Reconstructing the history of beech (Fagus sylvatica L.) in the north-western Iberian Range (Spain): From Late-Glacial refugia to the Holocene anthropic-induced forests
Introduction
The evolution of Fagus sylvatica in the Iberian Peninsula and its migration patterns during the late Quaternary have been investigated by several authors (Costa Tenorio et al., 1990, Costa Tenorio, 2001, Martínez Atienza and Morla Juaristi, 1992, Ramil-Rego et al., 2000). These studies have established an evolution model for beech forests which distinguishes three simultaneous origins (Agúndez et al., 1995): (1) the surviving populations from the Eastern Europe glacial refugia would have migrated westwards reaching the north of the Iberian Peninsula 3000 years ago (Huntley and Birks, 1983, Peñalba, 1989, Peñalba, 1994); (2) the local glacial refugia of the NW Iberian Peninsula, the Cantabrian Range, the Basque territories and Catalonia would have slightly expanded during the late glacial period and the early Holocene (Costa Tenorio et al., 1990, Martínez Atienza and Morla Juaristi, 1992, Ramil-Rego et al., 2000, Magri et al., 2006). It should be stressed that sporadic occurrences of Fagus pollen in these areas are documented, i.e. Mallarach et al. (1986), Ramil Rego (1992), Maldonado (1994), Martínez Atienza (1999); and (3) a combination of (1) and (2) (Martínez Atienza and Morla Juaristi, 1992, Rodríguez Guitián et al., 1993). Model (1) has been shown to be unlikely based on fossil records and genetic data (Magri et al., 2006), and its belief has been due to the insufficiency of available information (Ramil-Rego et al., 2000).
According to Huntley and Birks (1983), Costa Tenorio et al. (1990) and Martínez Atienza and Morla Juaristi (1992), Fagus forests appeared in the north of the Iberian Peninsula around 3000 BP. None of these works explores, however, the reasons, whether climatic or anthropic, for such spread. What is more, none of them takes into consideration the possible spread of Fagus from glacial refugia instead of the assumed east-west migration. Due to its conditions of natural enclave and its strategic position in the migratory routes of several taxa into the Iberian Peninsula (Gómez Lobo, 1993), the history of the north-western Iberian Range forests is key in the understanding of the vegetation dynamics of the whole Iberian Peninsula. In fact, the Iberian Peninsula is known as the Eurosiberian Island because, although located within the Mediterranean biogeographical area, it shows Eurosiberian features. Indeed it exhibits a great interest for both the particularities of its vegetation and the extension reached by different types of forests such as that of Fagus sylvatica.
The Iberian Range originated as a consequence of the raising of mountains, interrupted by wide fosses and depressions dominated by Mesozoic materials of marine origin (Gil García and Tomás Las Heras, 1996). Located in the north-central part of the Iberian Peninsula, between the Ebro and the Duero basins, it spreads for c. 150 km, in NW–SE direction. It contains three different morpho-structural units:
- (1)
the North-western sector (Sierras of La Demanda, Neila, Urbión and Cebollera);
- (2)
the North-eastern sector (Sierras of Moncayo, Gúdar, Maestrazgo and Javalambre); and
- (3)
the South-eastern sector.
The first two sectors are generally known as the north Iberian Range while the third one is known as the south Iberian Range. Our study focuses on the beech forests of the western sector (Fig. 1).
The high altitude of the north-western Iberian Range mountains (i.e. 2270 m a.s.l. in La Demanda, 2228 m a.s.l. in Urbión and 1881 m a.s.l. in Cebollera) together with its position in the north originated, during the coldest Quaternary periods, intense periglacial and glacial activities particularly above 1600 m a.s.l., where today there are still several lakes of glacial origin (Thornes, 1968, Ortigosa, 1994). In fact, some authors (Rodríguez Prieto et al., 1993) suggest that the glacial events in these mountains were of great intensity giving way to a high density of glacial phenomena in the area (probably the most intense of the whole Iberian Peninsula). It is clear, therefore, that although our area of interest is placed in the Mediterranean Region, its environmental conditions, and especially the rainfall levels, resemble very much those typical of the Eurosiberian zone. For this reason, the Iberian Peninsula offers good examples of different types of forest such as that of Fagus sylvatica (i.e. in Iregua and Urbión) or those of Pinus (Costa Tenorio et al., 1990). In effect, the annual rainfall in the Fagus forests of the north-western Iberian Range is relatively high, with an annual average of 910 mm (Agúndez et al., 1995). According to Rivas-Martínez (1987), one of the reasons explaining the occupation of the territory by beech forests is that, in these Sierras, the summer rains are relatively high, more than 141 mm per summer (Agúndez et al., 1995). In fact, some of the beech forests of the area under study like those from Sierra de Cebollera, are situated in shady areas supporting a major regime of rainfall (1500–1600 mm per year) (Navarro and Valle, 1987, Rivas Martínez, 1987).
