Long-term persistence of Mediterranean pine forests in the Duero Basin (central Spain) during the Holocene: The case of Pinus pinaster Aiton

Taphonomical considerations on soil charcoal deposition in sandy environments

In the sedimentary environment in which the palaeosols developed, the older sediment layers are below the more recent ones. This is supported by the radiocarbon dates obtained from different organic layers located at different depths of the same sedimentary profile (i.e. NAV-AS and PIN, see Table 1). In addition, two charcoal fragments (SANCH A1 and SANCH A2) from a single tilted palaeosol that were radiocarbon dated, yielded the same age (see Table 1), thus supporting the existence of this kind of stratification. This finding is of great importance when interpreting the soil charcoal record, as the charcoal fragments found in a charcoal layer seem to be contemporary. This is not the case in other environments where there is a lack of stratification with mixing of charcoal fragments of very different ages. This occurs for example in alpine and subalpine soils through soil bioturbation, tree uprooting and freeze-thaw processes (Carcaillet, 2001, 2007; Talon, 2010).

Long-term stability of Pinus pinaster forests in the Duero Basin

In this paper, we present the results of a comprehensive soil charcoal study showing that the maritime pine has been present over extensive areas of the southern Duero Basin along most of the Holocene (see Figures 1, 3). The genus Pinus was the only taxon identified. Because the analysed charcoal fragments were always longer than 1 mm, it is likely that their origin was very close to the deposition site (Clark et al., 1998; Lynch et al., 2004). The exclusively local origin, together with the homogeneous environment and the limiting conditions for vegetation development that are characteristic of sandy soils, may explain the low taxonomic diversity. This contrasts markedly with the anthracological record for Pintia (Figure 1), a Vaccaean (Iron Age) settlement where P. pinaster and the P. pinaster/pinea group were also the dominant taxa (Hernández et al., 2011) but the taxonomic richness is strikingly higher, probably because of environmental diversity (Hernández et al., 2011) and/or to anthropogenic bias (Rubiales et al., 2011).

Dominance of pine forests in this inland Iberian area throughout the Holocene has been previously shown in pollen sequences from Espinosa de Cerrato (Franco-Múgica et al., 2001), El Carrizal (Franco-Múgica et al., 2005; Figure 5b) and Camporredondo (García-Antón et al., 2011), where Pinus pollen was dominant for the last 10,000 years. On the basis of these sequences, it has been suggested that P. pinaster has been the main pine species since c. 8500–9000 cal. yr BP, following an expansion associated with the Holocene climatic amelioration (García-Antón et al., 2011). The soil charcoal record for Tierra de Pinares shows that the maritime pine has been the most widespread species on the dune fields and aeolian sand sheets of this region since at least c. 6500 cal. yr BP (Figure 5a), persisting until today. This new palaeobotanical evidence joins a small set of P. pinaster charcoal fragments from Hontalbilla (Figure 1) with an age 1700–1290 cal. yr BP (Alcalde et al., 2004). Data from archaeological sites also support the Holocene importance of the maritime pine in this geographical framework: P. pinaster charcoal fragments with an age of 7400–5300 cal. yr BP were recovered from Neolithic levels of the Cueva de la Vaquera (Figure 1) (López-García et al., 2003), whereas in the Iron Age (2800–2300 cal. yr BP) anthracological assemblage of Pintia (Figure 1), this pine species clearly represents the most common taxon (Hernández et al., 2011). Finally, the available historical record demonstrates considerable economic and ecological importance of the pine forests in this region since at least the Middle Ages (Calonge, 2003; Gil, 1991, 2008). The long-term persistence of these pine forests may be explained by the unsuitability of the sandy soils for crop cultivation and animal husbandry. Moreover, broadleaved trees are less competitive than conifers in continental Mediterranean areas, such as the eastern part of the Duero Basin (Costa et al., 1997).

Phylogeographical studies have also supported an ancient presence of the maritime pine in the Duero Basin. The gene pool of P. pinaster populations of the Duero Basin is different from that of P. pinaster stands from other relatively close areas, which could be due to long-term isolation during the Pleistocene (de Lucas et al., 2009). Unfortunately, it has not yet been possible to test this hypothesis, as there is no fossil evidence of P. pinaster during the Late Pleistocene in the Tierra de Pinares. The absence of Late Pleistocene sites is probably associated with arid climates.

