Ángel Martín Vicente

Long Term Persistence of Dehesas. Evidences from History

Dehesas are agrosilvopastoral systems characterized by a savanna-like physiognomy. Today the central problem for the continuity of the dehesas is the gradual decay of tree canopy. We have investigated the history of the vegetation and management of dehesas from the middle ages to the present day in order to know in which conditions they have persisted in the long run. The results show that the word dehesa has been used historically for private grazing lands, with no reference to any vegetation type. According with the resources used during the middle and modern times the vegetation was a mixture of grasslands, shrubs and trees. The first descriptions of the vegetation of SW Spain in the 18th century showed that open oak parklands were scarce, while oak shrublands were very common. After the privatization of the land in the 19th century large private farms called dehesas developed in SW Spain, that were devoted to livestock raising in combination with agriculture and forestry. The change in management practices favored open oak parklands over oak shrublands in the dehesas, becoming the dominant vegetation of these farms in the 20th century. Is in this moment when dehesa passed to mean open oak parklands devoted to grazing, cultivation and forestry. The effect of the change in vegetation structure and management practices on oak regeneration is discussed.

Long-term vegetation changes on the stabilized dunes of Doñana National Park (SW Spain)

Changes in the vegetation of stabilized sand dunes of Doñana National Park in the last four centuries have been analysed, combining ecological and historical sources of information. Quantitative vegetation data have been obtained from 5 inventories of a 15 km long transect, consisting of 34 points where nearby woody plants have been recorded. Inventories were taken in 1636, 1647, 1652, 1682 and 1859. The analysis of these data, combined with climatic and management information on the area, reveals that until 1636 a mediterranean forest dominated byQuercus suber, existed; it was used as a game preserve. From 1636 to 1682 vegetation underwent profound changes with a marked drop in trees and mature scrub. The vegetation was replaced by a pioneer scrub species. This rapid change was brought about by the introduction of cattle and pigs, timber and cork exploitation, and the use of prescribed fire. The climatic crisis that arose circa 1700 led to erosion of the sandy soils, which made the vegetation change irreversibly. Livestock pressure hampered natural forest regeneration until 1737 whenPinus pinea plantations started. From then onwards little change in vegetation is known to have occurred. It is suggested that the main steps of change in Doñana vegetation history may have occurred in many other areas of the Mediterranean Basin.