In the past, the distribution area of Fagus was smaller than today. However, there are still uncertainties relating to the existence of glacial refugia and to the different migration routes followed by beech within the Iberian Peninsula. Considering the available palynological data, our aim is recognizing the possible refugia areas. It should be borne in mind, however, that the detection of small populations is difficult using pollen analyses (Birks and Birks, 2000) and this is particularly true in unfavourable climatic contexts (Brubaker et al., 2005).
The present study is based on the compilation of the currently available pollen analysis data from the north-western Iberian Range, and aims to evaluate their potential for reconstructing the distribution of Fagus sylvatica in the past, its refugial zones, and the causes of its spread during the Middle Holocene.
Section snippets
Fagus sylvatica L. forests in the north-western Iberian range
Fagus sylvatica L. is mostly present in the north-central and north-eastern parts of the Iberian Peninsula. There are, nonetheless, other areas further south where beech forests are also found far from the main distribution area, i.e. the beech forest of Montejo in Sierra of Ayllón, (Costa Tenorio et al., 2001).
The north-western Iberian Range beech forests occupy a wide area which spreads from Sierra de la Demanda (in the most western part) to Sierra Cebollera (in the eastern part) (Navarro,
Methods
The collected pollen dataset consists of 24 sites located in the north-western Iberian Range (Fig. 1 and Table 1).The chronological setting of the study is provided by 58 14C dates (Table 2), calibrated using CALIB 5.0.2 program (http://calib.qub.ac.uk/calib/). In the text, calibrated radiocarbon dates are given as cal. BP. Ages in pollen diagrams have been obtained by linear interpolation between uncalibrated radiocarbon ages. In the sites without radiocarbon dates, the adopted chronology is
Results
Several authors (i.e. Huntley and Birks, 1983, Huntley, 1990, van der Knaap et al., 2005) have discussed the interpretation of different phases of tree migration through the analysis of pollen patterns. Based on these works, we have identified the following phases in the Fagus pollen curves: (1) scattered occurrences; (2) a phase of arrival/disappearance (arrival phase when beech pollen is less than 1%, representing the beginning of the continuous curve; disappearance phase when Fagus pollen
Discussion
The first Fagus occurrence in the north-western Iberian Range is documented in the Laguna Grande lacustrine core during the Upper Pleistocene (before 20,526–20,000 cal. BP) (Ruiz Zapata et al., 2002) (Fig. 3). The authors estimate that this event took place at the end of the Würm when, despite a mild climatic period, thermophillous species such as Corylus, Ilex and Quercus are recorded.
Beech percentages indicate its regional presence in the Sierra de Neila during the end of the Pleistocene.
Conclusions
Briefly, three main aspects characterized the spread of Fagus sylvatica throughout the north-western Iberian Range:
- (a)
The palaeobotanical record provides information on the location of, at least, a glacial refuge near to Laguna Negra in Sierra de Neila since the Upper Pleistocene.
- (b)
In the northern part of the Sierra de Cebollera, the high rainfall levels have favoured the spread of Fagus forests since c. 6400 cal. BP. The Atlantic influence coming from the Cantabrian Sea and the Foehn effect are
Acknowledgements
We would like to thank the three anonymous reviewers for their valuable suggestions which improved the manuscript. One of the authors (L. López-Merino) was supported by a CSIC-ESF (European Social Fund) research contract (I3P predoctoral program).
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