Dendrochronological evidence for long-distance timber trading in the Roman Empire.

An important question for our understanding of Roman history is how the Empire's economy was structured, and how long-distance trading within and between its provinces was organised and achieved. Moreover, it is still unclear whether large construction timbers, for use in Italy, came from the widespread temperate forests north of the Alps and were then transported to the sparsely-wooded Mediterranean region in the south. Here, we present dendrochronological results from the archaeological excavation of an expensively decorated portico in the centre of Rome. The oak trees (Quercus sp.), providing twenty-four well-preserved planks in waterlogged ground, had been felled between 40 and 60 CE in the Jura Mountains of north-eastern France. It is most likely that the wood was transported to the Eternal City on the Saône and Rhône rivers and then across the Mediterranean Sea. This rare dendrochronological evidence from the capital of the Roman Empire gives fresh impetus to the ongoing debate on the likelihood of transporting timber over long distances within and between Roman provinces. This study reconstructs the administrative and logistic efforts required to transport high-quality construction timber from central Europe to Rome. It also highlights an advanced network of trade, and emphasises the enormous value of oak wood in Roman times.

Introduction

“Mille praetera sunt usus earum, sine quis vita degi non possit”, (“Wood has thousands of uses, and without it, life would not be possible”, Pliny the Elder: Naturalis Historia XVI, 1–5). With this declaration, Pliny (23/24-79 CE) points out the value wood had for the Romans. Wood was important for any aspect of everyday life, ranging from the construction of buildings [1] to heating systems [2], and from shipbuilding [3] to metalworking [4]. In Latin, the distinction between firewood, lignum, and construction timber, materia, is indicative in this respect. The current Spanish word for wood is madera. However, in other languages the word “material” (in English) or Material (in German) has taken on a more general meaning, signifying “matter” or “substance”. Basically, wood was so important for the Romans that they considered it as “material” in the modern, English, sense of the word.

In Rome, timber requirements were immense [5,3]. The demand for timber led to the rapid depletion of the woodlands surrounding the capital and in much of the Apennines. As the Empire expanded, timber cutting continued abroad: in Pliny’s time (1st century CE), some of Algeria’s forests rich in sandarac trees (Tetraclinis articulata), a wood particularly appreciated by the Romans, had already been fully exploited so that its timber supply shifted to Morocco [1]. And Emperor Hadrian created an imperial forest, by fencing off the cedar of Lebanon woodland and marking its perimeter with inscribed boundary stones, in order to conserve those woods [6].

A great variety of tree species was available in Rome in large quantities: ebony (Diospyros spp.), cedar (Cedrus spp.), box (Buxus sempervirens L.), terebinth (Pistacia terebinthus L.), holm oak (Quercus ilex L.) and many others. Patrician houses commonly contained a wide choice of wood [7] and were adorned with other precious material like gold or ivory. For the construction of buildings, silver fir (Abies alba) was the preferred tree species. Vitruvius himself, in his treatise on architecture (De Architectura, II, chap. 9–10; 30–15 BCE), indicates the characteristics that make silver fir particularly valuable: its light wood and a large, regular stem. Archaeological finds in Pompeii and Herculaneum confirm this, where silver fir was the most common construction timber [8], followed by oak wood, which is heavier than silver fir and has a less regular stem, especially in the case of trees from the Apennines [4]. However, oak is stronger, harder and much more durable than fir. These characteristics made oak less suitable for providing long roof beams or roof trusses but perfect for all kinds of foundations in contact with the ground. Despite our understanding of the many uses of wood in Roman times, detailed insight into long-distance timber trading, the preferred tree species used and its sources of supply is still limited [1,9].

At the same time, recent advances in dendrochronology have made important contributions to archaeological research [10]. Given the right conditions, wood can be dendrochronologically dated to the calendar year [11,12]. Moreover, tree-ring research can determine the wood’s provenance [13], and sometimes it may even help to identify political and economic networks of commercial trade [14]. Unfortunately, in the Mediterranean region, the necessary conditions for dendrochronological analysis are rarely given [15,16]. Wood is preserved over a long period of time only in very humid or very dry locations, at very low temperatures, in contact with metal or in the form of charcoal [7,17]. In Mediterranean archaeological excavations, pottery and iron, for example, are easily found but wood is rare, and often it only occurs as minute fragments bonded to metal [18].

Due to the scarcity of datable wood, only a few and so far unpublished multi-millennial reference chronologies exist for Italy. This circumstance effectively hampers the dendrochronological assessment of Roman timbers. Hence, most studies on Roman timber constructions refer to archaeological sites outside of Italy concerning, for instance, the dating of ships [19,20,21,22], barrels [23] and the reconstruction of trade routes [24,25].

This study is, therefore, the rare exception of a successful dendrochronological investigation of archaeological timbers in the city of Rome, which has allowed us to: a) date these timbers by the method of dendrochronological cross-dating, b) determine the timbers’ geographical origin (provenance), and c) compare the dendrochronological results with those derived from historical and archaeological sources (multi-proxy comparison).

