INTRODUCTION

European oaks, represented mainly by pedunculate oak (Quercus robur) and sessile oak (Quercus petraea), belong to the most widely distributed tree species in Europe (EUFORGEN 2009a,b) and the most common species in dendrochronological research (Schweingruber 1996). The common distribution of pedunculate and sessile oaks covers all western and central Europe, but the easternmost distribution edge differs. Whereas distribution of sessile oak almost terminates at the eastern borders of Romania, Hungary, Slovakia, and Poland, pedunculate oak extends into Ukraine, Belarus, Baltic countries and partly western Russia (Figure 1).

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Given that both species are anatomically indistinguishable (Schoch et al. 2004) and TRW chronologies are well crossdated between these species (Dobrovolný et al. 2016), both oaks are often considered as one species in dendrochronological studies (Haneca et al. 2009). In Eastern Europe, several studies have focused on Quercus spp. in general (Ważny 1990; Pukienė 2003; Danek et al. 2007; Läänelaid et al. 2008; Bilyayava 2010; Yermokhin 2011; Chochorowski et al. 2014; Rădoane et al. 2015; Ustsky 2016; Nechita et al. 2017). All the other dendrochronological research in this area considered pedunculate oak and sessile oak separately (e.g. Sopushynskyy et al. 2008; Matison 2012; Sohar et al. 2014; Koval and Kostyashkin 2015; Yermokhin et al. 2017; Khasanov 2018; Netsvetov et al. 2019; Knysh 2019; Knysh personal communication). Only the research from Russian Krasnodar (Bitvinskas 1987) dealt additionally with pubescent oak (Quercus pubescens Willd.) (Table 1).

Table 1 List of Eastern Europe tree-ring width oak chronologies.

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Oak dendrochronology is a well-established and continuously evolving discipline that has become attractive for other research fields. The wide range of successful applications in archaeology, climatology and ecology have been situated mostly in western, southern and central Europe. Therefore, the aims of this paper are to overview the current state-of-the-art of oak dendrochronology and to outline the potential for tree-ring research in Eastern Europe, primarily in the countries of the former USSR.

CHRONOLOGY COMPILATION

The compilation of the TRW chronology is the first step before any dendrochronological analysis (Baillie 1995). The TRW chronology should be compiled for each species and each climatic area, as the tree-ring width is dependent on a lot of stand factors (Speer 2010). Oak is one of the most suitable European species for the compilation of multi-centennial and multi-millennial TRW chronologies because of its longevity, high durability, wide distribution, and widespread utilization in history (Schweingruber 1993). Most of the long chronologies were built in Western (e.g. Pilcher et al. 1984; Tegel et al. 2010), Central (e.g. Krąpiec 1998; Friedrich et al. 2004; Kolář et al. 2012; Prokop et al. 2016), and Southern Europe (e.g. Kuniholm 1987; Griggs et al. 2007).

Although Eastern Europe is largely forested and with many historical wooden buildings, long oak TRW chronologies in Eastern European countries are rare. The vast majority of oak chronologies in Eastern Europe are compiled with recent samples. Samples from Quercus robur are especially frequent, which corresponds with its distribution in Europe (Figure 1). In Eastern Europe several recent TRW chronologies from the area of Baltic States have been compiled (Table 1), but their extension is very difficult because of the rare occurrence of oak wood in historical constructions (Vitas 2008). Lithuania is also the only country of Eastern Europe where no papers on recent oak chronologies were found. The only crossdating of TRW from living trees from Lithuania was mentioned in studies concerning Baltic countries (Sohar et al. 2012; Sohar 2013) and the data were obtained from a database (samples taken in 1969–1997). TRW chronologies assembled from recent samples also dominate in Belarus, Russia, Moldova and Ukraine (Table 1). We can find only one long TRW chronology compiled from both recent and historical oak wood in western Ukraine (Kolischuk 2003). This oak TRW chronology goes back to AD 890 and even though accurate characteristics (e.g. origin of living trees, sample depth) are unknown, it is assumed that it is valid for the Lviv region based on the described origin of historical samples. Long TRW chronologies closest to Eastern European countries and potentially suitable for dendrochronological dating are in northern Romania (Nechita et al. 2018), southern Poland (Ważny 1990; Krąpiec 1996, 1998), Slovakia (Prokop et al. 2016), and Hungary (Grynaeus 2003).

