The meaning of Pinus sylvestris-type charcoal taphonomic markers in Palaeolithic sites in NE Iberia

Journal of Archaeological Science: Reports 30 (2020) 102231

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The meaning of Pinus sylvestris-type charcoal taphonomic markers in Palaeolithic sites in NE Iberia

T

⁎

Ethel Alluéa,b, , Bàrbara Masa a b

IPHES, Institut Català de Paleoecologia Humana i Evolució Social, Zona Educacional 4, Campus Sescelades URV (Edifici W3), 43007 Tarragona, Spain Àrea de Prehistòria, Universitat Rovira i Virgili (URV), Avinguda de Catalunya 35, 43002 Tarragona, Spain

A R T I C LE I N FO

A B S T R A C T

Keywords: Charcoal analysis Charcoal taphonomy Pinus sylvestris type Palaeolithic Fuel Environment

The aim of this work is to present the results of the classification of wood anatomy alterations on Pinus sylvestristype charcoal remains and to analyse the quantitative results from Palaeolithic sites in NE Iberia. The high relative percentages of Pinus sylvestris-type charcoal fragments in these anthracological records has enabled us to systematically observe the taphonomic markers producing alterations in the wood cell structure. The analysis is based on a large collection of Pinus sylvestris-type charcoal remains from eight sites. All the assemblages present values above 70%. The observed alterations have been regrouped into biological alterations related to natural processes, combustion alterations, and post-depositional alterations. The results show differences in the relative presence of the markers, which are difficult to interpret in terms of environmental or cultural processes related to fuel exploitation. However, this study suggests that charcoal taphonomy is a useful tool for supporting previous interpretations of fire-management activities related to firewood gathering.

1. Introduction

Woody plants that are used as fuel and recovered as charcoal in archaeological contexts undergo various processes from their growth to their recovery from an archaeological site (Fig. 1). These include the growth of a plant, its decay, carbonisation, post-deposition processes and recovery, and can give rise to a variety of alterations due to multiple, overlapping factors (Fig. 1). During the growth of trees and shrubs, modifications in the structure of the wood may be related to stress resulting from specific environmental conditions (Schweingruber, 1988, 2007). Tree management, such as pruning, can also cause variations in the wood structure, primarily affecting growth rings (Thiébault, 2006; Dufraisse et al., 2018) (Fig. 1). In today’s context, air pollution can also affect wood anatomy (Wodzicki, 2001). During the growth of the plant and after the wood is dead, it may be affected by macro- and microorganisms that alter the cell structure. Signs of decay, produced by organisms such as fungi and insects, are diverse and can affect anything from the entire plant to just a small part (Gorczynski and Molki, 1969; Schweingruber, 1988, 2007; Pyle and Brown, 1999; Henry and Théry-Parisot, 2014; Vidal-Matutano et al., 2019). Carbonisation causes shrinkage, fragmentation and deformation of the wood’s cell structure, depending on the previous state of the wood (Rossen and Olson, 1985; Prior and Gasson, 1993; Slocum et al., 1978; Loreau, 1994; Théry-Parisot and Henry, 2012) (Fig. 1). Post-depositional processes can affect fragmentation, charcoal dispersal, and cell-structure modification as a consequence of the effect of water, roots, freeze/thaw,

Charcoal taphonomy is an anthracological research method aimed at understanding the formation processes of charcoal assemblages, from the gathering of wood to its recovery at archaeological sites (ThéryParisot et al., 2010). Charcoal taphonomy issues are approached from different perspectives that aim to clarify methodological aspects and interpret anthracological assemblages. Methodological aspects involve overall fragmentation, which is the quantitative basis for the anthracological analyses, as well as charcoal preservation (Gorczynski and Molki, 1969; Chabal, 1992, 1997; Badal, 1992; Chabal et al., 1999; Chrzazvez et al., 2014; Arranz-Otaegui, 2017). Interpretative issues are primarily related to understanding the preservation and condition of the wood used, and these shed light on the use of fuel (Allué et al., 2007; Théry-Parisot, 2001; Allué, 2002a, 2002b; Allué et al., 2009, 2017a; Théry-Parisot et al., 2010; Alcolea, 2017, 2018; Alcolea et al., 2017; Vidal-Matutano et al., 2015; Vidal-Matutato, 2017, 2018). Some of these aspects have been analysed on the basis of charcoal cell structure alterations to identify different markers (Théry-Parisot, 2001, 2002; Allué, 2002a, 2002b; Théry-Parisot et al., 2010; Henry and Théry-Parisot, 2014). By understanding the agents, their mechanisms and formation processes, we are able to interpret the causes of the alterations and characterise the wood (Fig. 1) (Théry-Parisot, 2001, 2002; Allué, 2002a; Allué et al., 2009; Henry and Théry-Parisot, 2014). ⁎