Material and methods

Archaeological evidence

During the construction of Rome’s Metro line C (underground railway line) in 2014–16 CE, an archaeological excavation was carried out, covering an area of approximately 1,440 m2, in the gardens of via Sannio, next to the line. On this site, a total of twenty-four oak planks (Quercus sp.) were found under via Sannio (Fig 1), near the Basilica of San Giovanni in Laterano, just outside the ancient Aurelian walls. These planks had been part of the foundations of a richly-decorated portico (Fig 2), belonging to a vast and wealthy property [26]. All samples were well-preserved, as they were saturated with water. In many cases, the vessels of the wooden planks were filled with hard, glassy, translucent whitish mineral deposits deriving from the site. Six planks come from the south-eastern foundations (stratigraphic unit, s.u., 1141, Table 1), and consist of two non-continuous rows of horizontal planks, shored up by round posts (Figs 2 and 3). Most of the planks were about 3.60 m long, with the shortest one having a length of 1.15 m. The round posts were 60–66 cm long, with a diameter of 3.5–8 cm. Sixteen planks are from the north-western foundations (s.u. 1143, Fig 3), making up four rows of horizontal planks (1.67–3.57 m, though mostly about 3.50 m long; 25–30 cm wide). These planks were also shored up with 65 cm long posts of 4–9 cm diameter. Another sample, code A (Table 1) (s.u. 1286), belonged to the foundations of an older construction on the same site. Finally, sample B (s.u. 1383) was a support for a small bridge over a watercourse.

Fig 1

The archaeological site.

The map shows the site of the excavation in Rome, between the ancient Aurelian walls (marked by a black line) and Rome’s Metro line C (underground railway line), near the Basilica of San Giovanni in Laterano. Map sourced from http://dati.lazio.it/catalog/it/dataset/carta-tecnica-regionale-2002-2003-5k-roma. On a map of ancient Rome at the bottom right corner, the excavation site is indicated by a red dot.

Fig 2

The foundation of the portico.

Schematic section of the foundations of the south-eastern lateral portico (u.s 1141–1178) [26].

Table 1

Samples from the archaeological excavation.

Sample code	Stratigraphic unit	Position
A	M1286	Oldest foundation
B	1383	Canal bridge
C1	1359 (1143)	NW Foundation
C2	1359 (1143)	NW Foundation
C3	1359 (1143)	NW Foundation
C7	1359 (1143)	NW Foundation
C14	1359 (1143)	NW Foundation
C17	1359 (1143)	NW Foundation
C20	1359 (1143)	NW Foundation
C22	1359 (1143)	NW Foundation
C24	1359 (1143)	NW Foundation
C25	1359 (1143)	NW Foundation
C27	1359 (1143)	NW Foundation
C29	1359 (1143)	NW Foundation
C31	1359 (1143)	NW Foundation
C32	1359 (1143)	NW Foundation
C37	1359 (1143)	NW Foundation
C43	1359 (1143)	NW Foundation
C53	1178 (1141)	SE Foundation
C54	1178 (1141)	SE Foundation
C56	1178 (1141)	SE Foundation
C58	1178 (1141)	SE Foundation
C59	1178 (1141)	SE Foundation
C60	1178 (1141)	SE Foundation

Fig 3

Oak planks in situ.

At the bottom right, axe marks are visible on the planks [26].

A description of the archaeological site is found at the San Giovanni Station Museum in Rome, an exhibition showing the stratigraphy of the excavation and some of the objects found. All necessary permits for the described study were obtained from the Soprintendenza Speciale Archeologia, Belle Arti e Paesaggio di Roma.

Dendrochronological dating

Cross-sections of each plank were carefully prepared with a scalpel, and chalk was rubbed into the clean surface in order to highlight the anatomical tree-ring pattern. Tree-ring widths (TRW) were measured perpendicularly to the ring tangent, using a LINTAB device (LINear TABle, RinnTech, Heidelberg, Germany), with an accuracy of 0.01 mm. The TSAP-Win program [27] was used to record measurements and create individual TRW series. Where more than one TRW series was obtained from the same sample, these series were averaged. In oak trees, the ring border is often deformed by large parenchymatous rays, hence, the measurement direction required continuous adjustment. Where a cross-section also contained the pith, both radii were measured to exclude possible deformed rings. Another aspect that will affect ring-measurement precision on planks, though not on stem cross-sections, are anomalous, eccentric growth rings that are typical of oak. Each individual TRW series was compared visually and statistically (using PAST4 [28]) with all other individual series to ensure measurement accuracy and to identify possible xylological anomalies such as false or missing rings, or deformation caused by large parenchymatous rays. Finally, a mean TRW chronology was obtained by averaging TRW series with significant cross-correlation values (TBP > 4, see details below).

Each comparison was based on two statistical parameters: a) TBP and THO: t-value adapted to time-series by Baillie and Pilcher [29] and Hollstein [30]; b) Glk: Gleichläufigkeit and its value Gleichläufigkeitswert, as discussed by Eckstein and Bauch [31]. Glk is a non-parametric test, which, by comparing two time series at a given time-interval, represents the percentage of agreement between the growth sign (+ or -) from one year to the next. The significance level of the correlation coefficient is assessed at p = 0.05, 0.01 and 0.001, here indicated as *, ** and ***; c) overlap: the number of rings compared, to which the statistical tests refer.

Results

All samples belong to Quercus sp, subgenus Quercus. The wood’s decay rendered any microscopic distinction between the sections Quercus and Cerris [32] impossible.

Visual examination of the planks did not reveal any traces of previous usage, such as old dowels, engravings, marks from previous work or traces of insects. Only axe marks from very accurate cutting and wood preparation (Fig 3) were found, indicating that the timbers used were cut specifically for this construction.

