Late-glacial and holocene vegetation and environmental history of the Sierra de Urbión, North-West Central Spain

Abstract

This thesis presents the results of a palaeoecological investigation in to the Late-glacial and Holocene vegetation and environmental history of the Sierra de Urbion mountain range located in north-central Spain. The Sierra de Urbion is dominated by montane forest vegetation containing Pinus sylvestris, Fagus sylvatica, Quercus pyrenaica and Erica arborea up to an altitude o f c. 1800 m above see level (a.s.l.). Above this altitude sub-alpine shrub (with Juniperus communis, Calluna vulgaris and Vaccinium myrtillus) and grassland (with Festuca iberica) vegetation is dominant. Palaeoecological research in this thesis aimed to reconstruct the Late-glacial and Holocene vegetation history of the montane forests using fossil pollen analysis, macrofossil analysis and 14C dating of sedimentary sequences from a lake (Laguna Negra, 1760 m a.s.l.), marsh (Laguna Masegosa, 1600 m a.s.l.) and several small wet hollows taken from within the forests. Numerical analysis is used to examine the fossil pollen data and to compare the fossil data with the previously published fossil sequence from Quintanar de la Sierra, which is located in the neighbouring mountain of the Sierra de Neila. The fossil pollen data are also used to quantitatively reconstruct Holocene climatic history using pollen-climate response surfaces. At the close of the last (Weichselian) glaciation (prior to c. 13000 BP) a sparse vegetation cover of Poaceae and Artemisia dominated the region. During this phase a cold and arid climate can be inferred. The beginning of the Weichselian Late-glacial Interstadial (c. 13000-11000 BP) was marked by a substantial expansion of Juniperus scrub vegetation. This was succeeded by the development of an open Pinus-Betula woodland around the site studied (Laguna Masegosa). Pinus forests with Quercus probably expanded at lower elevations and Juniperus scrub and heath vegetation probably developed at higher elevations. Increased moisture availability and higher summer temperatures are inferred for this period. In the Weichselian Late-glacial Stadial (Younger Dryas, c. 11000-10000 BP) there was a reversion to open herbaceous vegetation containing Poaceae, Artemisia, Asteraceae and Chenopodiaceae. During this period a return to more arid conditions, with lower summer temperatures, is inferred, and this interpretation is reproduced by quantitative climatic reconstructions made from the fossil pollen data. The Late glacial vegetation successions revealed in this study match closely with previously published data from the Northern Iberian Mountains indicating regional homogeneity in Late-glacial vegetation history. The results from this study therefore confirm previous findings that the Northern Iberian Mountains were sensitive to the climatic influence of the North Atlantic Ocean during the Weichselian Late-glacial. The Holocene began (Laguna Masegosa and Laguna Negra) with a brief phase of Juniperus scrub vegetation, which was rapidly replaced, from c. 9950 BP, by Betula and Pinus woodland. This succession was primarily triggered by a marked increase in moisture availability at the Lateglacial/Holocene boundary. Betula and Pinus woodland was invaded by Quercus from c. 9900 BP and Corylus from c. 8900 BP. From then on until c. 4900 BP relatively stable and closed mixed deciduous forests with Pinus dominated the landscape. However, there were strong contrasts in vegetation between sites studied in this research, and with previously published research. A unique development of Ulmus and Fraxinus excelsior is recorded at Laguna Negra between c.8400 and c. 4900 BP. In addition Taxus, only sporadically recorded in previously published research, expanded from c. 6800 BP at both Laguna Negra and Laguna Masegosa. Ulmus, Fraxinus excelsior and Taxus thus had a localised distribution, and there therefore was an overall regional heterogeneity in vegetation in contrast to the Late-glacial period and early part of the Holocene. From the climatic perspective the early to mid Holocene fossil pollen records indicate relatively continental-type climatic conditions of cool winters and warm growing seasons. From c. 4900 BP marked changes in vegetation are indicated. Localised declines in Ulmus, Fraxinus excelsior and Taxus occurred in conjunction with a widespread decline in Corylus, during which there was a reduction in forest cover and site to site variation in forest composition. At the same time there was an expansion of heath and herbaceous vegetation, reflecting an increase in disturbance and instability. Fagus invaded these forests from c. 4500 BP. The changes in vegetation from c. 4900 BP may have been initiated by the onset of more oceanic climatic conditions at that time, which the quantitative climatic reconstructions indicate. However, these changes in vegetation may also have been caused by human interference. Widespread human activity, and its progressive intensification, can be inferred from c. 4900 BP by the expansion of Plantago lanceolata and cereals. The intensity of human activities increased further at c. 2600 BP following which expansions in Fagus woodlands, heath and herbaceous vegetation took place in conjunction with declines in Betula, Pinus and Quercus. In a final phase (from c. 1800 BP) which brings the pollen records up to the present Fagus vegetation declined and there was a regional expansion of Pinus forests. The late Holocene quantitative climatic reconstructions indicate a continuation in the trends that began in the mid Holocene (c. 4900 BP) with milder winter temperatures, lower growing season warmth and a reduced moisture availability indicated. However, there is greater doubt attached to these reconstructions due to the probable human interference in regional vegetation development in the late Holocene.