Corresponding author. E-mail address: [email protected] (E. Allué).

https://doi.org/10.1016/j.jasrep.2020.102231 Received 30 June 2019; Received in revised form 13 December 2019; Accepted 21 January 2020 2352-409X/ © 2020 Elsevier Ltd. All rights reserved.

Journal of Archaeological Science: Reports 30 (2020) 102231

E. Allué and B. Mas

Fig. 1. Processes affecting wood anatomy cell structure alterations from growth of the plant to recovery at the site (modified from Allué et al., 2009).

compared with other assemblages. The results and the assemblage comparison will enable us to evaluate the importance of the taphonomic marker values obtained and determine whether different sites can be directly compared.

and sediment pressure (Rodríguez-Ariza, 1993; Chrzazvez et al., 2014) (Fig. 1). Finally, fragmentation may also occur when charcoal is recovered from an archaeological site (Chrzazvez et al., 2014; ArranzOtaegui, 2017) (Fig. 1). Applying taphonomic techniques to charcoal from Palaeolithic sites, including classifying the anatomical features of the wood, was initially used to provide further information on wood-gathering strategies (Théry-Parisot, 2001; Théry-Parisot et al., 2010; Vidal-Matutano et al., 2015). The study of anatomical alterations in charcoal assemblages was first applied to Palaeolithic anthracological assemblages in experimental archaeology and ethnoarchaeology works (Allué et al., 2007a, 2007b; Caruso-Fermé and Théry-Parisot, 2011; Henry and ThéryParisot, 2014; Théry-Parisot, 2001; Mas et al., 2017). Ongoing research based on experimental approaches, mainly promoted by the CEPAM working group in Antibes (France), has provided important information on how alterations are formed, as well as their interpretation (ThéryParisot et al., 2010; Théry-Parisot and Henry, 2012; Henry and ThéryParisot, 2014; Vidal-Matutano et al., 2017; Caruso-Fermé and ThéryParisot, 2018). Specific descriptions and the study of charcoal assemblages from this perspective have helped shed light on certain significant aspects, such as wood decay and combustion processes (McParland et al., 2010; Moskal-del Hoyo et al., 2010; Théry-Parisot and Henry 2012; Henry and Théry-Parisot, 2014). In European contexts, conifers are widespread and most anthracological sequences contain significant quantities of montane pines (see among others: Théry-Parisot, 2001; Marquer et al., 2010; Badal et al., 2012, 2013; Allué et al., 2018). The most important sites are found in various locations on the Iberian Peninsula (Vidal-Matutano, 2017; Uzquiano, 2008, 2009, 2014; Uzquiano et al., 2008, 2012; VidalMatutano, 2018; Vidal-Matutano et al., 2015, 2017, 2018; Allué et al., 2012, 2013, 2017a, 2017b, 2018; Badal et al., 2012, 2013; Alcolea et al., 2017; Alcolea, 2017, 2018; Monteiro et al., 2017; Mas, 2018). The recurring presence of montane pines has enabled alterations to be identified based on equivalent wood anatomy. This facilitates systematic experimental work and improves descriptions of the anatomical features, in addition to comparison between assemblages. The objective of this paper is to present anthracological assemblages from NE Iberia in which alterations have been identified and quantified. The cell structure alteration in some of these assemblages has been described previously (Allué, 2002a, 2002b; Allué et al., 2007, 2009, 2017a, 2017b; Mas, 2018), but not evaluated quantitatively or

