Changes in Mercury Deposition in a Mining and SmeltingRegion as Recorded in Tree Rings

Maria Hojdová & Tomáš Navrátil & Jan Rohovec &

Karel Žák & Aleš Vaněk & Vladislav Chrastný &

Radek Bače & Miroslav Svoboda

Received: 16 March 2010 /Accepted: 3 June 2010 /Published online: 17 June 2010# Springer Science+Business Media B.V. 2010

Abstract Metal mining and processing in the centralCzech Republic has led to the contamination ofsurrounding soils and vegetation. In this study, Hgconcentrations were measured in spruce (Picea abiesL.) and beech (Fagus sylvatica L.) tree rings to monitorhistorical Hg deposition in the area. The highest Hgconcentrations were found in spruce at an HgSsmelting contaminated site (up to 15 ng g−1), probablyreflecting smelting activities at the end of the

nineteenth century. In the vicinity of a Pb smelter, Hgconcentrations increased from the 1950s to maxima(up to 8.4 ng g−1) in the 1970s, corresponding with apeak of metallurgical production and smelter emissionsin the mid 1970s. A decreasing trend in Hg concen-trations since the 1980s was probably related toimprovements in flue gas cleaning technologies. Thebeech trees, which grow at a site between two smeltersand range in age from 150 to 220 years, seem to reflectdeposition from both point sources. Mercury levels inbeech trees were lower, that resulting from their greaterdistance from pollution sources, but the concentrationtrend was strongly correlated with metal production.Analysis of nutrient elements (Ca, Mg, K and Mn) inwood revealed environmental changes related to aciddeposition, but a relation between concentration trendsof nutrients and Hg was not found. This study showsthat tree rings may be a good record of Hg depositionin areas affected by ore mining and smelting. Never-theless, further investigation of Hg cycling in trees isnecessary to satisfactorily interpret this particularhistorical Hg record.

Keywords Mercury . Dendrochemistry . Trees .

Geochemical archives . Czech Republic

1 Introduction

The behaviour and fate of mercury in the environmentis currently receiving increasing attention because of

Water Air Soil Pollut (2011) 216:73–82DOI 10.1007/s11270-010-0515-9

M. Hojdová (*) : T. Navrátil : J. Rohovec :K. ŽákInstitute of Geology, Academy of Sciences,Rozvojová 269,165 00 Prague 6, Czech Republice-mail: hojdova@gli.cas.cz

A. VaněkDepartment of Soil Science and Soil Protection,Czech University of Life Sciences Prague,Kamýcká 129, 165 21, Prague 6, Czech Republic

V. ChrastnýCzech Geological Survey,Geologická 6,152 00 Prague 5, Czech Republic

V. ChrastnýFaculty of Science, University of South Bohemia,Branišovská 31,370 05 České Budějovice, Czech Republic

R. Bače :M. SvobodaFaculty of Forestry and Wood Sciences,Czech University of Life Sciences Prague,Kamýcká 1176,165 21 Prague 6, Czech Republic

growing evidence of the potential threat Hg poses toecosystems and human health. Therefore, being ableto reconstruct the history of Hg inputs at sites thathave been contaminated by anthropogenic sourceswould be very valuable. Ice, peat and sediment coreshave often been used to reconstruct historical changesin atmospheric deposition. Tree rings have also beenused as geochemical archives in a number of studies ondifferent tree species and various elements (Watmoughand Hutchinson 2003; Bindler et al. 2004; Cheng et al.2007; etc.), though the use of dendrochemical techni-ques has had varying success (e.g. Eklund 1995;Hagemeyer 1995; Watmough and Hutchinson 2002).Factors influencing the applicability of dendrochemicalanalyses include differences in: (1) tree species andphysiology (in particular physiological differencesbetween sapwood and hardwood), (2) the metal ofconcern, (3) deposition levels and (4) soil chemistry(Cutter and Guyette 1993; Watmough 1999).

Studies on past Hg pollution recorded in tree ringsare rather rare. One pioneer study focused on acomparison of the temporal and spatial evolution ofHg concentrations in tree rings from sites differing inthe degree of Hg soil contamination (Zhang et al.1995). That study proved that Hg in tree rings wasdeposited from the atmosphere onto the tree surface—there was no evidence of Hg uptake from soil. Solarradiation, temperature and geographic latitude seemedto affect Hg uptake. More recently, tree rings andlichens were used by Becnel et al. (2004) to monitorHg pollution in an industrial area. A strong correla-tion was found between Hg in tree cores and lichens,suggesting a similar mechanism of uptake from theatmosphere.