TELECONNECTION OF THE OAK CHRONOLOGY

In recent years, a large network of TRW chronologies in Europe has been compiled, and there is an effort to find correlation and teleconnection among them. Therefore, investigations focused on teleconnections of oak chronologies have been performed in southeastern Europe and the Baltic area. The study on teleconnection of oak TRW chronologies in southeastern Europe includes 26 sites mostly with samples of Quercus robur, Quercus petraea and Quercus cerris L. in southern countries along the transect extending from Poland to northwestern Turkey (Ważny et al. 2014). A significant correlation among the oak site chronologies was demonstrated, where southeastern European dendrochronologies act as a “bridge” between north-central chronologies and eastern Mediterranean chronologies. In the Baltic area, research concentrated on all countries together with Finland (Sohar et al. 2012), including the research investigating the correlation among oak TRW chronologies in these countries (Sohar 2013; Sohar et al. 2012). A significant correlation among all countries was found; however, the Finnish oak chronology resembled the Estonian chronology more closely than that of southern regions. Northern Poland was established as the southern limit of regions with similarity to the Estonian chronology.

DENDROARCHAEOLOGY

Because long oak TRW chronologies are less available in Eastern Europe, only a few examples concentrating on dendroarchaeology were found. Although Eastern Europe is rich in historical wooden objects (Buxton 1981; Lakotka 2003; Taras 2015), oak is less represented in constructions in some regions, mainly in Baltic countries (Vitas 2008). Successful dating of oak relicts according to available sources was recorded only in Lithuania, Estonia, Belarus, and Ukraine. In addition, TRW chronologies from other areas and/or radiocarbon methods were used.

There are three dendroarchaeological studies from Lithuania. The first Lithuanian study (Pukienė 2003), based on the analysis of subfossil oak trunks discovered in peaty ground in Biržai forest, presented two floating chronologies dated by radiocarbon. The first, a 207-year-long mean chronology, dated to ca. 4600–4400 BC, and the second, from a tree that could not be dated against the remaining samples and which grew on the same site several centuries earlier, dated to ca. 5100–5000 BC. The second investigation into historical timbers was carried out in the Vilnius Lower Castle (Pukienė 2007). The study provided the absolutely-dated 217-year-long mean oak chronology dated using oak TRW chronologies from Poland (Ważny 1990), along with an oak chronology compiled from samples found in England but of the Baltic origin (authors J. Hilliam, I. Tyers, D. Mills) and confirmed by radiocarbon methods to AD 1202–1418. In the third study, in Lithuania 79 wooden samples, including three oak samples, were discovered in the Kegai mire (Vitas 2009). The oldest and longest tree-ring series spans 116 years and was dated with radiocarbon methods to the period 3437–3322 BC.

In Belarus in the 1960s and the 1970s, oak subfossil trunks were excavated in the Smurgainiai district (northwestern Belarus) (Bitvinskas et al. 1972; Bitvinskas et al. 1978) and in total, 129 samples were obtained (Vitas et al. 2014). Some of the samples were dated by the radiocarbon method in the 1970s and the 1980s (Bitvinskas et al. 1978; Bitvinskas 1984), and the majority of samples have been dated recently (Vitas et al. 2014). In total, TRW series resulted in 2 floating and one absolutely-dated TRW chronologies, the latter crossdating with the Polish TRW oak chronology (Ważny 1990) to AD 778–1326. The floating chronologies dated with radiocarbon methods fell within the wide period from the 6th Millennium BC to the 18^th Century AD, and the longest chronology spans 549 years (Table 1). We also found a dendroarchaeological study focusing on dendrochronological dating of the Church of Savior Transfiguration in Polotsk (Yermokhin et al. 2016), the only preserved 12^th Century church in Belarus. The chronology compiled from 4 samples was dated by the Smarhoń chronology (Bitvinskas et al. 1978), which was confirmed by radiocarbon dating and covers the period from AD 869 to 1122 (Table 1). The date of construction of the church (AD 1124–1137) perfectly corresponds with historical literature resources.