Pinus sylvestris L. and Pinus nigra Arnold have also been present in the pine forests of the Duero Basin during the Holocene. Dominance of these species during the early Holocene has been suggested based on regional pollen sequences. From the early- to mid-Holocene transition onwards, these cryophilous pines would have been replaced by the maritime pine, a more mesic pine (García-Antón et al., 2011). Furthermore, the macrofossil record provides evidence of the presence of P. sylvestris and P. nigra at several low-altitude sites on the eastern edge of the Duero Basin, during both the early Holocene (García Antón et al., 1995) and the mid to late Holocene (García-Amorena et al., 2011; Roig et al., 1997). In our study area, the P. sylvestris/nigra type dominates the anthracological assemblage of the Cueva de la Vaquera (mid Holocene; López-García et al., 2003), it is also present in Hontalbilla (late Holocene; Alcalde et al., 2004) and is well represented in Pintia, an Iron Age settlement (late Holocene; Hernández et al., 2011). Our soil charcoal data set lacks the Pinus sylvestris/nigra type, which is a remarkable difference compared with those sites discussed previously. This striking feature of the Tierra de Pinares soil charcoal record could be caused by the fact that both the Cueva de la Vaquera and Pintia are sites where human inhabitants collected the wood over a potentially wide area. This area could have included a variety of habitats with different soil properties and topographic features, where there would have been sites where humidity and/or soil moisture was higher (e.g. north-facing slopes, riparian corridors, alluvial plains), allowing the survival of this/these pine species. Thus, the relict stands of P. sylvestris and P. nigra that currently remain along the River Cega are located at the bottom of the valley or on slopes where groundwater discharge occurs (Guerra et al., 2006). In contrast, the soil charcoal data presented in this report are of a purely local origin and were collected in a completely homogeneous environment. The finding of several P. sylvestris/nigra type charcoal fragments in Hontalbilla suggests a very localised regional persistence of this taxon until the late Holocene at sites other than the relict populations of the River Cega and Navazo de los Valsaínes (Soriano et al., 2002).

Our data significantly extend the time period for the Holocene presence of P. pinaster in the Duero Basin in an extensive area of approximately 375 km². The presence of the maritime pine in the Tierra de Pinares area since at least 6000 cal. yr BP constitutes definitive proof of its autochthony, which was previously suggested on the basis of palaeobotanical (Franco-Múgica et al., 2005; García-Antón et al., 2011; Hernández et al., 2011), historical (Calonge, 2003; Gil, 1991, 2008) and geobotanical data (Costa et al., 1997; Font Quer, 1954). The evidence contradicts the opinion expressed by several phytosociologists who assumed an artificial origin (Navarro and Valle, 1987) or a limited importance with respect to the ‘potential natural vegetation’ (Rivas Martínez, 1987) for these pine forests.

Pinus pinaster: a ‘versatile’ species during the Iberian Holocene

Maritime pine is currently the most widespread conifer on the Iberian Peninsula, where it grows in a great diversity of habitats and geographical areas (Alía et al., 1996; Costa et al., 1997). Its extensive natural distribution area was increased significantly by widespread afforestation during the 20th century (Alía et al., 1996). This is probably the main reason for the widespread opinion that exists discounting an important role of P. pinaster under ‘natural’ conditions (e.g. Peinado et al., 2008; Rivas Martínez, 1987), despite its dominance in several Iberian natural forest stands (Costa et al., 1997). Consequently, the only potential vegetation series presenting maritime pine forests as the most developed vegetation type (Valle, 2003) is restricted to areas with serpentine soils, which are characterised by poor productivity and toxicity for plants (Brady et al., 2005). However, the palaeobotanical record is helping to clarify the Holocene distribution of P. pinaster in Iberia, as well as its importance in Iberian forest ecosystems.

Anthracological investigations from central and northern Portugal have shown that the maritime pine has been widespread throughout the Holocene (Figueiral, 1995; Figueiral and Bettencourt, 2004; Tereso, 2009; Figure 6). There is evidence of a local presence even in the River Tagus estuary where García-Amorena et al. (2007) reported finding several P. pinaster stumps with an age of 7430–7930 cal. yr BP (García-Amorena et al., 2007; Figure 6). The anthracological studies suggest that this pine was present at many sites throughout Portugal and that it was an important species in the landscape, usually accompanied by other Mediterranean trees and shrubs (Figueiral, 1995). In this geographical framework, P. pinaster has demonstrated ecological plasticity, as there is Holocene evidence supporting its presence from the Atlantic shoreline to the mountains in the north, where its lower abundance could be due to the suitability of these areas for deciduous forests (Figueiral, 1995).

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Figure 6.

