succession -change in tree species composition and a drastic reduction in ecosystem productivity (Janik et al., 2008). This combination of climate change and groundwater table decline has contributed to a decline in the population of oaks and ashes in the floodplain forests of the South Moravian Region (Šenfeldr et al., 2021), and throughout Europe (Okoński, 2017;Mikac et al., 2018;Roibu et al., 2020). The current climatic conditions are more suitable for mesohydric species such as hornbeam or field maple (Janik et al., 2008;Mikac et al., 2018). ...
Article
Recent changes in the floodplain forests of Central Europe, caused mainly by changes in hydrological management and the increased frequency of droughts due to climate change, have led to severe degradation of floodplain ecosystems. Our main objective was to determine the sensitivity of trees to drought by observing the response of the tree phenology, stem radial growth, and physiology (sap flow) of three predominant tree species, namely English oak, narrow-leaved ash, and common hornbeam, to the environmental variables (climate). Stem radial growth began before bud break in ring-porous oak and ash, whereas in diffuse-porous hornbeam, growth onset occurred after leaf formation. The early onset with intense growth during favorable months (April–May) observed in ring-porous species was a major prerequisite for the successful growth of oak and ash at this site. Tree water deficit (TWD), an indicator of stem hydration, was triggered by decreasing soil moisture in all species, and was most prominent in ash, followed by oak. Intriguingly, sap flow was decoupled from TWD in all species and was driven primarily by evaporative demand from the atmosphere. Oak was the least conservative in regulating sap flow under atmospheric drought followed by hornbeam, whereas ash was most restricted and reduced its transpiration during dry periods. In contrast, ash was characterized by the highest radial growth and growth-based water-use efficiency. The lower water storage capacity of oak and ash is likely compensated by deep rooting and drought avoidance strategies, respectively. Tree species that tend to use surface soil water could be severely limited by more extractive species such as hornbeam. Despite the contrasting leaf and wood phenology, stomatal control, and rooting depth among the studied floodplain tree species, they exhibited analogous sap flow and water storage dynamics responses to drier conditions that enabled them to co-exist in the South Moravian Region. Nevertheless, our results suggest that the severe droughts and human-induced alterations in groundwater pose serious threats to floodplain forests in Central Europe, with certain tree species being unable to adapt to these altered conditions.
... The high tree species richness of floodplain forests (Ward et al., 1999) makes them ideally suited for comparative studies of tree species reactions to consecutive droughts as they are one of the few systems where coexisting mature trees spanning an entire gradient of hydraulic behaviours can be found. Floodplain forests rank among the most rapidly disappearing ecosystems due to land conversion and drainage (Leuschner & Ellenberg, 2017;Mikac et al., 2018) and novel climatic conditions-like prolonged droughts-may amplify this trend through changing the hydrological regimes on which these forests depend. For instance, sinking groundwater levels may increase tree growth sensitivity to drought and susceptibility to drought-induced dieback (Mikac et al., 2018;Skiadaresis et al., 2019) and this might bring these forests, which are among the most dynamic, productive and diverse Central European habitats (Kowalska et al., 2020;Tockner & Stanford, 2002), closer to a tipping point. ...
... Floodplain forests rank among the most rapidly disappearing ecosystems due to land conversion and drainage (Leuschner & Ellenberg, 2017;Mikac et al., 2018) and novel climatic conditions-like prolonged droughts-may amplify this trend through changing the hydrological regimes on which these forests depend. For instance, sinking groundwater levels may increase tree growth sensitivity to drought and susceptibility to drought-induced dieback (Mikac et al., 2018;Skiadaresis et al., 2019) and this might bring these forests, which are among the most dynamic, productive and diverse Central European habitats (Kowalska et al., 2020;Tockner & Stanford, 2002), closer to a tipping point. ...
Article
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Droughts increasingly threaten the world's forests and their potential to mitigate climate change. In 2018–2019, Central European forests were hit by two consecutive hotter drought years, an unprecedented phenomenon that is likely to occur more frequently with climate change. Here, we examine tree growth and physiological stress responses (increase in carbon isotope composition; Δδ13C) to this consecutive drought based on tree rings of dominant tree species in a Central European floodplain forest. Tree growth was not reduced for most species in 2018, indicating that water supply in floodplain forests can partly buffer meteorological water deficits. Drought stress responses in 2018 were comparable to former single drought years but the hotter drought in 2018 induced drought legacies in tree growth while former droughts did not. We observed strong decreases in tree growth and increases in Δδ13C across all tree species in 2019, which are likely driven by the cumulative stress both consecutive hotter droughts exerted. Our results show that consecutive hotter droughts pose a novel threat to forests under climate change, even in forest ecosystems with comparably high levels of water supply.
