The primeval vegetation is that present in the territory before the terrestrial ecosystems began to be profoundly transformed, about 6000 years ago, with the arrival of livestock and agriculture, that is, a period that covers, at least, the last 60,000. The evolution of these forests is one of the lines of research of the project Evaluation and contextualization of methane reservoirs of the Galician estuaries in relation to variations in sea level and environmental changes during the post-glacial period, developed by the XM-3 Análises de Concas group, deals with the evolution of these forests. The journal Vegetation History and Archaeobotany has just published an article that includes some results of this study, carried out in collaboration with scientists from Ibader (Institute of Agrarian Biodiversity and Rural Development of the USC), the Ikerbasque Foundation, the University of the Basque Country, and the Institute of Plant Sciences and Oeschger Centre for Climate Change Research, of the Bern University. This project also gave rise to the doctoral thesis of Iria García-Moreiras, defended in November.
In this work, the researchers focus on revealing what happened to the hornbeam population (Carpinus betulus L.), a deciduous tree common in Europe and Asia Minor but of which only a few Iberian populations currently exist. “Before the last glaciation it was quite common in the Iberian northwest, but it began to decline 60,000 years ago”, points out the researcher from the XM-3 group Castor Muñoz. To find out what caused this gradual decrease, the sediments of the Vigo estuary offer an important source of information, since during the last glacial cycle (between 110,000 and 10,000 years ago) the sea level was located at least a few tens of meters below the current average level. “Many of the secrets of those times were stored and preserved in strata that are now under the sea”, explains Castor Muñoz. Thus, the sediments of the estuary contain Carpinus pollen “in coherent sequences and dated between 60,000 and less than 9000 years ago” that show the history of this tree, “quite different and much more complex than previously believed”.
45,000 years ago
The researchers involved in this study draw a story in four episodes. The first of these takes place 45,000 years ago, when the sea was 40 meters below its current level and barely penetrated beyond Toralla and Punta Balea, in Cangas. “In this scenario, the estuary housed a wealth of ecosystems that is difficult to imagine today,” explains Muñoz. Large volumes of sand accumulated in front of the coastal plains of southern Galicia and northern Portugal, forming extensive beach-dune ecosystems, “which penetrated inland in the form of fixed dunes and sandy soils colonized by coastal pines and cypresses.” Behind these dune ecosystems emerged the ancient valley, covered with meanders, marshes, transitional wetlands and coastal heaths. Upstream, willows and alders appeared in the riverbeds, and on the higher banks of these forests the first hornbeams were already beginning to appear. On higher, sunnier slopes, hornbeams gave way to deciduous trees such as ash, hazel, oak, etc. sclerophyllous trees such as holly, cork oak or ergot.
20,000 years ago
A second stage takes us 20,000 years ago, during the coldest period of the last glacial maximum, when the relative sea level was 120 metres below the current level. At that time, explains Castor Muñoz, “we could have walked to the Cies Islands” and, with a colder climate, the maximum altitude of the forests decreased, so that many of the vegetation belts were moved along the old valley, towards the sea. The intense cold caused the beech trees to disappear and, in some parts of the Rías Baixas, coastal birch forests appeared. The rest of the forest types persisted, including hornbeam, which was moved down the valley towards the current mouth of the estuary.
10,000 years ago
A new increase in temperature brought more changes, when at the end of the last glaciation the average summer temperature increased by about 6 °C on the coast of southern Galicia. “The melting of glaciers caused a rapid rise in sea level, and about 10,000 years ago the sea began to flood and erode the old plains located at the northern mouth of the current estuary”. 9000 years ago the sea had already reached A Guía and this movement drastically reduced the surface area on the banks of the estuary, especially the dune systems colonised by pines and junipers, and the wetlands and coastal heath areas. The frequency of riverbank overflows also increased and this favoured species that better withstand frequent and prolonged flooding, such as alders, willows and ash trees, compared to hornbeam, a tree more suited to sub-humid and temperate or cool climates, which, as Muñoz points out, “only tolerates occasional flooding and for short periods”. The data handled by researchers from the XM-3 group show that the hornbeam survived the last glaciation in the coastal valleys of northwest Iberian Peninsula and only disappeared later, during the first millennia of the Holocene.
The human impact during the Holocene
The fourth stage corresponds to the last part of the Holocene, when the sea level gradually rose to its current position. The intertidal mudflats, marshes and alluvial forests gradually retreated until they were restricted to the mouths of the Verdugo-Oitavén and Alvedosa-Redondela basins in the innermost area; and the Lagares and Miñor rivers on the southern bank. Scientists also point out that the regional climate, markedly oceanic, favored the development and predominance of mesothermophilous deciduous forests with oak, linden, ash, hazel, birch, maple, lamiguero, chestnut, alder, willow, etc. However, signs of human transformation of forests already began to become evident in the Holocene Climatic Optimum period (between 8000 and 3000 years ago), explains Muñoz, and from then on they became more pronounced, “due to a certain climatic deterioration and, above all, to the successive cultural changes of the Bronze and Iron Ages, Romanization, etc.” Thus, the previously forested land became areas of crops, pastures or population centers and only some riverside forests of alders and willows and small areas of other deciduous forests mixed with conifers and sclerophylls were preserved, generally confined to the most abrupt, inaccessible or inhospitable areas. Thus, the hornbeam gradually became a marginal tree in this area.
En conclusión, os resultados confirman que “os cambios climáticos poden ter efectos drásticos sobre os nosos ecosistemas e biodiversidade e, a partir de aí, sobre todo o que de eles poida depender”.