To date, no studies have reported a Hg time trendin tree rings at sites historically contaminated byanthropogenic sources whilst taking into account theproduction of the point sources. In this study,concentrations of Hg and nutrient elements (Ca, Mg,K and Mn) were measured in tree rings of Norwayspruce (Picea abies L.) and European beach (Fagussylvatica L.) sampled in an area contaminated bymining and smelting. The aim of this work was to: (1)test the applicability of dendrochemical analysis toexamine historical Hg deposition by comparing Hgin tree rings with available data on metal produc-tion and (2) assess the record of historical Hgdeposition in the central Czech Republic, a region

known for its history of ore mining and smeltingsince medieval times.

2 Experimental

2.1 Site Description

Tree ring samples fromNorway spruce (P. abies L.) andEuropean beech (F. sylvatica L.) were collected fromthree sites in the central Czech Republic (CR; Fig. 1).

The first sampling site was located in the closevicinity of the Příbram smelter (Fig. 1), which hasbeen in operation for over 220 years. Historically, thesmelter processed Pb–Ag–Zn±Sb ores mined in thearea; since the cessation of mining in 1972, the plantrecycles Pb from scrap materials, mainly car batteries(Vurm 2001). Although Hg-bearing minerals were notcommon in the ores processed in the Příbram oreregion, trace Hg admixtures have been detected ingalena (PbS) and sphalerite (ZnS) that were histori-cally mined and processed at Příbram (Bambas 1990).Ores from other areas of the CR were smelted inPříbram during the twentieth century as well (Bambas1990), but the Hg content in these imported ores isnot known. Historical metal production from thePříbram ore region and changes in smelter emissionsare shown in Fig. 2. Mercury contamination in thePříbram region has been documented by severalstudies (e.g. Rieuwerts and Farago 1996; Ettler et al.2007), but precise information on Hg emissionsfrom the smelter remains unknown. Dendrologicalcores were collected from three spruce trees (PB1,PB2 and PB3) located approximately 0.5 km (PB1and PB2) and 2.5 km (PB3) from the smelter stack(Fig. 1).

The second sampling site is aptly named JedováHora (translated in English to “poison mountain”).Cinnabar (HgS), as well as native Hg to some extent,was mined at the site from the eighteenth century until1870 (Sattran et al. 1978), mostly as a by-product ofiron ore mining. Accurate information on total Hgproduction is not available; what is known is given inTable 1. Local processing of the Hg ore was done inthe nearby village of Komárov (Fig. 1; Velebil 2003).Dendrological cores at Jedová Hora were taken fromthree spruce trees (JH1, JH2 and JH3). Tree JH1 isgrowing in the soil substrate that has developed on

74 Water Air Soil Pollut (2011) 216:73–82

mine waste material. Trees JH2 and JH3 were situated0.5 km from the centre of the mining area.

The third site (Ohrazenice) was located between thesetwo ore processing and smelting areas, ∼9 km north ofthe Příbram smelter and ∼4 km from Jedová Hora

(Fig. 1). The site was selected because of the presenceof 150- to 220-year-old beech stands (F. sylvatica L.),covering a relatively long history of metal deposition inthe central part of the CR. Cores were taken from threebeech trees (OH1, OH2 and OH3).

Fig. 1 Location of sam-pling sites, smelters andmining areas in the centralCzech Republic

Fig. 2 Historical Pb and Ag yearly production from the Příbramore district (POR; data from Bambas 1990; compiled from Žáket al. 2009). a Ores from other ore districts that were smelted in

Příbram during the twentieth century, not included in the figure. bTotal Pb yearly production and Pb emissions from the Příbram Pbsmelting and recycling plant after 1945 (data from Vurm 2001)

Water Air Soil Pollut (2011) 216:73–82 75

2.2 Sampling and Sample Preparation

We selected dominant and co-dominant trees to avoidthe possibility of missing tree rings. Tree cores weretaken using a 5-mm stainless steel Haglöf incrementalborer at chest height (∼1.5 m). Diameters of maturespruce and beech trees ranged from 51 to 68 cm andfrom 76 to 108 cm, respectively. All cores werecollected from the SW, in agreement with theprevailing winds in the area and locations of thepollution sources. The PIAP model was used tocalculate wind frequency diagrams for individual sitesaccording to Hanslian and Pop (2008). After eachcoring, the borer was cleaned with ethyl acetate(CH3COOCH2CH3) and deionised water (Millipore®).The tree cores were transported in plastic drinkingstraws and stored at −15°C in the freezer until furtherpreparation and analysis.