Map showing the main palaeobotanical sites where P. pinaster has been identified in Iberia during the Holocene. Macrofossil sites: A and B: Central Portugal, several sites (Figueiral, 1995); C: Northern Portugal, several sites (Figueiral, 1995; Figueiral and Bettencourt, 2004; Tereso, 2009); D: Tierra de Pinares, several sites (this study); 1: Barreiros (García-Amorena et al., 2007); 2: Teleno (Domergue and Herail, 1978); 3: Castillejos II (Rubiales et al., 2009); 4: Mora de Rubielos (Richter and Eckstein, 1986); 5: Yecla (Alcalde et al., 2004); 6: Santa Bárbara (Rodríguez-Ariza, 2000). Pollen sites: 7: Lanzahíta (López-Sáez et al., 2010); 8: Navarrés (Carrión and van Geel, 1999); 9: Siles (Carrión, 2002); 10: El Sabinar (Carrión et al., 2004); 11: Cañada de la Cruz (Carrión et al., 2001); 12: Sierra de Baza (Carrión et al., 2007); 13: Sierra de Gádor (Carrión et al., 2003)

There are other sites in western Iberia where P. pinaster charcoal has been recovered, for instance from the Roman period at the foothills of the Teleno Mountains in NW Spain (Domergue and Herail, 1978) in an area with acidic and coarse-grained soils and a sub-Mediterranean climate. Farther south, Rubiales et al. (2009) identified P. pinaster charcoal in the Iron Age archaeological site of Castillejos II (c. 2200 cal. yr BP) at 550 m a.s.l. Charcoal from other plants, such as sclerophyllous Quercus and Cistus species, were also abundant. This site is especially interesting from a biogeographical point of view because of the presence of pines where natural pine is now extremely rare (Rubiales et al., 2009). There is also macrofossil evidence of P. pinaster in the eastern half of Iberia (Figure 6), including sites in the Supra-Mediterranean (Mora de Rubielos, 3200–3500 cal. yr BP; Richter and Eckstein, 1986), Meso-Mediterranean (Yecla, 730–570 cal. yr BP; Alcalde et al., 2004) and Thermo-Mediterranean belts (Santa Bárbara, Chalcolithic; Rodríguez-Ariza, 2000).

Several palynologists have distinguished a P. pinaster pollen type based on identification criteria provided by Roure (1985) and Carrión et al. (2000). Thus, the palynological record suggests that the maritime pine has played an important role in the Holocene landscapes of several areas of the Betic Ranges (Carrión, 2002; Carrión et al., 2001, 2003, 2004, 2007), in Navarrés, at the foothills of the Iberian Range (Carrión and van Geel, 1999), and on the southern slopes of the Gredos Mountains (López-Sáez et al., 2010). Most of these pollen sequences show that the expansions of the maritime pine are synchronous with those of several sclerophyllous trees and shrubs, particularly evergreen Quercus species (e.g. Carrión, 2002; Carrión and van Geel, 1999). In contrast, the sequence from the Gredos Mountains (López-Sáez et al., 2010) suggests a clear long-term dominance of P. pinaster, associated with an understorey rich in Mediterranean shrubs. In NW Iberia, several pollen sequences show a certain presence of P. pinaster type throughout the Holocene, but always discontinuously and in low percentages (e.g. Muñoz Sobrino et al., 2004; Ramil-Rego et al., 1998).

In summary, the Holocene fossil record on the Iberian Peninsula shows that the maritime pine has been present in very diverse habitats and geographical regions, including some areas where it is absent today. Palynological and anthracological records demonstrate that this pine has been commonly associated with Mediterranean trees and shrubs. The soil charcoal data from the sandy soils of Tierra de Pinares presented in this report provide novel and valuable information suggesting that maritime pine forests were present throughout the Holocene, thus notably spatially expanding its Holocene record. In addition, our data also suggest that Pinus pinaster formed pure stands under the limiting conditions of the Duero Basin (deep sandy soils and a continental Mediterranean climate). These findings complete the vegetation histories based on regional pollen sequences both spatially and taxonomically (Franco-Múgica et al., 2005; García-Antón et al., 2011), supporting the local presence of pines over wide areas and detailing the particular species that produced at least a portion of the pollen signal. There is a striking difference between the palynological record and the soil charcoal record of Tierra de Pinares. The soil charcoal only includes pines, whereas the pollen shows woody plants other than Pinus. These trees and shrubs could have been present in the understorey of the pine forest or growing on more favourable soils outside of the dune fields and sand sheets. Additionally, the difference between the records might have been caused by the different source areas of the pollen and soil charcoal studies.