In Estonia many dendroarchaeological investigations have been done, but most of them focused on pine (Pinus sp.) and spruce (Picea sp.) samples (Läänelaid 2002). However, some oak wood art items were successfully dendrochronologically dated (Art museum of Estonia 2015). For example, tree-ring series of boards of H. van Essen's painting “Still life with a lobster” were successfully crossdated with a chronology of Baltic origin (authors J. Hilliam, I. Tyers, D. Mills) and were dated to AD 1600 (Läänelaid 2006). When the approximate number of sapwood rings was added, because all of sapwood rings had been removed from the boards, the most probable time of creation was determined to be between AD 1617 and 1623. This paper represents the first successful dendrochronological dating of an oak wood old painting panel in Estonia. We found a dendrochronological investigation focusing on two partly charred oak wooden book covers (Läänelaid 2017). Both books were printed in AD 1879. The first cover, of the book “Mineia for February”, was dated by comparison with an East Pomerania (Poland) oak chronology to the 14^th Century, which can be explained by the re-use of an old cover. The book-cover age of the second one, “Mineia for April”, was determined as AD 1857 using the same chronology, which corresponds to the print year.

Despite the fact that oak is rare in archaeological sites in European Russia (Khasanov et al. 2020), there are three dendroarchaeological studies from this area. The first study (Khasanov 2002) presents the radiocarbon dating of 22 subfossil oak (Quercus robur) trunks recovered from the deposits of the upper Zapadnaya Dvina River (Tverska region). The oldest tree was formed in the middle of the 5^th Millennium BC, most of samples (16 trunks) were dated to period from AD 600 to the 14^th Century and the youngest trunk is from 15^th Century. The second research (Karpukhin et al. 2020) has been carried out on 39 oak boards from graves found in Vyazma (Smolensk region). This new chronology from Vyazma significantly correlates with the originally floating chronology from Zapadnaya Dvina. Both chronologies were dated with the Smarhoń chronology (Bitvinskas et al. 1978). Vyazma chronology spannned the period from AD 1074 to 1306, which was confirmed by radiocarbon dating. The summary and presentation of these chronologies (the one from Zapadnaya Dvina covering the period from AD 649 to 1382) together with the chronology from Novgorod represents the last study from this area (Khasanov et al. 2020). The Novgorod chronology was compiled from 11 archaeological samples from various parts of house constructions and covers the period from AD 1059 to 1386. All three chronologies were dated using the Polotsk chronology (Yermokhin et al. 2016) and confirmed by Smarhoń and Baltic 1 chronologies.

Dendroarchaeological studies in Ukraine presented the absolutely dated TRW chronologies for the Odessa region (Bilyayeva et al. 2010), for the Podolia region (southwestern Ukraine near the north border of Moldova) (Ważny et al. 2014), and a short floating TRW chronology for Mielniki (central Ukraine) (Chochorowski et al. 2014). The samples from the Odessa region were obtained from the Akkerman fortress and were divided into two groups, Akkerman-early and Akkerman-late. The Akkerman-early chronology was crossdated with oak TRW chronologies from Slovakia (Prokop et al. 2016) and Poland (Ważny 1990; Krąpiec 1998) as a reference chronology and dated to AD 1356–1507. The Akkerman-late chronology was dated in Ważny's paper (2014) to AD 1677–1792. The chronology from the Podolia region (Ważny et al. 2014) was assembled from historic samples and extends back to AD 1643. In Mielniki, 20 charred pieces of oak wood of a defensive settlement were excavated. A 62-year-long mean chronology was compiled from 12 crossdated tree-ring series. A floating chronology dated by the radiocarbon method has been created and the most likely period of the tree felling based on historic sources is 665–630 BC.