A total of thirteen planks was successfully dated. The individual TRW series ranged from 27 to 284 years, with a mean segment length of 135 rings and an average growth increment of 1.1 mm. Four of twenty samples had more than 250 rings (Table 2). The TRW patterns of thirteen series were very similar, which is reflected in their significant, positive inter-series correlation (Rbar = 0.4, Table 2 and Fig 4). These individual TRW series form the mean site chronology RMC1 (Roman Metro Chronology 1) that spans 320 years (Fig 4). Given the lack of a solid Italian oak reference chronology for the Roman period, RMC1 was compared with several floating Italian chronologies in the Roman period (Martinelli N., unpublished data). However, no significant correlation was found. Hence, RMC1 was further compared with central European oak chronologies [30,33], which indicates a statistically significant correlation for the years 279 BCE-40 CE.

Table 2

Dendrochronological correlation values of each individual TRW series against the mean chronology of all other samples.

Code	Stratigraphic unit	Distance to pith (cm)	Sapwooda(no. of rings)	Tree rings(no.)	Dating(last ring)	TBP; Glk (%)***
A	M1286	10	35	81	//b
B	1383	5	0	44	//
C1	1359 (1143)	3	15	284	40 CE	12.20; 65.50***
C2	1359 (1143)	>10	0	278	//
C3	1359 (1143)	5	0	283	3 CE	9.21; 65.90***
C7	1359 (1143)	10	0	269	14 CE	7.30; 61.90***
C14	1359 (1143)	10	0	//	//
C17	1359 (1143)	>10	12	174	40 CE	7.36; 63.90***
C20	1359 (1143)	5	0	//	//
C22	1359 (1143)	1	1(?)	100	23 CE	5.77; 75.00***
C24	1359 (1143)	10	0	60	54 BCE	4.28; 62.50*
C25	1359 (1143)	10	0	78	//
C27	1359 (1143)	>10	0	102	84 BCE	5.36; 69.10***
C29	1359 (1143)	0	1(?)	114	21 CE	4.72; 68.40***
C31	1359 (1143)	5	0	//	//
C32	1359 (1143)	1	0	69	15 CE	4.52; 62.30*
C37	1359 (1143)	8	10	97	34 CE	8.52; 64.40**
C43	1359 (1143)	>10	0	27	//
C53	1178 (1141)	1	0	61	11 BCE	4.76; 73.00***
C54	1178 (1141)	>10	20	81	30 CE	4.08; 51.90*
C56	1178 (1141)	3	1(?)	//	//
C58	1178 (1141)	>10	0	141	//
C59	1178 (1141)	2	0	59	4 BCE	5.15; 71.20***
C60	1178 (1141)	0	0	28	//

aSapwood is the outer layer of recently formed wood, usually of lighter colour, between the heartwood and the bark, containing the functioning vascular tissue.

b// indicates: no results

Fig 4

The TRW visual comparison.

Visual comparison of all dated individual Roman oak TRW series (A), and of the mean “Rome Metro Site Chronology” (RMC1, in red) with the “Moissey-La Tuilerie Site Chronology” (B) from the French Jura, demonstrate the highest correlation values for the year 40 CE (TBP = 6,57, Glk 64,40***, see Table 3).

Table 3

Correlation values of the Rome Metro Site Chronology (RMC1) in comparison with European reference chronologies for the year 40 CE.

Region/Site	Reference	Overlap	TBP	THO	Glk (%)
Moissey-La Tuilerie (Jura)	Charlier 2001 [34]	260	6.1	6.1	64.8***
Alsace	Tegel et al. 2016 [35]	260	5.5	6.0	65.6***
Lorraine	Tegel et al. 2016 [35]	260	3.3	4.1	55.6*
Champagne	Tegel et al. 2016 [35]	260	3.6	4.2	61.0**
Burgundy	Lambert and Lavier 1991 [36]	260	4.7	5.6	58.5**

Having successfully dated the Roman oak planks against central European reference chronologies (above), a closer look at regional reference chronologies points to the timber’s provenance in eastern France, i.e. Alsace (Upper Rhine Valley), Lorraine, Champagne and Burgundy.

Indeed, high correlation values were obtained with a reference chronology for the Alsace region (THO 6.0), followed by Burgundy (THO 5.6). By contrast, regions located further north such as Lorraine and Champagne show less significant correlations (Table 3). RMC1 has the highest correlation values (THO and TBP 6.1) with the Moissey "La Tuilerie" Chronology, Dép. Jura [34], which confirms that the timber originates from the Jura mountain range (Fig 5).

Fig 5

The timber road.

Map of Roman provinces in today’s France and Germany, with the probable provenance of the Roman oak Metro samples. Some important Roman towns are indicated (Colonia Agrippina = Cologne; Augusta Treverorum = Trier; Divodurum = Metz; Dorocortorum = Reims; Augustobona = Troyes; Lugdunum = Lyon; Arelate = Arles; Aquae Sextiae = Aix-en-Provence and Massilia = Marseilles), as well as the regions where the reference chronologies (Table 3) come from and also the rivers (Saône and Rhône) leading to the Mediterranean Sea. Map sourced from https://mapswire.com/europe/.

Sapwood was identified in eight samples (Table 2). The presence of sapwood was also microscopically verified by the absence of tyloses in large vessels of the porous rings. Even if a sample only contained sapwood remains, this narrows down the estimate of the tree’s felling date. As a general rule, mature oaks develop between 10 and 30 sapwood rings [30]. Hence, the felling date can be estimated with a precision of ±10 years. On the basis of these calculations, and considering that sapwood estimates may vary geographically, all dated trees were felled between 40 and 60 CE (Table 2).

