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This paper examines the mid- to late-Holocene (c. 6850-1160 cal. yr BP) environmental history of Sierra de Gádor in southern Spain. The local vegetation dynamics are reconstructed through the palaeoecological record obtained from a lacustrine deposit situated at 1530 m. Various hypotheses are considered to explain the vegetation dynamics apparent in the palaeoecological evidence, including climatic change, fire occurrence and human activity. Although the vegetation in this region is sensitive to climatic change, threshold events driven by ecological factors are also apparent. Climatic events include a thermo-mesophytic optimum with abundance of deciduous trees and maximum lake water level found from c. 6850 to 5500 cal. yr BP. In contrast, changes in the frequency of major fire episodes appear to have shaped interspecific relationships and vegetation change, especially from c. 4200 cal. yr BP onwards. Biotic properties of the ecosystem such as the inertia of established tree populations, interconnected with competition adjustments, appear also to have played a role. Over the last two millennia, overgrazing, combined with natural and/or human-set fires, appears to have pushed mountain forests over a threshold leading to the spread of grassland, thorny scrub, junipers and nitrophilous communities. [ABSTRACT FROM AUTHOR]

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. [Copyright &y& Elsevier]

We examined the variations in vegetation, organic carbon isotopic compositions and biomarker distributions spanning a well-constrained climatic shift in a Sub-Boreal Dutch raised bog. The macrofossils record a vegetation shift from a predominance of degraded Ericaceae rootlets to a predominance of well-preserved Sphagnum species, first S. cuspidatum, followed by S. imbricatum. This shift, as well as evidence from other macrofossils and fungal spores, indicates that the local climatic conditions changed from relatively dry to relatively wet, with the wettest local conditions occurring at the base of the transition. At this boundary, a positive shift occurs in the δ[sup 13]C values of bulk peat (c. 4‰) and compounds derived from specific peat-forming plants (1 to 2‰). This is attributed to a combination of factors, including selective preservation of certain compound classes, changes in precipitation and its effect on the water table, and plant growth rates. The changes in peat preservation also indicate that redox changes occurred across the climatic change event. Indeed, biomarker transformations thought to reflect past redox conditions or bog water acidity - the 1,2,9-trimethyl1,2,3,4-tetrahydropicene to ursolic acid ratio, diacid to hydroxy acid ratio, and 17β,21β(H)-homohopane to total homohopane ratio - all exhibited shifts spanning the climatic change transition. Although some of these trends indicate the need for more fundamental research on biomarker transformations in peat, some, such as the 17β,21β(H)-homohopane to total homohopane ratio, show promise as indicators of past bog water conditions that could complement macrofossil analyses. [ABSTRACT FROM AUTHOR]