The key factor for a precise dendrochronological dating is the presence of the outermost tree ring (waney edge). However, as the outermost tree ring is not always preserved on historical samples, the analysis of the number of sapwood rings is crucial to determine at least the period when the tree was felled down. Owing to the fact that the sapwood estimates are highly variable both within an area and between areas, it is necessary to estimate the number of sapwood rings for every area separately (Baillie 1995). Because of a tendency of the decreasing number of sapwood rings from west to east (Baillie 1995), the compilation of oak TRW chronologies is usually accompanied by the analysis of the sapwood ring numbers. A general trend has also been confirmed in the Eastern European countries where the number of sapwood rings ranged from 6 to 19 (Table 1), which is significantly lower compared to central and western Europe (Ważny 1991; Miles 1997; Haneca et al. 2009). Nevertheless, sapwood rings were counted only in the Baltic States (Sohar et al. 2012; Sohar 2013) and Belarus (Yermokhin et al. 2016), so their number remains unknown for other countries of Eastern Europe.

DENDROCLIMATOLOGY

Because of the high sensitivity of tree-ring formation to climate variability (Fritts 1976), dendroclimatology is one of the most common disciplines. Recently, many dendroclimatic studies using either tree-ring width or intra-annual anatomical structure have been performed throughout Eastern Europe.

Owing to the fact that Eastern Europe largely belongs to the same climate region (humid continental climate based on Köppen classification), a similar climate-growth pattern can be expected across the territory. However, slightly different results were found between northern and southern part of Eastern Europe. Positive correlations between TRW and summer precipitation (Läänelaid et al. 2008; Yermokhin 2011; Matisons et al. 2012b; Sohar 2013; Sohar et al. 2014; Netsvetov et al. 2017; Helama et al. 2018; Knysh 2019) or positive correlations with precipitation during the whole vegetation period (Vakoljuk 2009; Mazepa et al. 2010; Koval and Kostyashkin 2015) were found across all Eastern Europe. In the southern part of Eastern Europe (Ukraine), positive correlations between TRW and summer precipitation were detected in earlier months than in the northern part (Koval et al. 2015; Netsvetov et al. 2017; Netsvetov et al. 2018), and further to the south we can find this correlation in even earlier months in northern Romania (Nechita et al. 2017). A positive correlation with summer temperature (Yermokhin 2011; Matisons 2012a; Sohar, 2013; Sohar et al. 2014; Knysh 2019) was established in the northern part of Eastern Europe whereas in the southern part the positive correlation was detected in spring months (Netsvetov 2017; Knysh 2019). A negative correlation was only found between TRW and temperature in the period from the previous summer to the summer of the current growing season (Läänelaid et al. 2008; Vakoljuk 2009; Matisons 2012a,b; Matisons et al. 2013; Koval and Kostyashkin 2015; Koval et al. 2015; Nechita et al. 2017; Netsvetov et al. 2018; Knysh 2019).

Most of the studies in countries of Eastern Europe are based on the effects of climatic factors on tree-ring width. Among them are a few papers on different sensitivity of wood anatomical measurements and tree-ring width to climate variability. All of the studies (Matisons 2012a,b; Matisons et al. 2013; Pritzkowa et al. 2016; Netsvetov et al. 2019) revealed significantly stronger correlations of climatic factors with the earlywood vessel area than TRW. In addition, Vakoljuk (2009) and Netsvetov et al. (2019) found that earlywood width is more climate-sensitive than TRW. Vakoljuk (2009) also determined that the ratio of earlywood to the total width of tree rings increases from north to south.

In Russia, at the easternmost edge of oak distribution where temperature conditions are growth-limiting, we can find papers that investigate the formation of severe winter rings (small-sized cells shaped in a distinct band) caused by winter temperature dropping below –42°C (Khasanov 2013) and very narrow earlywood created in years with a combination of extreme weather events in winter and spring (Khasanov 2011). The expression of these phenomena associated with extreme weather allows reconstruction of these extreme events.