Discussion

The Romans based their hegemony on an imposing road system, which enabled long-distance trade and the massive exploitation of resources in the regions they controlled [37]). Metal, pottery, marble and many other luxury goods were transported, regardless of distance and possible geographical barriers. This is fairly easy to imagine in the case of luxury goods and non-perishable food but it is more complicated to achieve for imposing blocks of marble for Roman squares or with the wild animals to be killed by gladiators in Roman amphitheatres [38]. Hence, the infrastructures of the Roman Empire, combined with advanced logistical skills, require our admiration and respect to the present day.

The principal routes for commercial transport in Roman times are well known [37,39], and there are detailed descriptions of the Romans’ means of acquisition as well as the exact origin of the goods [1,38]. However, very little is known about timber trading to Rome and the existence of commercial routes for this purpose [40,9]. Particularly valuable tree species, or those used as status symbols, like the sandarac tree (Tetraclinis articulata) or ebony (Diospyros sp.), were imported but there are no documents in support of this, and there is no evidence or any proof at all that would indicate long-distance timber trading to Rome for construction purposes [9]. We do know that wood was transported over long distances after the fall of the Roman Empire. Timber, coming from the Alps and already in the form of long beams, was shipped right across the Mediterranean Sea to be used in Palestine [14].

Our research has shown that in Roman times central European wood was used for construction purposes in central Rome, and a commercial route of transport has been identified for this scope. In fact, a comparison between the Roman oak planks examined and existing site chronologies (Table 3) does not leave any doubt that the provenance of the oaks used for building the patrician porch is today’s north-eastern French region of the Jura. This statement is based on the following results:

There is no statistically significant correlation between the Rome Metro Chronology (RMC1) and any dendrochronological series of Italian oak wood from Roman times. Although it is difficult to find wooden samples in archaeological excavations in Italy, numerous valid dendrochronological series have, in fact, been obtained for ancient times (Martinelli N., Wazny T. pers. com.) but they had to be radiocarbon-dated.

Comparing the Roman oak chronology with non-Italian site chronologies has, on the other hand, produced increasingly significant results as one moves towards Central Europe, where the highest correlation values were obtained for site chronologies from north-eastern France. These correlation values are even better if one considers that the Rome Metro Chronology only consists of thirteen individual time series, whose total length of 320 tree rings is reduced to 280 rings if only the well-replicated part of the chronology is considered (Table 3, Fig 4).

It is well-known that Central European oak growing in an oceanic or continental climate is very different from that in the Mediterranean [41]. Hence, Mediterranean site chronologies from Roman times cannot be successfully cross-dated with the long central European oak reference chronologies [41].

As our TRW series cross-dated best with the Moissey site chronology in the French Jura, it can be assumed that the oaks originate from this region, which is also close to the most important–and in great part navigable–commercial trade routes of ancient Gaul [40]. Moissey is located 60 km north-east of Chalon-sur-Saône (Cabillonum), at the onset of the Belfort Gap (Fig 5), which separates the drainage basins of the rivers Rhine and Rhône (Doubs, Ognon and Saône) between the low mountain ranges of Vosges and Jura. Due to its outstanding topographic location, this was an important area, connecting the provinces of Gaul and Germania Superior. The timber was most likely transported by road or rafted and floated downstream on the Saône river up to Chalon-sur-Saône, an important commercial centre of Gaul and, at the time, the administrative headquarters of a river fleet [40]. From this point, the Saône is then navigable southwards, down to the river Rhône. At the confluence, there is Lyon (ancient Lugdunum), the most important commercial and administrative centre of the province of Gaul (Gallia Lugdunensis). Even large ships could transport the timber to Rome’s ancient port of Ostia, via the river Rhône and the Ligurian Sea, and then up the river Tiber [42].

The dendrochronological dates are confirmed by archaeological finds such as large quantities of ancient, red Roman pottery, which contains many marked fragments that were found underneath the pavement and date to the same time period.

It is interesting to note how the wooden samples that contain a few sapwood rings are correctly aligned towards the end of the site chronology. This proves [19] that the samples belong to a single wood lot and that, therefore, they have the same provenance.

The samples’ high reciprocal correlation values further confirm this, which has also led to the hypothesis that some planks may come from the same tree. Samples C1 and C3, in particular, have TBP-values above 8 (8.15), if some external rings are excluded from the calculation. Similar correlation values are also found for samples C7 and C37 and the TBP-values between individual tree-ring series and the site chronology of the remaining samples are rather high (Table 2).

Although four oak samples have more than 250 rings (Table 2), no sample extended from pith to bark, indicating that some of the oaks must have been fairly long-lived. Other trees from the same lot have considerably fewer rings. This might indicate that the trees came from near-natural oak woodlands, with an uneven age structure and very little human disturbance.

The vast forests in the northern provinces would have supplied seemingly endless raw material that, during the initial occupation of the new territories, was locally used to construct encampments and to supply the necessary war machinery, but at the end of the war was also employed for the upkeep of various strategic fortifications of the Empire, the fleet and for construction purposes [9]. Cutting the trees, stockpiling and working, and then transporting the timber; everything had to be organised in such a way that the half-prepared timber travelled safely from the French woodlands to the heart of Rome.