Two cores from each tree were taken close to eachother. Individual cores were cut into 5-year segmentsin a laminar box using a stainless steel knife and air-dried. The segments (with weights ranging from 0.03to 0.5 g) were digested in PTFE Savillex® beakerswith a mixture of 65% HNO3 (Merck, Suprapur®)and 30% H2O2 (Merck, Suprapur®) at 150°C over-night prior to analysis of nutrient elements. Since themethod for analysis of Hg used (see below) does notrequire any sample pretreatment, individual segmentswere measured directly after drying to constantweight.

The soils were classified as Haplic or DystricCambisols (FAO 2006) and were sampled in closeproximity to the studied trees. Soil samples werecollected from 1 × 1-m-wide pits from the majormorphological horizons, put into plastic bags andstored in the dark at −15°C. Prior to analysis, allsamples were freeze-dried, sieved (2 mm) and

homogenised. Soil pH was measured in a 1:2.5 (v/v)ratio of soil to deionised water suspension (Pansu andGautheyrou 2006).

2.3 Analytical Procedures

Concentrations of total Hg were determined on anAMA-254 cold vapour atomic absorption Hg analyzer(Altec Co., Czech Republic). Analyses of standardreference materials NCS DC73351 (tea leaves) andriver stream sediment 1 (Analytika, Czech Republic)were used for quality control of Hg measurement. Theaccuracy of all measurements was <5% RSD. Con-centrations of Ca, Mg, K and Mn in the digests weredetermined by inductively coupled plasma–opticalemission spectrometry (Thermo-Elemental IRIS In-trepid II) under standard analytical conditions.

2.4 Statistical Data Treatment

Measured data were tested using the followingstatistical methods. The ANOVA test (main effectscheme) was used to compare the amount of Hg at thestudied sites (the factor of year was omitted). The ttest for dependent samples (each pair was the averageHg concentration at two localities tested in theappropriate year) was used to compare the amountof Hg in tree rings. The Pearson correlation test wasused to analyse: (1) the relation between tree ring Hgconcentration and data about mining near the studiedlocalities in the appropriate year and (ii) the relationsbetween Hg concentrations at all sites. All statisticaltests were performed using Statistica 6.0 (StatSoft) ata significance level of 5%.

3 Results and Discussion

3.1 Historical Hg Deposition Recorded in Tree Rings

From studies on Hg deposition rates to the forestfloor, moss and peat, it has become clear that long-term ore mining and processing elevated the historicalbulk Hg deposition in the central part of the CR(Suchara and Sucharová 2004; Ettler et al. 2008).Trends of Hg concentrations recorded in tree ringsfrom our three studied sites are shown in Fig. 3. Treesfrom the Příbram site (PB) were younger than atJedová Hora. Thus, the deposition record from tree

Table 1 Available data on Hg production at Jedová Hora(according to Velebil 2003)

Year Cinnabar (tons) Smelted Hg (tons)

1778 3.1 n.a.

1779 4.7 1.75

1830 n.a. 1.4

1851 n.a. 1.4

1854–1858 3.4 n.a.

n.a. data not available

76 Water Air Soil Pollut (2011) 216:73–82

rings at PB covers only a short interval (50–80 years)of the time period the smelter was in operation. Allthree tree cores showed similar a trend in Hgconcentration distribution (Fig. 3a), with concentra-tions increasing steadily from the 1950s to themaxima (up to 8.4 ng g−1) in wood formed in the1970s. Although Hg emission rates are not known,

this peak in Hg concentrations corresponded wellwith the peak of Pb emissions in 1970 (Vurm 2001)and with peak in Pb smelter production in the 1970s(Fig. 2b). Such a sharp increase of Hg depositionfrom the 1950s, with maximum net Hg accumulationsbetween the 1960s and 1980s, was also observed byEttler et al. (2008) in peat cores in the vicinity of thePříbram smelter.