2. Materials and methods The sites included in this study are located in NE Iberia, in different biogeographical areas from the coast to the Pre-Pyrenees (Table 1, Fig. 2). The sequences are framed within the Upper Pleistocene, with chronocultural layers corresponding to the Late Middle Palaeolithic, Early and Late Upper Palaeolithic, and Epipalaeolithic (Table 1). These assemblages contain a high number of Pinus sylvestris-type fragments, with a relative occurrence above 70% (Table 1). The number of fragments analysed per layer varies between 101 and 570 (Table 1). The recurrence of wood from Pinus sylvestris-type allows us to compare taphonomical markers affecting equivalent cell structures. Pinus sylvestris-type is a taxonomic category comprising at least three different mountain pines, and it has frequently been used in anthracological records in various ways (Pinus sylvestris/nigra ssp. salzmannii, Pinus sylvestris/nigra, Pinus sylvestris-type or Pinus t. sylvestris). This category includes the Pinus nigra spp. salzmanni, Pinus sylvestris and Pinus uncinata that are currently distributed across NE Iberia according to the present-day biogeographical setting (Folch i Guillén, 1986; Quézel and Médail, 2003; Roiron et al., 2013). Pinus nigra spp. salzmannii is found at elevations of between 500 and 1200 m.a.s.l. in the Supra-Mediterranean or Oro-Mediterranean belts. Pinus sylvestris has the widest distribution, probably favoured by the action of humans, and is found primarily at elevations of between 800 and 1700 m.a.s.l. in the Oro-Mediterranean belt. Pinus uncinata is restricted to the Pyrenees, at elevations above 1800 m.a.s.l. (Folch i Guillén, 1986; Quézel and Médail, 2003). The wood of these three species shares the same anatomical properties (Schweingruber 1990; Esteban et al., 2000): It is homoxylous with resin canals in the late wood; in tangential section, it presents uniseriated rays of up to 10 cells, with occasional tangential resin canals; the radial section presents fenestriform pits in the crossfields and dentate traqueid walls; and the longitudinal traqueids bear areolate punctuations. We observed and quantified the following alterations: Tension wood or compression wood is an alteration usually observed in conifer traqueids. It is a growth of the traqueid walls in a 2

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Table 1 Chronology, biogeographical data and anthracological results regarding the values of Pinus sylvestris type in the sites analyzed in this paper. Pinus sylvestris type Site

Chrono-culture

Altitude m.a.s.l.

Layer

N.

%

Total

Sampling method

Reference

Abric Romaní

Middle Palaeolithic

350

1157

Allué et al., 2013

Epipalaeolithic Early Upper Palaeolithic Middle Palaeolithic Late Upper Palaeolithic

760 120

Handpicking/flotation Hand picking

Allué et al., 2007 Allué et al., 2017b

Handpicking/flotation

Allué et al., 2018

Cova Gran

Middle Palaeolithic

480

Molí del Salt

Late Upper Palaeolithic

350

Handpicking/flotation

Allué et al., 2010

Parco Parco

Epipaleolithic Late Upper Palaeolithic

420

Handpicking

Allué et al., 2013

Riera dels Canyars

Middle Palaeolithic

28

254 497 195 567 203 318 302 284 1171 447 282 210 485 101 127 231 379 294 367 277 205

Hand picking/flotation

Balma del Gai (I) Coll Verdaguer (1) Coll Verdaguer (2) Cova Gran

92.52 80.68 87.69 86.77 83.74 98.11 96.03 98.59 38.60 81.21 84.04 87.62 83.71 87.13 67.72 66.67 82.32 46.94 80.11 86.28 87.32

Allué et al., 2017a

Epipalaeolithic

235 401 171 492 170 312 290 280 452 363 237 184 406 88 86 154 312 138 294 239 179