The felled trees were most likely split to make planks on site and were further processed by axes or adzes, as proven by characteristic toolmarks (Fig 3). Their plain surface indicates the use of freshly cut oaks. Also for practical reasons, we assume that the planks were prepared before the timber was transported over long distances. The dimension and weight of the oak planks suggest river and maritime rather than land transport. A dense network of waterway trade routes were intensively used to connect the Roman Empire.

Considering the distances, calculated to be over 1700 km, the timber’s dimensions, road transport with all the possible obstacles along the way, floating the timber down rivers and finally shipping it across the sea, the logistic organisation of the Romans must have been formidable. Even more so as, in this case, the buried oak planks would not even have been visible, being part of the foundations.

It would probably have been more difficult, and less convenient, to find solid oak with the necessary characteristics in Apennine forests, rather than making them arrive from the occupied provinces, where a large workforce and the abundance of raw material made it easier to obtain wood of the best quality. In the province of Gaul, it was only necessary to manage the workforce but that was organised by Roman logistics.

Conclusions

Although enormous quantities of wood were used in Roman times, south of the Alps dendrochronological analysis is difficult because few remains have survived intact [15]. So far, dendrochronological dating of southern Alpine oak timbers has proved to be difficult due to the lack of long reference chronologies. Here, the first successful cross-dating results of thirteen, on average 3.6 m long oak planks, from the foundations of a patrician villa in Rome, are presented. The planks are from up to 300-year old oak trees, felled between 40 and 60 CE in north-eastern France, possibly in near-natural woodlands of the Jura Mountains. The timber was rafted and floated down the principal waterways of the province of Gaul, the Saône and Rhône rivers, and then crossed the Mediterranean Sea to be finally taken up the river Tiber to Rome. For the first time, the use of oak trees grown in the Roman provinces north of the Alps has been proved for the construction of buildings in ancient Rome. The transport routes have been reconstructed on the basis of still existing waterways.

Thirteen individual tree ring series from Rome and the resulting mean chronology.

Species: Quercus sp. Tucson format.

(PDF)

Click here for additional data file.

12 Aug 2019

PONE-D-19-20970

DENDROCHRONOLOGICAL EVIDENCE FOR LONG-DISTANCE TIMBER TRADING IN THE ROMAN EMPIRE

PLOS ONE

Dear Dr. Bernabei,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

==============================

All three reviewers think that the manuscript is interesting for publishing in PLOS ONE. However, they require minor revision before it can be accepted for publication.

In addition, please note that manuscripts reporting paleontology and archaeology research must adhere to the PLOS ONE policies described at http://journals.plos.org/plosone/s/submission-guidelines#loc-paleontology-and-archaeology-research. Specifically, appropriate specimen numbers should be provided, and the specimens should be publicly deposited or available for replication of the study.

==============================

We would appreciate receiving your revised manuscript by Sep 26 2019 11:59PM. When you are ready to submit your revision, log on to https://www.editorialmanager.com/pone/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

If you would like to make changes to your financial disclosure, please include your updated statement in your cover letter.

To enhance the reproducibility of your results, we recommend that if applicable you deposit your laboratory protocols in protocols.io, where a protocol can be assigned its own identifier (DOI) such that it can be cited independently in the future. For instructions see: http://journals.plos.org/plosone/s/submission-guidelines#loc-laboratory-protocols

Please include the following items when submitting your revised manuscript:

A rebuttal letter that responds to each point raised by the academic editor and reviewer(s). This letter should be uploaded as separate file and labeled 'Response to Reviewers'.

A marked-up copy of your manuscript that highlights changes made to the original version. This file should be uploaded as separate file and labeled 'Revised Manuscript with Track Changes'.

An unmarked version of your revised paper without tracked changes. This file should be uploaded as separate file and labeled 'Manuscript'.

Please note while forming your response, if your article is accepted, you may have the opportunity to make the peer review history publicly available. The record will include editor decision letters (with reviews) and your responses to reviewer comments. If eligible, we will contact you to opt in or out.

We look forward to receiving your revised manuscript.

Kind regards,

Michal Bosela, Ph.D.

Academic Editor

PLOS ONE

Journal Requirements:

1. When submitting your revision, we need you to address these additional requirements.

Please ensure that your manuscript meets PLOS ONE's style requirements, including those for file naming. The PLOS ONE style templates can be found at

http://www.journals.plos.org/plosone/s/file?id=wjVg/PLOSOne_formatting_sample_main_body.pdf and http://www.journals.plos.org/plosone/s/file?id=ba62/PLOSOne_formatting_sample_title_authors_affiliations.pdf

2. Thank you for stating the following in the Acknowledgments Section of your manuscript:

WT received funding from the German Research Foundation (DFG, TE 613/3-2). UB received funding from the Czech Republic Grant Agency (17-22102s).

We note that you have provided funding information that is not currently declared in your Funding Statement. However, funding information should not appear in the Acknowledgments section or other areas of your manuscript. We will only publish funding information present in the Funding Statement section of the online submission form.

Please remove any funding-related text from the manuscript and let us know how you would like to update your Funding Statement. Currently, your Funding Statement reads as follows:

The author(s) received no specific funding for this work.

3. In your manuscript, please provide additional information regarding the specimens used in your study. Ensure that you have reported specimen numbers and complete repository information, including museum name and geographic location.

If permits were required, please ensure that you have provided details for all permits that were obtained, including the full name of the issuing authority, and add the following statement:

'All necessary permits were obtained for the described study, which complied with all relevant regulations.'

If no permits were required, please include the following statement:

'No permits were required for the described study, which complied with all relevant regulations.'

