Analyse pollinique d'une tourbière holocène dans les Montes do Buio : Cuadramón (Galice, nord-ouest de l'Espagne) [Pollen analyse of holocene peat-bog in the Montes do Buio : Cuadramón (Galice, N. W. of Spain)] - article ; n°3 ; vol.11, pg 257-268

Quaternaire - Année 2000 - Volume 11 - Numéro 3 - Pages 257-268
L'analyse pollinique de deux sondages effectués dans un site tourbeux (Cuadramón I et II) dans les Montes do Buio (nord-ouest de l'Espagne) a permis de retracer l'évolution de la végétation holocène régionale.
Une comparaison avec les données palynologiques de la tourbière proche de Lobeiras, dont les zones polliniques sont corrélables, et qui présente deux datations ¹⁴C, permet de proposer l'hypothèse chronologique suivante : le début des dépôts tourbeux serait contemporain de la fin de la période préboréale et antérieur à la datation obtenue de 8770±170 BP. Les conditions climatiques régionales n'ont vraisemblablement pas permis l'installation d'une chênaie dense mais favorisé l'expression de formations assez ouvertes mêlant Quercus, Corylus et Betula. Les défrichements sub-atlantiques ont fait diminuer l'étendue des formations forestières au bénéfice de matorals à Ericacées.
El análisis polinico de dos sondeos realizados sobre una misma turbera de los Montes del Cuadramón (Lugo, España) ha permitido conocer la evolución de la vegetación holocena.
La comparación con la turbera próxima de Lobeiras, datada por radiocarbono, y la similitud de sus zonaciones polinicas han permitido establecer una hipótesis cronológica y situar el comienzo de la deposición turbosa a finales del periodo Preboreal. Las condiciones climáticas no parecen haber permitido la instalación de un robledal denso, sino que han favorecido la expansion de formaciones bastante abiertas, en las que conviven Quercus, Corylus y Betula. Las roturaciones y enfnamientos subatlánticos destruyeron las formaciones boscosas en beneficio de los matorrales con Ericaceae.
The palynological analysis of two sedimentary sequences in the same peat bog (denominated Cuadramon I and Cuadramon II, situated at an altitude of 700 m in the Cuadramon mountains (Lugo, N.W. Spain) within the Eurosiberian biogeographical regional framework) has permitted an evolutional knowledge of vegetation during the last nine millennium. Specifically, the intention was to determine the presence, diffusion and expansion of certain taxons, such as Quercus, Corylus, Betula, Alnus, Pinus, and even Fagus, throughout the successive vegetational stages that occurred in this area over a period of time.
The ¹⁴C data obtained in this peat bog (8770±170 BP in the CAII-2 zone of Cuadramon II) and others nearby, together with similarity of their palynological zonation, has permitted the establishment of a chronological hypothesis that positions its foundation at the beginning of the Holocene Epoch. Initially, climatic conditions do not appear to have favoured the installation of dense oak forests, but have favoured the expansion of open formations, in which Quercus, Corylus and Betula coexist.
The optimum arboreal, in which the major expansion of oak is observed, occurs in the Boreal, beginning around 8770±170 BP. Later on, the Atlantic Period would respond to an arboreal predominance, during which the combination Corylus-Quercus-Betula dominates the landscape and the first signs of an incipient anthropeic activity appears (occasional presence of Cerealia in Cuadramon I). The Subatlantic Period would suppose the substitution of forests for thickets, fundamentally composed of Ericaceae, Calluna and Poaceae.
The episodes that imply an arboreal decrease, of both climatic or anthropic origin, entails the increase of Poaceae and Ericaceae, above all, in levels that represent preactual or recent vegetation characteristics. These latter increments are parallel to the expansion of Castanea, the continual presence of Cerealia and the increase of ruderal and hydrophilous taxa.
The low relevance of Pinus in the landscape is notable. In contrast, it is a major component of the regional vegetation in other mountains of the Northern Iberian Peninsular.
On the other hand, the results obtained in this study, as well as those in other nearby locations, could indicate that Fagus is not an habitual component of Galician vegetation.
The representation of Mediterranean taxons such as Oleaceae, taking into account knowledge of the actual climate and vegetation of the zone, could be explained by the capacity of pollen from these taxons to travel long distances from lower latitudes.
In synthesis, this palynological study, correlated with those earned out in geographically proximate and well dated chronologically peat bogs such as Lobeiras (Gonzalez, 1996), makes a contribution to the understanding of the evolution of the Holocene vegetation of the Northwest Iberian Peninsula. After taking into account the characteristics of an open landscape together with the fresh and well ventilated nature of the Galician Atlantic climate, it demonstrates the importance of arboreal taxons, represented by deciduous trees : Quercus, Betula and Corylus.
A notable factor of the arboreal history of Galicia is, for historic reasons and distance of refuges, the low relevance of taxa such as Fagus and Abies during the Holocene Epoch. The presence of Pinus is also very weak. This behaviour would explain, in part, the importance of Corylus and Betula. The progressive decrease of oak, during the period following its arboreal optimum, benefits Corylus throughout the Atlantic Period in which a discreet presence of cultivated plants and accompanying ruderal was established, observing finally, in the second part of the Subatlantic, the expansion of degradation thickets.
12 pages
Source : Persée ; Ministère de la jeunesse, de l’éducation nationale et de la recherche, Direction de l’enseignement supérieur, Sous-direction des bibliothèques et de la documentation.