... While Quercus robur grows at diverse site conditions, with an associated better adaptation to lowering water availability, Fraxinus angustifolia grows predominantly at sites with easily available water [37]. Studies focused specifically on Fraxinus angustifolia growth response, as well as in a comparison with Q. robur, are rare and originated predominantly from primeval forests in Southeastern Europe [38]. ...
... Our results indicate obvious differences between FRAN and QURO, as we detected higher sensitivity of FRAN to spring precipitation, spring, and summer SPEI and groundwater level, with the difference in sensitivity between the species being greatest for groundwater level (Figure 3). This species-specific climate sensitivity is consistent with similar studies from Southeastern Europe [38]. Our initial hypothesis (H1) that FRAN would show higher climate sensitivity was thus supported. ...
Article
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European floodplain forest is facing increasingly frequent and severe drought events related with ongoing climate change. Moreover, this ecosystem type was frequently affected by river regulation, leading to groundwater table lowering; however, river revitalization has, in some locations, achieved some restoration of groundwater levels. In this study, we investigated the growth–climate sensitivity and growth modulation after groundwater-level manipulation for Fraxinus angustifolia Vahl. and Quercus robur L. in one of the most important floodplain forest complexes in Central Europe. We constructed three different types of tree ring chronologies to reflect the high frequency variability, medium-low frequency variability, and basal area increment. We found F. angustifolia to be more sensitive than Q. robur to both drought and groundwater level fluctuations. Moreover, F. angustifolia showed more pronounced short-term and long-term growth decreases after artificial ground water level alteration than did Q. robur. We also found that the groundwater level increase due to river revitalization reduced the climate sensitivity for both F. angustifolia and Q. robur. The decrease in climate sensitivity associated with revitalization was more pronounced for F. angustifolia which, moreover, showed a greater basal growth after river revitalization. Our results suggest that F. angustifolia will be more threatened than Q. robur by the diminution in groundwater availability and increase in drought with ongoing climate change. They also show that river revitalization can be a suitable management tool to help the adaptation to climate change.
... On some wetlands, the decomposition is very slow, and when plant productivity exceeds it, carbon accumulates [6]. In the course of ongoing climate change with more often occurring extreme events, the role of floodplain forests as carbon stores and their role in nutrient cycles to mitigate climate change has become even more crucial [7][8][9][10]. Changing temperature and precipitation regimes due to changing climatic conditions can shift the balance between the rate of R eco and GPP, which can cause wetlands to become a carbon source [11,12]. ...
Article
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Environmental conditions influence forest ecosystems and consequently, its productivity. Thus, the quantification of forest CO2 exchange is a critical requirement to estimate the CO2 balance of forests on a local and regional scale. Besides interpreting the annual CO2 exchange corresponding to environmental conditions over the studied years (2015–2020) at the floodplain forest in Lanžhot, Czech Republic (48.6815483 N, 16.9463317 E), the influence of below-above canopy air mass decoupling on above canopy derived CO2 exchange is the focus of this study. For this purpose, we applied the eddy covariance (EC) method above and below the forest canopy, assessing different single- and two-level flux filtering strategies. We focused on one example year (2019) of concurrent below and above canopy EC measurements. We hypothesized that conventional single-level EC flux filtering strategies such as the friction velocity (u*) filtering approach might not be sufficient to fully capture the forest CO2 exchange at the studied ecosystem. Results suggest that decoupling occurs regularly, but the implication on the above canopy derived EC CO2 fluxes appears to be negligible on an annual scale. We attribute this to the open canopy and flat EC tower surrounding terrain which inhibits horizontal removal of below-canopy respired CO2.
... To our knowledge, xylem conductivity loss of U. laevis has not yet been studied in situ and, hence, a direct comparison with Q. robur regarding vulnerability to cavitation is not possible. However, a strong drought adaptation ability of Q. robur was reported by Mikac et al. (2018), while Venturas et al. (2013) in their studies on elm seedlings, characterize U. laevis to be highly vulnerable to cavitation. This might indicate what still needs to be evidenced by further studies: root distribution patterns are pivotal for fine root mortality, consequently, for cavitation risk and, thus, for survival of trees under progressive drought incidences. ...