FOREST HISTORY AND DENDROHYDROLOGY

Various dendroecological studies have been found across Eastern Europe. In Belarus, a reconstruction of mixed forest species historic composition and structure revealed that the formation of all stand types was caused by the rapid disintegration of the original stand in response to either logging or massive spruce die-back. The result is that all stands consist of trees mainly of the same age with a small proportion of older trees; the study also suggests that a change in forest composition can be expected in the coming decades (Yermokhin et al. 2017). Khasanov (2018) studied the impact of insect infestation (cockchafers, moths and oak leaf roller) on the oak TRW and anatomical wood structure (intra-annual density fluctuations and abrupt growth changes) obtained in Russia (Kaluga region). The investigation was based on the similarity of tree-ring series of host and non-host species (Swetnam et al. 1985) and reported that insect infestation is responsible for growth interruptions. Other external stress factors can intensify the impact of insect outbreaks, but would not separately cause growth interruptions.

The influence of recreation on radial width growth of oak and beech was investigated in Ukraine (Derekh 2014). The percentage of tree damage produced by human recreation in Lviv is 40–100%, and a change in taxonomic structure and quantitative composition of vegetation cover was found. The research assessing the difference between the TRW of oaks damaged by hoarfrost and those unaffected (Vakoljuk 2009) shows that TRW of trees at the epicentre of a hoarfrost event decreased in the following year by 15% relative to the TRW of trees unaffected by hoarfrost. Netsvetov's (2016) research defined the age-to-diameter ratio in an urban forest in Kyiv (Ukraine) to 2.13 years per cm. Thanks to this result it was possible to determine the age of the oldest trees in the stand without sampling, and the determination was confirmed by historical resources.

Also in Estonia, the age of mostly hollow old oak trees in the Tallin district was estimated by two methods, one using radial increment data and the other using a bark method elaborated by M. Rohtla (Läänelaid et al. 2001). Both were compared with known written references on the history of the estates. The oldest living tree in Tallin was found in Kardriorg Park with a circumference of 520 mm and an age of about 360 years.

Two dendrohydrological studies exploring the relationship between the radial increment and river level (Koval et al. 2015; Ustsky et al. 2016) were carried out in the Zhytomyrska region (northern Ukraine). Research from the Novograd-Volynsky physiographic region (Koval et al. 2015) found a relationship between the oak and ash (Fraxinus excelsior L.) radial increments and groundwater level. They found that the deterioration of oak plantations growing under conditions of drainage reclamation conducted in the 1960s caused a sharp decrease in the level of groundwater. Additionally, they determined negative correlations between oak TRW and groundwater levels, especially during February–March. Further, it was found that oak trees were able to adapt to a change in groundwater level and ash trees were not. Research based on recent pine and oak samples from the Chernihiv region (Utsky et al. 2016) determined that reclamation significantly positively influenced the TRW of oak trees. An increase in oak latewood increment of almost 80% was noted, and TRW increased by 70%. The radial growth of oak stands outside the influence of reclamation canals (control) gradually decreased over the whole period.

CONCLUSIONS

Most of dendrochronological studies done in Eastern Europe are conducted using living trees and with focus on dendroclimatology and dendroecology (Table 1). The only oak TRW chronologies for the easternmost distribution of oak in Europe that were assembled from both living trees and historical resources are from Ukraine (Kolischuk 2003; Ważny et al. 2014). The analysis of the number of sapwood rings, which allows a more accurate estimation of the year of felling for trees with incomplete sapwood, was carried out only in studies from Estonia (Sohar et al. 2012) and Belarus (Yermokhin et al. 2016).

A compilation of new oak TRW chronologies including an analysis of the number of sapwood rings and also an extension of already assembled oak TRW chronologies in Eastern Europe is highly important for the improvement and refinement of dendrochronological investigations. To the best of our knowledge, the network of recent oak chronologies in some regions of Eastern Europe is relatively sparse. The extension of this network using living oak trees from unexplored regions would facilitate spatiotemporal dendroclimatological analyses, which should provide for the regional diversity of oak growth and show the territory where regional TRW chronologies can be used for dating. Moreover, very well-preserved historic buildings in some areas (e.g. wooden churches in western Ukraine) indicate a great potential to build long chronologies that would be useful in dendroarchaeology and paleoclimatology.