For more information on PLOS ONE's requirements for paleontology and archaeology research, see "" ext-link-type="uri" xlink:type="simple">https://journals.plos.org/plosone/s/submission-guidelines#loc-paleontology-and-archaeology-research.""

4. We note that Figure 1 in your submission contain [map/satellite] images which may be copyrighted. All PLOS content is published under the Creative Commons Attribution License (CC BY 4.0), which means that the manuscript, images, and Supporting Information files will be freely available online, and any third party is permitted to access, download, copy, distribute, and use these materials in any way, even commercially, with proper attribution. For these reasons, we cannot publish previously copyrighted maps or satellite images created using proprietary data, such as Google software (Google Maps, Street View, and Earth). For more information, see our copyright guidelines: http://journals.plos.org/plosone/s/licenses-and-copyright.

We require you to either (1) present written permission from the copyright holder to publish these figures specifically under the CC BY 4.0 license, or (2) remove the figures from your submission:

You may seek permission from the original copyright holder of Figure 1 to publish the content specifically under the CC BY 4.0 license.

We recommend that you contact the original copyright holder with the Content Permission Form (http://journals.plos.org/plosone/s/file?id=7c09/content-permission-form.pdf) and the following text:

“I request permission for the open-access journal PLOS ONE to publish XXX under the Creative Commons Attribution License (CCAL) CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). Please be aware that this license allows unrestricted use and distribution, even commercially, by third parties. Please reply and provide explicit written permission to publish XXX under a CC BY license and complete the attached form.”

Please upload the completed Content Permission Form or other proof of granted permissions as an "Other" file with your submission.

In the figure caption of the copyrighted figure, please include the following text: “Reprinted from [ref] under a CC BY license, with permission from [name of publisher], original copyright [original copyright year].”

If you are unable to obtain permission from the original copyright holder to publish these figures under the CC BY 4.0 license or if the copyright holder’s requirements are incompatible with the CC BY 4.0 license, please either i) remove the figure or ii) supply a replacement figure that complies with the CC BY 4.0 license. Please check copyright information on all replacement figures and update the figure caption with source information. If applicable, please specify in the figure caption text when a figure is similar but not identical to the original image and is therefore for illustrative purposes only.

The following resources for replacing copyrighted map figures may be helpful:

USGS National Map Viewer (public domain): http://viewer.nationalmap.gov/viewer/

The Gateway to Astronaut Photography of Earth (public domain): http://eol.jsc.nasa.gov/sseop/clickmap/

Maps at the CIA (public domain): https://www.cia.gov/library/publications/the-world-factbook/index.html and https://www.cia.gov/library/publications/cia-maps-publications/index.html

NASA Earth Observatory (public domain): http://earthobservatory.nasa.gov/

Landsat: http://landsat.visibleearth.nasa.gov/

USGS EROS (Earth Resources Observatory and Science (EROS) Center) (public domain): http://eros.usgs.gov/#

Natural Earth (public domain): http://www.naturalearthdata.com/

5. We note that you have stated that you will provide repository information for your data at acceptance. Should your manuscript be accepted for publication, we will hold it until you provide the relevant accession numbers or DOIs necessary to access your data. If you wish to make changes to your Data Availability statement, please describe these changes in your cover letter and we will update your Data Availability statement to reflect the information you provide.

Additional Editor Comments:

Please note that manuscripts reporting paleontology and archaeology research must adhere to the PLOS ONE policies described at http://journals.plos.org/plosone/s/submission-guidelines#loc-paleontology-and-archaeology-research. Specifically, appropriate specimen numbers should be provided, and the specimens should be publicly deposited or available for replication of the study.

[Note: HTML markup is below. Please do not edit.]

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Yes

Reviewer #2: Partly

Reviewer #3: Yes

**********

2. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Yes

**********

3. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Yes

**********

4. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Yes

**********

5. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: The study presents an original research on dendrochronology of oak from an ancient Roman construction recently excavated in Rome. To my knowledge this is the first successful dendrochronological dating of oak from the given period in Rome. In addition to dating of the construction, the investigation enabled to define the origin of wood and revealed information on wood transport in Roman time, which has not been reported in archived documents. The study presents an excellent collaboration between wood science (dendrochronology) and humanities (archaeology) and provides enrichment for both disciplines.

Specific comments:

Lines 54, 55: please give also scientific names of wood species ebony, cedar, box, terebinth, holm oak

Lines 81-86

You can also mention recent successful application of dendrochronology related to Roman period south of the Alps like

Cufar et al. 2014. Journal of Archaeological Science (Roman barge in the Ljublanica river…)

Cufar et al. 2019. Les / Wood (Research potential of wood of barrels from Roman water wells)

Figure 1. map on the Left and on the Right

On both maps please use larger font to mark an important location (e.g. San Giovanni in Laterano) in the current version of the maps the font is too small to read the inscriptions

Line147 …. Finally, a mean TRW sample chronology

Change to …. Finally, a mean TRW chronology…

Line 149 Each comparison is based on …

Change to …was based on…

Figure 5 – Can you add a scale bar (in kilometres)? Or somewhere in the text mention approximate distance between the area of timber source and Rome

Reviewer #2: I found this an interesting, informative paper that set the results in good context, allowing the reader to understand the importance of the findings to wider fields of study. It is well written. I have a minor problem with the specificity of the conclusion that trees were felled between 40 and 53 CE - which, unless I am missing something, is not fully supported by the evidence in Table 2. If one takes the quoted sapwood range of 10-30, then C1 could go to 55CE, C17 to 58CE and C37 to 54CE, and given that sapwood estimates vary a little geographically, I think it unwise to stick rigidly to 53CE as the end point - would it not be safer to say 60CE, but then comment about any justification you may have for trimming that outer point?

line 89 - the ref to Corona 1974 does not fit comfortably in the sentence as it stands, needs a little editing so that it is clear just what is meant - presumably Corona - in a rather old reference now, states how difficult results are to obtain in Rome? Suggest this is re-worded.

line 160 Cerris? are you talking Q cerris? Unclear.