Article
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Key message Extremely high fine root mortalities were observed under drought in 2018, increasing fine root mortality in young trees could be explained by differences in root distribution compared to old trees. Abstract Juvenile trees in floodplains are prone to high mortality leading to a low success in reforestation in these habitats. One of the reasons contributing to high mortality of juvenile trees could be limited water and nutrient uptake due to a high level of fine root mortality, especially during summer droughts on higher terraces of floodplains. Strategies of different tree species in hardwood floodplain forests (HFFs) on keeping fine roots alive are still poorly understood. During the record-breaking summer drought of 2018, we examined the relationship between tree age and fine root dynamics of Quercus robur and Ulmus laevis along the Elbe River in Germany. Root area index (RAI), live root density (LRD) and relative root mortality (RRM) of young and old Q. robur and U. laevis were analysed by taking soil cores three times during the progression of a summer drought. Old oaks had a lower RAI in the upper soil than young oaks, while RAI of elms did not differ between young and old trees. RRM was very high during summer reaching more than 100% on average. RRM of young trees of both species increased with increasing drought during summer, while RRM of old trees did not change. We argue that differences in the response of RAI between oaks and elms reflect the sink competition between growing deep roots and shallow roots, which is pronounced in oaks due to their characteristic tap root system. Differences in root distribution patterns and the ability to perform hydraulic redistribution may explain the differences in RRM between trees of different species and ages.
... It is estimated that about 40% of industrial losses resulting from natural disasters have been caused by floods (Poprawa and Rączkowski, 2003). High variability and decreasing stability of river flow regimes, and thus the growing intensity of extreme hydrological events, increases the impact of floods as an ecological stressor for forests (Arnell, 1994;Hannah et al., 2006;Mikac et al., 2018;Stahl et al., 2010). Over the last 30 years there have been numerous flood events in valleys of medium and large rivers in Poland. ...
Article
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Climate change observed in recent years has led to a rise in the frequency and magnitude of meteorological phenomena such as violent storms, intense rainfall or heat events, which have triggered floods, mass land movement, droughts, and wildfire. The intensification of severe weather conditions has put a strain on forest stand regions in Europe. Weather-related stressors are considered the leading factor in the degradation of forest stands and the growing susceptibility of trees to pathogenic agents. The new dynamic situation has substantially altered the external conditions of forest management in Poland. Long-established management methods have had to be revised, and new approaches developed to face the emerging or aggravated threats to forests. It has become apparent in many cases that sole silvicultural techniques will not be sufficiently effective and should be supported or supplemented by technical solutions to respond adequately. We discuss our research and findings in the domain of engineering solutions applied in forests to optimise the road network, limit the effects of natural hazards, implement hydrological restoration, alter water storage and assess the impact of engineering infrastructure on the environment. The article presents the management of natural hazards and adopted engineering approaches from a Polish perspective. Case studies of applied solutions to prevent and limit the damaging effects of natural hazards and disasters on forests are included. Climate change observed in recent years has led to a rise in the frequency and magnitude of meteorological phenomena such as violent storms, intense rainfall or extreme heat events, which have triggered floods, mass land movements, droughts and wildfires. The intensification of severe weather conditions has put a strain on forest stands in many regions of Europe. Weather-related stressors are considered a leading factor in the degradation of forest stands and the growing susceptibility of trees to pathogenic agents. The new dynamic situation has substantially altered conditions for forest management in Poland. Long-established management methods have had to be revised, while new approaches need to be developed to face the emerging or aggravated threats to forests. It has become apparent in many cases that silvicultural techniques alone will not be sufficiently effective and thus should be supported or supplemented by technical solutions to respond adequately to the altered conditions. This study and the obtained findings are discussed in terms of engineering solutions applied in forests to optimise the road network, limit the effects of natural hazards, implement hydrological restoration and enhance water storage. The article presents management of natural hazards and adopted engineering approaches from a Polish perspective. Case studies of applied solutions to prevent and limit the damaging effects of natural hazards and disasters on forests are included.
... The increased FAI and EQ values shown in Figures 2 and 3 indicate that parts of Serbia, especially the southern and southeastern parts, are likely to be severely impacted by climate change. However, local and international studies indicate that the reactions of forests to climate change are likely to differ depending on the overall ecological needs and provenance characteristics of their species (Stojanović et al. 2015a, Mikac et al. 2018, Horváth and Mátyás 2016, Matović et al. 2018, Rybníček et al. 2012, Čermák et al. 2017, Castagneri et al. 2015, Tikvić et al. 2008, Jović et al. 2018, Gavrilov et al. 2019, Horák et al. 2014, Stjepanović et al. 2017, Carrer et al. 2012, Spathelf et al. 2013, Führer et al. 2016, Isaac-Renton et al. 2018, George et al. 2015, Taeger et al. 2013, Arend et al. 2011, Robson et al. 2012, Rose et al. 2009, Matías et al. 2016). This should be kept in mind because increasing aridity levels do not explicitly mean the disappearance of species. ...