Hand picking

Balma de Guilanyà

M O Oinf P Q E J K I U. 1 U. 2 5P 6P S1C S1D B1 B2 II III IV I

Handpicking

Allué et al., 2017b

480

and Théry-Parisot, 2014). Four different states of decay have been defined in conifers, from AL0 to AL3, where AL0 represents undecayed wood and AL3 is highly decayed wood (Henry and Théry-Parisot, 2014). Although this classification has not been used in this study, the presence of collapsed cells has been recorded, corresponding to level AL3 (Fig. 3c). The most common and well-described alterations produced by combustion are radial cracks. This type of alteration has been related to the combustion of green wood, according to the number of radial cracks per cm3 (Henry and Théry-Parisot 2014). However, applying this to small samples is difficult, and it is challenging to quantify the cracks with sufficient accuracy and to demonstrate a direct relationship between the presence of these cracks and the use of green wood (Henry and Théry-Parisot, 2014). In this work, we have considered the presence of radial cracks and other combustion alterations to be related to the general deformation of the structure (Fig. 3d). The identification of combustion alterations is based on the observation of experimental materials (Allué, 2002a; Allué et al., 2007). However, sometimes this can be inaccurate as combustion overprints previous alterations. Positive identification can only be made when the deformation is clearly an effect of the combustion process (Fig. 3d). Vitrification is a recurrent alteration involving the fusion of cells, resulting in a glassy appearance (Fig. 3e). Vitrification has been associated with several processes, contexts, and characteristics of the wood (e.g., green wood or kilns) (Théry-Parisot, 2001; Marguerie and Hunot, 2007; Allué et al., 2009; McParland, et al., 2010; Courty et al., 2012; Vidal-Matutano et al., this volume; Courty et al., 2020). However, to date, experimental work has not succeeded in reproducing the effect. It is likely that this alteration is only produced under specific temperatures and combustion conditions. In this study, post-depositional alterations have not been specifically identified (Fig. 3f). This category includes effects produced by roots, sediment inclusion related to the action of water, and freeze/ thaw processes. These were identified as a generic post-depositional alteration category as the specific origin cannot always be accurately determined. Fragmentation occurs during combustion, the post-depositional period, and sampling, and is one of the most complex and important aspects of anthracology (Chabal, 1992; Chabal et al., 1999; ArranzOtaegui, 2017), as quantification is based on the number of fragments present. Indeed, quantification based on fragment numbers is a widely

Fig. 2. Map showing the location of the sites included in this study.

helical thickening-like pattern. It is related to stress and might occur more often in branches, but it is also seen in the trunks of specimens growing on pronounced slopes (Schweingruber, 1988) (Fig. 3a). Wood decay can be caused by physical and chemical processes and by attack from wood-decomposing fungi and xylophagous insects (Gorczynski and Molki, 1969; Moskal-del Hoyo et al., 2010; CarusoFermé, 2012; Henry and Théry-Parisot, 2014; Vidal-Matutano et al., 2017). Microorganisms can affect uncombusted wood as well as charcoal remains (Badal 2004; Moskal-del Hoyo et al., 2010; Caruso-Fermé, 2012). In this work, we only consider pre-burning alterations. In the assemblages from this study, these are identified in different forms in charcoal remains as occasional alterations caused by fungi, showing degradation that is either partial or extended throughout the wood cell structure, modifying the traqueids (Fig. 3b). Collapse of the cells primarily affects late wood; this deterioration of the wood depends on chemical and physical changes, not the effects of microorganisms. According to Gorczynski and Molki (1969), cellulose chains in secondary cell walls depolymerise quickly, and cellulose loses its physical bonds with lignin leading to a loss of rigidity. The only quantitative indicator that has been developed to date is the “alteration level” (AL), corresponding to different degrees of wood decay (Henry 3

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Fig. 3. SEM and ESEM images showing different alterations on Pinus sylvestris type cell structure. a) Radial section of a Pinus sylvestris type charcoal fragment from Abric Romaní showing tension wood. b) Cross section of a Pinus sylvestris from Abric Romaní type charcoal fragment showing punctual sings of decay. c) Cross section of a Pinus sylvestris type fragment from Balma del Gai showing collapsed cells. d) Cross section of a Pinus sylvestris type fragment from Abric Romaní showing radial cracks. e) Cross section of a Pinus sylvestris type fragment from Abric Romaní showing vitrification. f) Cross section of a Pinus sylvestris type fragment from Balma del Gai showing a postdepositional alteration.

was identified in most of the layers, with the exception of Balma del Gai, and the percentage of fragments affected never exceeds 10% (Fig. 6). Vitrification was observed in most of the layers, affecting between 0.4% and 13% of the fragments (Fig. 7). Significant differences were observed at Abric Romaní, for example, where some layers presented very low percentages of vitrification while others had higher values. Combustion effects, including cracks, were also distributed irregularly among the samples (Fig. 8). The highest number of fragments affected, more than 20%, occurs at Parco II. Differences were also observed between layers from the same site, for example, at Abric Romaní and Balma de Guilanyà. Finally, post-depositional alterations irregularly affect the different layers. Layers J and K from Balma de Guilanyà present values above 30%, whereas the other sites and layers have lower values, between 0% and 22% (Fig. 9). The sites studied were processed using different sampling methods. In addition, the number of fragments differs from layer to layer. The comparison between the number of taxa and number of charcoal fragments varied according to factors such as the number of fragments studied, the sampling methods, and the diversity of the original plant community. Less diversity was documented in the Middle Palaeolithic (MP) and Early Upper Palaeolithic (EUP) assemblages, whereas the Late Upper Palaeolithic (LUP) and Epipaleolithic (EPI) sites presented more diversified assemblages (Fig. 10). In samples from sites where flotation techniques were employed, the diversity is generally higher (Fig. 10).