A significant decrease in Hg concentrations sincethe 1980s was found in tree cores PB1 and PB3. Thisdecrease was probably related to improvements influe gas cleaning technologies launched in 1982 thatconsiderably decreased the amount of dust emissions(Vurm 2001; compare Figs. 2b and 3a). A local Hgconcentration peak appeared in the tree core PB2 inthe 1990s. Higher Hg deposition rates in the late1990s were also observed in nearby peat cores (Ettleret al. 2008).

Mercury concentrations in spruce tree rings atJedová Hora (the historical HgS mining area) aredepicted in Fig. 3b. The oldest tree rings (core JH1)showed the highest concentration (up to 15 ng g−1) inthe 1890s. This could be evidence of HgS mining andprocessing at that time, though there might also be apossible influence of Hg emissions from soil sub-strates with elevated Hg concentrations. The potentialcontribution of soil Hg emissions to within-canopyrecycling of Hg between the forest floor andvegetation was mentioned by Lindberg et al. (1992).However, a number of more recent studies (e.g. Reaet al. 2002; Bushey et al. 2008) have indicated thatHg accumulated in the canopy is “new” Hg ratherthan Hg recycled within forest ecosystems.

The maximum Hg concentration in tree ring JH1was followed by a sharp decrease after the 1890s,probably due to the cessation of mining and smelting.Tree cores JH2 and JH3 did not extend back to thatperiod. Local maxima in Hg concentrations in thesetwo cores appeared in the twentieth century. Cinnabarmining at JH ceased in 1870 (Sattran et al. 1978);therefore, these concentration peaks must be due toother sources, most likely the Příbram smelter.Similarly to the PB2 tree core, local Hg concentrationmaxima were again observed in the 1990s in all JHtree cores.

Evidence of Hg deposition since the eighteenthcentury was found in the beech tree cores from theOhrazenice site (OH; Fig. 3c). Considering thelocation, Hg concentrations in the OH trees could be

Fig. 3 Concentrations of Hg in spruce tree rings in the vicinityof the Pb smelter (a), in the historical Hg ore mining area (b)and in the old beech stand located between these sites (c)

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influenced by three factors: (1) the mining andsmelting of local ores in Příbram, (2) the smelting ofimported ores with unknown Hg content during thetwentieth century in Příbram and (3) cinnabar miningand processing at Jedová Hora.

Mercury concentrations at OH were lower than atJH and PB, on average (ANOVA, p<0.001), as wouldbe expected considering the larger distance from bothpollution sources. Local Hg concentration peaksappeared at the beginning of the ninteenth century,in the 1840s and 1860s. Considering the steadilyincreasing metal production at Příbram, these maximaprobably reflect non-continuous (irregular) HgSsmelting at the JH site. According to historicalsources, cinnabar was not smelted continuously, butrather ore was first accumulated to sufficient amountsand then smelted at once (Velebil 2003).

From the second half of the nineteenth century, Hgconcentrations increased steadily up to 9.3 ng g−1 inthe 1910s. It was impossible to distinguish betweenthe two pollution sources; nevertheless, ore mining atPB steeply increased from the 1880s (Fig. 2a), whilstsmelting at JH declined at that time. Therefore, theconcentration maxima most probably reflect theactivity of the Příbram smelter.

A further concentration peak in the 1930s wasfollowed by a sharp decrease, probably caused by adecrease in mining during World War II. Similarly tothe cores from Příbram (Fig. 3a), the production peakin the mid-1970s was reflected in beech trees atOhrazenice.

Mercury concentration trends in tree rings werecompared with the known production from thePříbram smelter point source to asses the reliabilityof this geochemical archive. A strong correlation(r = 0.7, p<0.001) was found between the Hgconcentration trends in tree rings at OH and theproduction of Pb and Ag in Příbram. This suggeststhat the chemical composition of beech tree ringsaccurately reflects the smelting activity in the region.However, the Hg concentration trends in spruce treerings at PB did not correlate with metal production.This may be explained by the short time span of thedata (due to younger trees at PB) or by the chemicalbehaviour of Hg. Mercury is highly volatile and maybe transported for long distances (Fitzgerald et al.1998). Though heavy dust particles may have beendeposited close to the smelter stack, much of the Hgdeposition probably took place at greater distances

and is thus better reflected in the tree rings at OH.Differences in the accumulation of Hg from theatmosphere between the tree species studied (beechand spruce) cannot be excluded.