Article
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The survival of forest tree species within certain areas of their distributions is in question due to the increasing occurrence of disturbances and degradation processes in forest ecosystems due to climate change. The aim of this paper is to predict changes in the spatial distribution of the ten most important tree species in Serbia (European beech, Pedunculate oak, Austrian oak, Hungarian oak, Sessile oak, Narrow-leafed ash, Silver fir, Norway spruce, Black and Scots pine) using climate indices (Forestry Aridity Index, FAI and Ellenberg Quotient, EQ) with up-to-date climate observations (E-OBS, covering the time period 1990-2019) and projections of future climate conditions (RCP 4.5 and RCP 8.5 scenarios, split into two time periods 2041-2070, 2071-2100). The computation of the area under the receiver operating characteristic (ROC) curves has shown that both FAI and EQ have a “fair” to “excellent” ability to predict the occurrence of five out of ten species (European beech, Silver fir, Norway spruce, Black and Scots pine), with EQ having a slightly better predicting ability. EQ-based projections from mid (2041-2070) to late 21st century (2071-2100) under RCP 4.5 predict that reduction rates will not exceed 25%. Similar reduction rates are given by FAI-based projections until 2070, while rates increase to approximately 35% towards the end of this century. For RCP 8.5, FAI-based projections are significantly worse than EQ-based projections. Irrespective of the used index, projections until 2070 suggest that 55-75% of existing habitats will remain intact. Towards the end of the century, however, our analysis indicate that 75-85% (EQ) to 90-100% (FAI) of the five analyzed species habitats, will be found outside of their current climate space.
... Within the lifecycle of trees, the seedling stage is often the most vulnerable period (Kitajima and Fenner 2000). Numerous factors influence seedling emergence, establishment, growth, and survival with the potential of shaping tree species composition long-term (Jones et al. 1989;Mikac et al. 2018). Different adaptations among floodplain tree species enhance seedling survival in floodplain systems; these adaptations reflect species' distinctive regeneration niches and affect the chance of a new individual replacing a mature member of its own species, i.e., regenerating (Grubb 1977). ...
Article
The regeneration process is a sensitive period within life cycles of floodplain tree species and can strongly influence forest community composition. Yet, fundamental information remains limited on the relationship between regeneration processes and the flood disturbances that, together, construct floodplain forest landscapes. In a controlled greenhouse experiment we tested the effects of complete submergence on six temperate floodplain forest species to understand how flood timing and duration influence seedling survival. Groups of overcup oak (Quercus lyrata), Nuttall oak (Quercus texana), willow oak (Quercus phellos), sugarberry (Celtis laevigata), green ash (Fraxinus pennsylvanica), and American elm (Ulmus americana) seedlings were submerged for either 5, 15, 25, or 0 days (control) at the ages of 3-weeks, 6-weeks, and 9-weeks post-emergence. All species demonstrated a higher sensitivity to flooding at age 3-weeks compared to 6- and 9- weeks, indicating substantial changes in seedling resilience within the first months following emergence. Additionally, the heavier-seeded Q. lyrata, Q. texana, and Q. phellos were less or equally vulnerable to flooding compared to the lighter-seeded C. laevigata, F. pennsylvanica, and U. americana across all age groups, especially at 3-weeks post-emergence. The results of this study have implications for understanding woody species regeneration ecology and changes in floodplain forest composition, particularly in the context of hydrologic modifications.
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SAŽETAK Klimu kao prosječno stanje vremena određuju srednje vrijednosti, ali i kolebanja klimatskih elemenata. S obzirom na to da je klima najvažniji stanišni čimbenik, nužno ju je motriti, opisivati, analizirati i pratiti njezine promjene. Prikupljeni su podaci o vrijednostima temperature zraka (°C), količine oborine (mm) i relativne vlažnosti zraka (%) za meteorološke postaje Grič, u središtu grada, i Maksimir, u suburbanom dijelu grada. Na osnovi proma-tranih i mjerenih klimatskih elemenata izračunati su različiti klimatski indeksi koji pobliže opisuju klimu grada Zagreba. Analiziran je i trend Palmerova indeksa oštrine suhoće (scPDSI). Temperatura zraka (°C), količina ob-orine (mm), kao i iznos potencijalne evapotranspiracije (mm) imali su znatno više vrijednosti u središtu grada u odnosu na suburbani dio grada. Suburbani dio grada humidniji je u odnosu na središte grada. Trend Palmerova indeksa oštrine suhoće za travanj, srpanj i kolovoz upućuje na povećanje aridnosti na području Zagreba.
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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.
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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.
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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.
Article
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.
Article
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.
Article
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.
Article
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.
Article
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.