Fig 4 - presumably all DATED individual Roman oak series?

Throughout - use decimal point in t values, 6.57 not 6,57 (inconsistent at present)

The following references are used in the text but do not appear in the References list:

Janssen et al 2017

Eackstein and Bauch 1969

Momigliano and Schiavona 1988

Rea 2001

Reviewer #3: Highly interesting paper - the first scientific evidence of Roman timber trade from north-eastern France to Rome and solid confirmation of Roman long-distance timber trade. This is a confirmation of high level of wood technology and wood-working skills of the Romans, and their enormous organizational level. A few of my suggestions of minor improvements concern the introductory part:

- page 3, line 71: reference to Buengten et al. 2018 is not the happiest in this context. There were numerous scholars before, who stressed annual precision of dendrochronological dating - Douglass, Huber, Becker, Eckstein, etc.;

- page 4, line 84: Guibal focused his research on shipwrecks found south of the Alps;

- page 4, lines 88-89: Corona's investigation published in 1974 was not very successful, and had nothing to do with this study.

In general I would like to suggest to be more careful with references, and to reduce their number. Not every sentence must be supported by references.

**********

6. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #1: No

Reviewer #2: No

Reviewer #3: No

[NOTE: If reviewer comments were submitted as an attachment file, they will be attached to this email and accessible via the submission site. Please log into your account, locate the manuscript record, and check for the action link "View Attachments". If this link does not appear, there are no attachment files to be viewed.]

While revising your submission, please upload your figure files to the Preflight Analysis and Conversion Engine (PACE) digital diagnostic tool, https://pacev2.apexcovantage.com/. PACE helps ensure that figures meet PLOS requirements. To use PACE, you must first register as a user. Registration is free. Then, login and navigate to the UPLOAD tab, where you will find detailed instructions on how to use the tool. If you encounter any issues or have any questions when using PACE, please email us at figures@plos.org. Please note that Supporting Information files do not need this step.

17 Sep 2019

RESPONSE TO REVIEWERS

Reviewer #1 (R1): The study presents an original research on dendrochronology of oak from an ancient Roman construction recently excavated in Rome. To my knowledge this is the first successful dendrochronological dating of oak from the given period in Rome. In addition to dating of the construction, the investigation enabled to define the origin of wood and revealed information on wood transport in Roman time, which has not been reported in archived documents. The study presents an excellent collaboration between wood science (dendrochronology) and humanities (archaeology) and provides enrichment for both disciplines.

Author response (AR): we thank the Reviewer 1 for his nice words and the following improving observations.

Specific comments:

R1: Lines 54, 55: please give also scientific names of wood species ebony, cedar, box, terebinth, holm oak

AR: the suggestion has been accepted and the scientific name inserted in the revised text.

R1: Lines 81-86: You can also mention recent successful application of dendrochronology related to Roman period south of the Alps like

Cufar et al. 2014. Journal of Archaeological Science (Roman barge in the Ljublanica river…)

Cufar et al. 2019. Les / Wood (Research potential of wood of barrels from Roman water wells)

AR: the suggestion has been accepted and the references inserted.

R1: Figure 1. map on the Left and on the Right

On both maps please use larger font to mark an important location (e.g. San Giovanni in Laterano) in the current version of the maps the font is too small to read the inscriptions

AR: the suggestion has been accepted and a new Fig 1, clearer and better defined, has been changed. The new map is under CC-BY 4.0 license. Please visit: http://dati.lazio.it/catalog/it/dataset/carta-tecnica-regionale-2002-2003-5k-roma

R1: Line147 …. Finally, a mean TRW sample chronology

Change to …. Finally, a mean TRW chronology…

AR: the suggestion has been accepted and the text corrected.

R1: Line 149 Each comparison is based on …

Change to …was based on…

AR: the suggestion has been accepted and the text corrected.

R1: Figure 5 – Can you add a scale bar (in kilometres)? Or somewhere in the text mention approximate distance between the area of timber source and Rome

AR: At the end of the discussion a mention of the distance (over 1700 km) has been inserted. We guess that this observation has improved the text and we thank the Reviewer 1.

Reviewer #2 (R2): I found this an interesting, informative paper that set the results in good context, allowing the reader to understand the importance of the findings to wider fields of study. It is well written. I have a minor problem with the specificity of the conclusion that trees were felled between 40 and 53 CE - which, unless I am missing something, is not fully supported by the evidence in Table 2. If one takes the quoted sapwood range of 10-30, then C1 could go to 55CE, C17 to 58CE and C37 to 54CE, and given that sapwood estimates vary a little geographically, I think it unwise to stick rigidly to 53CE as the end point - would it not be safer to say 60CE, but then comment about any justification you may have for trimming that outer point?

AR: We approve this comment. We based our estimation on simple calculations which probably do not represent well the possible dates. We agree to change our estimation from 40 to 60 CE. The text has been changed accordingly.