used technique that has been proved to be the best method for validating a reliable charcoal assemblage (Chabal, 1988, 1992). Anthracological analyses also require a minimum number of fragments to produce a reliable data set, generally between 250 and 400 fragments per layer (Chabal, 1992; Théry-Parisot et al., 2010). Context and geographical area might affect the minimum number of charcoal fragments that need to be studied to obtain a reliable record (Théry-Parisot et al., 2010; O’Carroll and Mitchell, 2012). In Palaeolithic sites, the scarcity of charcoal and diminished diversity found in some cases has led to the presence of fewer fragments being considered reliable (Uzquiano, 1997; Allué et al., 2017a). Sampling methods based on water sieving and size classes (2 mm, 3 mm, 4 mm) can also affect diversity, and therefore reliability, and provide greater accuracy in anthracological studies of Palaeolithic sites (Vidal-Matutano, 2018). In this study, we evaluated the quantitative results of the anthracological records with regard to the number of charcoal fragments, sampling methods, and diversity.

3. Results The results of the observations are given as relative values of each alteration type (as described in the Materials and Methods section). Across all the samples, tension wood represents 10–45% of the alterations (Fig. 4). Layers K and J from Balma de Guilanyà, Parco layer III, and Molí del Salt present the highest values, between 30% and 45%. Lower values, between 10% and 20%, are seen in Abric Romaní, Cova Gran, and Later E from Balma Guilanyà. Signs of decay were observed irregularly across the various sites (Fig. 5, Fig. 6). For example, at Abric Romaní, the values vary from 60% in layer M to 20% in layer Qa. Other sites present similar decay values between the different layers at the same site, with values lower than 20%. Decay alterations rarely affect an entire assemblage. Cell collapse

4. Discussion The anthracological records from NE Iberia reveal a significant occurrence of Pinus sylvestris-type fragments at sites dated between 60 and 11 ky BP (Allué et al., 2012, 2013, 2017a, 2017b, 2018; Mas, 2018). 4

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Fig. 4. Graph representing values of tension wood identified in the studied sites. AR: Abric Romaní; BG: Balma de Guilanyà; GAI: Balma del Gai; CV: Coll Verdaguer; CG: Cova Gran; MS: Molí del Salt; PAR: Parco; RC: Riera dels Canyars.

Fig. 5. Graph representing values of decay markers identified in the studied sites. AR: Abric Romaní; BG: Balma de Guilanyà; GAI: Balma del Gai; CV: Coll Verdaguer; CG: Cova Gran; MS: Molí del Salt; PAR: Parco; RC: Riera dels Canyars. 5

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Fig. 6. Graph representing values of collapse cells evidences identified in the studied sites. AR: Abric Romaní; BG: Balma de Guilanyà; GAI: Balma del Gai; CV: Coll Verdaguer; CG: Cova Gran; MS: Molí del Salt; PAR: Parco; RC: Riera dels Canyars.

other species (Chrzazvez et al., 2014; Allué et al., 2007). In Balma del Gai, charcoal fragments were both counted and weighed and only very slight differences were found according to the sampling and counting method (Allué et al., 2007) (Fig. 11). The values are higher when counting the number of fragments from the sieved fraction. In contrast, the values were lower with the weight measurement. The fragility of Pinus sylvestris-type wood affects the values when studying samples from the sieving fraction. This is different for other taxa, for example, for Acer sp. the values from the handpicked count based on the number of fragments are higher than the weight values (Allué et al., 2007). This difference was equivalent for the two layers analysed (Fig. 11). Prunus shows differences in total counting and weight values. The values obtained based on number of fragments are over 20%, whereas the weight values are less than 20%. This suggests differences between the species and their corresponding fragmentation rates. However, in general, these results do not affect the final interpretation (Allué et al., 2007). Finally, the data suggest that sieving leads to the recovery of a greater

