The biogeographical role of Pinus forest on the Northern Spanish Meseta: a new Holocene sequence

Abstract

The aim of this paper is to reconstruct the variation in the vegetation during the Holocene on the Northern Meseta through pollen analysis in order to determine the roles played by Pinus and Quercus in the vegetation dynamics. A new pollen sequence, Camporredondo, in the centre of the Duero River basin, Northern Meseta, Spain, is presented. It covers the period from ca 11,250 to 1630 cal yr BP. The Camporredondo sequence suggests that a forested landscape existed from the start of the Holocene with values of approximately 90% of arboreal pollen mainly derived from pine forests in the surroundings. Shortly after 8600 cal yr BP pine forests continued to dominate but the landscape became more open while stands of deciduous and evergreen Quercus developed. Around 4500 years ago, coinciding with the start of a period of less rainfall, the lacustrine areas in these territories began to silt up, favouring hydrophytic taxa such as Salix or Cyperaceae. A comparison with other Holocene pollen sequences from the central-eastern part of the Northern Meseta confirms the presence of pine forests in the whole region since the early Holocene. We discuss the variations of the general vegetation pattern in terms of the local or regional conditions at each site. Throughout the Holocene, Pinus forests dominated at least in the eastern half of the Duero River basin. The typical Holocene substitution of conifers by broadleaf species as recorded in other parts of the Iberian Peninsula and Europe, never takes place in the study area. The results of the present study suggest that the current view of the potential vegetation in the Spanish Northern Meseta should be reviewed.

Introduction

The Mediterranean area constitutes a unique biogeographic region in Europe (Breckle, 2002). In comparison with the uniform vegetation developments in central or northern European territories, the Mediterranean Basin presents highly complex vegetation dynamics during the Holocene. Climate changes in the last 11,500 cal yr BP are accompanied by different patterns at each location (e.g. Jalut et al., 2009, Tinner et al., 2009, Carrión et al., 2010). In the case of the Iberian Peninsula, geographic position, mesoclimatic variations, altitudinal differences or type of substrate, among other factors, together with the strong anthropogenic influence in more recent times, are aspects that apparently led to unique responses by the vegetation (Costa Tenorio et al., 1990, Costa Tenorio et al., 1996, Carrión et al., 2000, García Antón et al., 2002, Morla Juaristi, 2004).

The Northern Meseta (Central Spain) is an inland area of the Iberian Peninsula within the Duero basin. It is a sedimentary plateau that comprises altitudes between 600 and 1000 m a.s.l. representing a singular region in Europe. The area itself is a vast territory with a continental Mediterranean climate which is surrounded by mountains that isolate it from the oceanic influence. In fact, similar conditions beyond the European Mediterranean area can only be found in the inland plains in central Anatolia (Turkey). However, lower precipitation and temperature values (especially the minima temperature values) in Anatolia than those registered in the Northern Iberian Meseta determine a vegetation characterised by non arboreal communities (Quézel and Médail, 2003).

The Meseta is an area that has undergone profound vegetation changes (Ruiz de la Torre, 1986–2002) as a result of anthropogenic activities. Traditionally, these mid-altitude areas with flat topography are considered suitable for broadleaf species, especially Quercus ilex L. subsp. ballota (Desf.) Samp., Quercus faginea Lam. and Quercus pyrenaica Willd. forests, whereas the different Pinus species (Pinus pinaster Aiton, Pinus nigra Arnold and Pinus pinea L.) currently coexisting with these oaks, have been assigned a successional role in forest dynamics or questioned its naturalness (Rivas Martínez, 1987). Little palaeobotanic information is currently available, as opposed to the relatively abundant data for adjacent mountain systems (Martínez Atienza, 1999, Carrión et al., 2010, Rubiales et al., 2010). Recent pollen results in these mid-altitudes (García Antón et al., 1995, Franco Múgica et al., 2001, Franco Múgica et al., 2005), however, call for a very different interpretation and question this model, which is widely accepted among the scientific community. Older studies, such as the one by Menéndez Amor and Florschütz (1975), highlighted the predominance of the pine forests, although due to the lack of absolute datings, the authors considered them to be of the preboreal–boreal age, as they expected a substitution by broadleaf species at this stage of climate deterioration in the Holocene.

A new pollen sequence for the Northern Meseta region is needed in order to understand the vegetation dynamics for this Mediterranean area of the Iberian Peninsula. It is located in the most inland part of the Duero basin and its marginal position on the sandy beds that characterise a part of the Meseta, provide a broader reference framework for interpreting Holocene changes in vegetation. The objectives of this paper are i) to analyse a new pollen sequence in order to establish if pine forests occurred throughout the Holocene or not ii) to use accurate chronological control to compare the events taking place in this sequence with the existing ones in the Duero Basin.