Environmental factors and land use determining the conservation and recovery of semi-arid Mediterranean vegetation in the middle Ebro Valley

Arid and semi-arid vegetation is largely determined by environmental constraints, such as climate, topography and lithology. Moreover, these areas have usually a intense human use. In this study I examined the main factors related to the conservation and recovery of vegetation in the Middle Ebro Valley, one of the most arid areas in Spain (350 mm year-1). The study was developed at two different scales. At landscape scale, a Geographic Information System (GIS) was developed to evaluate the relationship between the vegetation conservation, environmental factors (aridity, slope angle, slope aspect and lithology) and human use (grazing and human settlements). The role of environmental factors and human use on the vegetation dynamics (1957-1998) was also analysed. At plant community scale, I studied the influence of water availability, grazing and fragmentation on the structure and specific composition of the most common plant communities (dwarf-shrubland and garrigues). I also studied the old-field succession undergone in the bottom valleys under moderate grazing conditions. Results showed that aridity is an important factor determining the presence of less developed plant communities, such as garrigues, and it impedes the recovery of the open woodland in the area. At plant community scale, low water availability decreases species diversity and vegetation coverage. Fragmentation is the most important consequence of human use in the area, as it hampered the vegetation recovery in the landscape, and decreased the species diversity. Close to human settlements, human use had a negative effect on vegetation recovery and conservation, but extensive grazing had not an important effect on vegetation health and plant succession after land abandonment. Finally, gypsophile vegetation was affected by topography and grazing, because they alter the rigors of the gypsipherous soil.