The diversity of these assemblages is low, but becomes increasingly higher in Late Glacial deposits. Changes in diversity are related to the composition of the forests responding to environmental changes. Additionally, the type and length of an occupation, as well as other aspects related to wood selection for fuel, might affect the composition of an assemblage (Uzquiano, 2008, 2009, 2014; Uzquiano et al., 2008, 2012; Vidal-Matutano et al., 2015, 2017; Vidal-Matutano, 2018; Allué et al., 2012, 2017a, 2017b, 2018; Badal et al., 2012, 2013; Alcolea et al., 2017; Alcolea, 2017, 2018; Monteiro et al., 2017). However, fragmentation caused by different processes might also affect the representation of these records (Vidal-Matutano et al., 2015, 2017; VidalMatutano, 2018). Furthermore, for anthracological analyses, the number of charcoal fragments studied from a certain size class (4 mm or 2 mm) has been defined as a major determinant of diversity (Badal, 1992; Chabal, 1988, 1992; Théry-Parisot et al., 2010). Pinus sylvestris-type wood can produce fragile charcoal that can be easily hyper-fragmented and provide higher numbers of fragments than

Fig. 7. Graph representing values of vitrification identified in the studied sites. AR: Abric Romaní; BG: Balma de Guilanyà; GAI: Balma del Gai; CV: Coll Verdaguer; CG: Cova Gran; MS: Molí del Salt; PAR: Parco; RC: Riera dels Canyars. 6

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Fig. 8. Graph representing values of combustion alterations identified in the studied sites. AR: Abric Romaní; BG: Balma de Guilanyà; GAI: Balma del Gai; CV: Coll Verdaguer; CG: Cova Gran; MS: Molí del Salt; PAR: Parco; RC: Riera dels Canyars.

Fig. 9. Graph representing values of postdepositional alterations identified in the studied sites. AR: Abric Romaní; BG: Balma de Guilanyà; GAI: Balma del Gai; CV: Coll Verdaguer; CG: Cova Gran; MS: Molí del Salt; PAR: Parco; RC: Riera dels Canyars. 7

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Fig. 10. Graph representing the relation between the number of fragments and number of taxa in the studied sites according to the period and sampling methods.

Fig. 11. Graph representing Pinus sylvestris type values according to the type of counting and sampling from two layers from Balma del Gai.

et al., 2012, 2013, 2017a, 2017b, 2018; Badal et al., 2012, 2013; Alcolea et al., 2017; Alcolea, 2017, 2018; Monteiro et al., 2017). In NE Iberia, even though there may have been biogeographical differences, montane pines were distributed from the coast to the Pre-Pyrenees (Allué et al., 2018). These forests were probably groves on slopes, forming part of a mosaic landscape that also comprised other plant communities, such as river forests or open grasslands (Burjachs et al., 2012; Allué et al., 2018). These other plant communities are usually not recorded in anthracological records as they do not include woody plants, or because trees found in the river forests were not abundant near the occupation sites. According to our results, the only alteration described that is directly related to general environmental constraints is compression wood, which is produced under conditions of growth stress and predominantly occurs in branches that grow under pressure, for example, the burden of permanent heavy snow or on slopes. The results show no specific differences between sites that would indicate differences according to the geomorphology of the site locations. Although the sites are located at different altitudes and, more precisely, on slopes or lower valleys, there is no correspondence between biogeographical changes and the percentage values of tension wood. However, this evidence could be related to which branches were preferentially gathered for fuel. At Abric Romaní, the evidence of wood imprints suggests the use of branches ranging in size from small twigs to others up to 10 cm