R2: line 89 - the ref to Corona 1974 does not fit comfortably in the sentence as it stands, needs a little editing so that it is clear just what is meant - presumably Corona - in a rather old reference now, states how difficult results are to obtain in Rome? Suggest this is re-worded.

AR: We agree with this observation. The citation of Corona refers to an old attempt of dating a single board found in the Colosseum. This citation is useless here and has been removed.

R2: line 160 Cerris? are you talking Q cerris? Unclear.

AR: We refer to the taxa “section”, not species. The “section” Cerris, to which belongs Q. cerris, includes also Q. trojana and Q. aegilops. According to Cambini (1967), it shows anatomical features slightly different from section Robur (Q. robur, Q. petraea, Q. pubescens, Q. farnetto). We just tried to identify the section of the samples because it may be of some interest about the origin of the timber (some species are only Mediterranean).

R2: Fig 4 - presumably all DATED individual Roman oak series?

AR: the suggestion has been accepted and the text corrected.

R2: Throughout - use decimal point in t values, 6.57 not 6,57 (inconsistent at present)

AR: the suggestion has been accepted and the text corrected.

R2: The following references are used in the text but do not appear in the References list:

Janssen et al 2017 AR: inserted.

Eackstein and Bauch 1969 AR: inserted.

Momigliano and Schiavona 1988 AR: changed in “Momigliano, 2016”.

Rea 2001 AR: inserted.

Reviewer #3 (R3): Highly interesting paper - the first scientific evidence of Roman timber trade from north-eastern France to Rome and solid confirmation of Roman long-distance timber trade. This is a confirmation of high level of wood technology and wood-working skills of the Romans, and their enormous organizational level. A few of my suggestions of minor improvements concern the introductory part:

R3: - page 3, line 71: reference to Buengten et al. 2018 is not the happiest in this context. There were numerous scholars before, who stressed annual precision of dendrochronological dating - Douglass, Huber, Becker, Eckstein, etc.;

AR: we perfectly agree with R3. We know that we can start from the real beginning of dendrochronology (Douglass…). Anyway, we would like to stress the recent applications of this principle, which sometimes is still questioned. This is a very important issue: a lot of scientists of other disciplines still continue to have doubts about the effectiveness of dendrochronology. This citation demonstrates its effectiveness at a planetary level.

R3: - page 4, line 84: Guibal focused his research on shipwrecks found south of the Alps;

AR: the suggestion has been accepted and the text corrected.

R3: - page 4, lines 88-89: Corona's investigation published in 1974 was not very successful, and had nothing to do with this study.

AR: we accept this suggestion. For a clarification, see the answer to the R2. The citation has been cancelled.

R3: In general I would like to suggest to be more careful with references, and to reduce their number. Not every sentence must be supported by references.

AR: we do agree with this sentence!

We accepted the suggestion and we removed the citations redundant or unuseful.

Citations removed:

- Corona, 1974

- Haneca et al, 2009

- Hughes et al., 1981

- Jansma et al., 2014

- Sass-Klassen et al., 2008

- Schweingruber 1990

- Wigley et al., 1984.

Final considerations

We are grateful to the reviewers, which with their comments, suggestions and corrections helped us to improve our manuscript.

Best regards

San Michele all’Adige, 29/08/2019

Mauro Bernabei

(On behalf of all of the co-Authors)

Submitted filename: Response to Reviewers.docx

Click here for additional data file.

7 Oct 2019

DENDROCHRONOLOGICAL EVIDENCE FOR LONG-DISTANCE TIMBER TRADING IN THE ROMAN EMPIRE

PONE-D-19-20970R1

Dear Dr. Bernabei,

We are pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it complies with all outstanding technical requirements.

Within one week, you will receive an e-mail containing information on the amendments required prior to publication. When all required modifications have been addressed, you will receive a formal acceptance letter and your manuscript will proceed to our production department and be scheduled for publication.

Shortly after the formal acceptance letter is sent, an invoice for payment will follow. To ensure an efficient production and billing process, please log into Editorial Manager at https://www.editorialmanager.com/pone/, click the "Update My Information" link at the top of the page, and update your user information. If you have any billing related questions, please contact our Author Billing department directly at authorbilling@plos.org.

If your institution or institutions have a press office, please notify them about your upcoming paper to enable them to help maximize its impact. If they will be preparing press materials for this manuscript, you must inform our press team as soon as possible and no later than 48 hours after receiving the formal acceptance. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information, please contact onepress@plos.org.

With kind regards,

Michal Bosela, Ph.D.

Academic Editor

PLOS ONE

Additional Editor Comments (optional):

Reviewers' comments:

9 Oct 2019

PONE-D-19-20970R1

Dendrochronological evidence for long-distance timber trading in the Roman Empire

Dear Dr. Bernabei:

I am pleased to inform you that your manuscript has been deemed suitable for publication in PLOS ONE. Congratulations! Your manuscript is now with our production department.

If your institution or institutions have a press office, please notify them about your upcoming paper at this point, to enable them to help maximize its impact. If they will be preparing press materials for this manuscript, please inform our press team within the next 48 hours. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information please contact onepress@plos.org.

For any other questions or concerns, please email plosone@plos.org.

Thank you for submitting your work to PLOS ONE.

With kind regards,

PLOS ONE Editorial Office Staff

on behalf of

Dr. Michal Bosela

Academic Editor

PLOS ONE