3.2 Mercury Concentrations in Soils and Availabilityto Trees

Our understanding of trace metal cycling and themechanisms by which metals become incorporatedinto wood is still limited. Previous studies haveshown that Hg in leaf and wood tissues is largelyderived from atmospheric sources (e.g. Ericksen et al.2003; Rea et al. 2002). Root uptake of Hg from soilsand translocation to needles seems to be a minorsource of foliar Hg (e.g. Bishop et al. 1998; Graydonet al. 2009). Zhang et al. (1995) suggested that Hgentered the wood of black spruce mainly through barkand foliage, whilst root uptake was negligible. Theuptake of Hg by ground vegetation also seems to beof minor importance (e.g. Schwesig and Krebs 2003).

Mercury concentrations in soils differed among thethree sampling sites. Generally, the highest Hgconcentrations in soils were found in the upperorganic and A horizons and decreased sharply in themineral horizons. At Příbram, higher Hg concentra-tions were found in soils located closer to the Pbsmelter (sampling sites PB1 and PB2; Fig. 1). Hgconcentrations in organic horizons reached up to4,165 ng g−1; in mineral horizons, they ranged from149 to 437 ng g−1. Mercury concentrations of1,011 ng g−1 in the organic horizon and from 91 to100 ng g−1 in mineral horizons were found at sitePB3, located farther from the smelter stack (Fig. 1).Elevated Hg concentrations in forest soils in thePříbram district (ranging up to 6,485 ng g−1) havealso been reported by Ettler et al. (2007).

The highest soil Hg concentrations from the threesites were recorded at Jedová Hora. The soil sampled∼0.5 km from the mining area only containedelevated Hg concentration in the upper Oa and Ahorizons (up to 6,542 ng g−1), decreasing to627 ng g−1 Hg in mineral horizons. Relatively highHg concentrations in both organic and mineral soilhorizons were found directly in the mining area,reflecting contamination from mining and smeltingoperations at the site. Mercury concentrations reachedup to 9,946 ng g−1 in A horizons and varied from2,794 to 8,951 ng g−1 in mineral horizons. Concen-

78 Water Air Soil Pollut (2011) 216:73–82

trations and speciation of Hg in soils at Jedová Horaare described in Hojdová et al. (2009) in more detail.

At Ohrazenice, located farther from the pointsources of pollution, Hg concentrations were lower,only reaching 462 ng g−1 in organic and 160 ng g−1 inmineral horizons. These values are similar to Hgconcentrations reported for upland soils in centralEurope by Schwesig et al. (1999).

The soils were mostly acidic, with pH valuesranging from 3.2 to 5.9. The only exception was thesoil substrate at JH1, with pH ranging from 6.9 to 8.1.

At our studied sites, there did not seem to be anyrelationship between Hg concentrations in tree ringsand concentrations in soils. Similar concentrationtrends at sites with elevated soil Hg concentrations(PB, JH) and at the site with relatively low soilconcentrations (OH) support this. Moreover, tree JH1growing in soils developed on mine waste materialdid not show elevated wood Hg concentrations incomparison to other studied trees in the area.

3.3 Radial Distribution of Nutrient Elements in TreeRings

Concentrations of nutrient elements in tree rings areusually attributed to their content and availabilityfrom the exchangeable complex in soils (e.g. Meerts2002). The content of basic cations in smelteremissions has been several times lower than heavymetal concentrations (Ettler et al. 2005). Nevertheless,smelting of sulphidic ores has been an importantsource of SO2 emissions in the CR (Kopáček andVeselý 2005). These emissions have been one of themajor acidifying pollutants associated with the deple-tion of nutrient elements and their availability tovegetation.

The radial distributions of nutrient elements (Ca,Mg, K and Mn) at all sampling sites are shown inFig. 4 and show differences between the two treespecies studied (spruce and beech). Such a relationbetween differences in nutrient concentrations and lifeform (gymnosperms vs. angiosperms) has beenpreviously reported by Meerts (2002).