diversity of taxa, but can also favour fragmentation of the charcoal, leading to slightly higher values. Beyond the number of fragments, the extent of the sampling across an excavation surface involving either part or all of an occupation layer could also affect diversity (Mas, 2018; Vidal-Matutano et al., 2017). However, Pinus sylvestris-type charcoal values in Palaeolithic contexts from Iberia are still significant according to data from a number of sites where different sampling methods are used (Table 1). However, the diversity of the assemblages may be affected by the sampling method, with greater diversity been recorded at LUP and Epipaleolithic sites where flotation techniques have been employed. In contrast, the diversity recorded at Middle Palaeolithic sites does not appear to depend on the sampling method (Fig. 10). The diversity observed increases significantly at LUP and Epipaleolithic sites as a consequence of environmental changes that caused the spread of mesophyllous taxa (Allué et al., 2007, 2010, 2012). Environmental records from the Upper Pleistocene based on various proxies suggest climate variations (see among others Carrión et al., 2003; Tzedakis et al., 2007; González-Sampériz et al., 2010; Burjachs et al., 2012; López-García et al., 2014; Carrión et al., 2018). Most of the environmental records indicate generally colder temperatures than today, favouring the spread of conifer communities throughout the Iberian Peninsula (Uzquiano, 2008, 2009, 2014; Uzquiano et al., 2008, 2012; Vidal-Matutano et al., 2015, 2017, Vidal-Matutano, 2018; Allué

8

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76760-C3-1-P and the Generalitat de Catalunya research project 2017SGR836. Fieldwork and research from the sites included in this paper is supported by Spanish Government projects (HAR2016-75124, HAR2016-76760-C3-1-P, HAR2017-86509), Generalitat de CatalunyaAGAUR (2017SGR-0011, SGR2017-1357) and Servei d’ArquelogíaGeneralitat de Catalunya (CLT009/18/00053, CLT009/18/00054, CLT009/18/00022; 2014/100639, CLT009/18/000242014/100479). We would like to thank the editors in chief of this journal, the co-editors of this volume and the two anonymous reviewers that provided comments that helped to improve the manuscript.

diameter (Solé et al., 2013). This might be related to gathering strategies according to the abundance and availability of pine wood. Wood anatomy alterations allow us to characterise the wood and identify selection patterns beyond taxa (Théry-Parisot et al., 2010). The comparison between different sites related to alteration according to the use of different wood qualities is uneven. Signs of decay, including collapse and other features, present low values and do not show a specific pattern (Figs. 5, 6). The data suggests that there was no specific selection regarding wood quality, and decayed wood was neither rejected nor favoured. The length of the occupations and the timing of the abandonment of the settlements could be related to the fuel available in the surroundings of the sites. In the first phase of an occupation, there would be more dead wood available, and this would decrease throughout the occupation. However, the evidence is unable to demonstrate any direct relationship between the percentages of decayed wood and occupation length. It is not expected that green wood would be gathered in forests with high rates of dead wood, such as montane pine forests. In this sense, the occurrence of cracks cannot be explained by the presence of green wood. Other combustion alteration effects, such as vitrification, do not show specific patterns and are present in low values at all the sites. In general, sites with higher rates of decayed wood, show higher combustion alteration values. Both alteration types probably indicate the quality of the wood in terms of decay and preservation. In summary, the alterations observed in these assemblages reinforce the hypothesis on the gathering of branches and twigs, which states that, when selecting firewood, the quality related to the degree of decay is random. Further analyses for each site including spatial analyses (Vidal-Matutano et al., 2017; Mas, 2018), or comparisons with the type and length of the occupations, might shed light on the interpretation of specific aspects regarding selection of wood for fuel.

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5. Conclusion Previous studies have established the use of taphonomy in anthracology as an important tool for understanding the formation of charcoal assemblages. The observation and quantification of wood anatomy alterations as a consequence of natural or anthropogenic agents can yield information related to the environment and fuel gathering strategies among hunter-gatherers. The identification of alterations at eight Palaeolithic sites in NE Iberia has enabled us to evaluate the relative values of the taphonomic markers. The comparison between sites shows no patterns related to environmental constraints, where tension wood is the only identified marker. Other markers such as post-depositional processes related to environmental constraints have not been identified. Low percentages of wood decay have been identified, suggesting that there are no specific patterns within the sites. However, the random presence of tension wood, as well as decayed and undecayed wood, suggests that fuel gathering was based on the utilisation of the most abundant and freely available wood. Post-depositional processes and sampling have very little effect on these assemblages, enabling anthracological analyses to be performed on all of them. Further experimental studies, spatial analyses, and precise identification of the causes of cell structure alteration will provide new insights into the taphonomical markers found in charcoal assemblages. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements Research by E. Allué is funded by the Spanish government research projects MICINN-FEDER PGC2018-093925-B-C32, MINECO HAR20169

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