Calcium concentrations in spruce trees at PB andJH were relatively stable at both sites, though a slightincrease was observed in the 1970s at PB (Fig. 4a).This increased Ca uptake is assumed to be due to theacidifying effect of increased smelter emissions in thisperiod (Fig. 2), with consequent Ca release from soils.

A similar increase in Ca concentrations in spruce treeswas also observed by Mihaljevič et al. (2008) in thevicinity of the Příbram smelter.

The Mg concentration pattern in spruce trees wasrather variable. Magnesium concentrations were rela-tively constant at PB, with higher values in the corePB3. A slightly decreasing concentration trend to-wards the bark (i.e. in youngest segments of trees)was found at JH. Significantly higher Mg concen-trations were found in beech trees than in spruce(Fig. 4c). Higher concentrations of Mg and K in thexylem of beech compared to spruce have also beenreported by Skřivan et al. (2002), in line with thegenerally lower concentrations of all mineral nutrientsin gymnosperms compared to angiosperms alreadymentioned (e.g. Meerts 2002).

The K concentration in spruce trees increasedsteadily, with maxima in the youngest wood. Com-parable patterns in conifers have been found by otherauthors (e.g. Watmough and Hutchinson 2002;Mihaljevič et al. 2008) and may be influenced bydifferences between sapwood and heartwood chemis-try. Similarly to Mg, K concentrations in beech treeswere significantly higher than in spruce trees.

Manganese concentrations slightly decreased inyounger wood for both spruce and beech trees andwere probably related to decreases in acid depositionsince the 1980s (Kopáček and Veselý 2005). Theperiod of increased smelter emissions in the 1970swas reflected in increased Mn concentrations in treerings at JH and OH (Fig. 4b, c). Such elevated Mnuptake could be related to the high Mn2+ supply in thesoil solution associated with increased acidification(e.g. Augustin et al. 2005). Low Mn concentrations inthe spruce JH1 (Fig. 4b) were most likely related tothe relatively high substrate pH (6.7–8.1). Higher soilpH is connected with restricted soil cation mobility.Similar Mn patterns related to soil conditions werealso observed in a forested catchment by Navrátilet al. (2007).

There were large variations in Mn, K and Mgconcentrations in beech until the middle of thetwentieth century. The concentrations of nutrients inwood are to a great extent influenced by climatefluctuations (higher precipitation levels, etc.), andthese factors were probably responsible for theobserved variations.

We studied the distribution of these selectedcations because of their importance for tree growth

Water Air Soil Pollut (2011) 216:73–82 79

and metabolism. Nevertheless, no relation betweenthe temporal trends of nutrients and Hg concentra-tions was found. Mercury is a non-essential elementand thus plays no specific physiological role in trees.Also, unlike some other trace elements (e.g. Rb, Sr,Ba, Cd, Tl and others), it is not a homologue ofsignificant nutrient elements. Therefore, Hg does notundergo the internal metabolic cycles of forestvegetation.

4 Conclusions

This study shows that tree rings may be anappropriate record of Hg deposition in an areaaffected by ore mining and smelting. A strongcorrelation was found between the Hg concentrationtrend in beech trees and metal production inPříbram. However, no correlation was found inspruce trees closer to the smelter stack. This

Fig. 4 Concentrations of Ca, Mg, K and Mn in spruce tree rings in the vicinity of Pb smelter (a), in the historical Hg ore mining area(b) and in the old beech stand located between these sites (c)

80 Water Air Soil Pollut (2011) 216:73–82

absence of a correlation may be explained by the shortdata interval or by the chemical behaviour of Hg.

Temporal trends in the nutrient elements analysedreflected environmental changes connected with highdeposition loads, but no relations were found betweenconcentration trends of these nutrients and Hg.

The limited amount of data precludes us fromdrawing definitive conclusions. Further research onHg cycling in trees will be necessary to betterinterpret historical Hg record in this geochemicalarchive.

Acknowledgements This research was funded by the grant ofthe Czech Science Foundation GAČR, (no. 526/09/P404) andthe project of the Ministry of the Environment of the CR (SP/2d2/111/08). Long-term financial support was provided by theInstitute of Geology of ASCR (project no. AV0Z30130516).We thank Dr. Zuzana Chládová, Institute of AtmosphericPhysics ASCR for computing wind directions and Dr. PetrSkřivan for helpful comments on manuscript.

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