Old Field Dynamics on the Dry Side of the Mediterranean Basin: Patterns and Processes in Semiarid Southeast Spain

Abstract

Euro-Mediterranean countries have a long history of land use changes and urbanization processes (Antrop 2004). During the past century, with the ad-vent of industrial and tourist development, these countries have experienced an important intensification of land use changes related to the abandonment of rural livelihoods and city sprawl in coastal areas. On the northern (European) rim of the Mediterranean basin, the socio-economic changes taking place during this period promoted a dramatic rural exodus, with the consequent abandonment of large cultivated areas (Lepart and Debussche 1992) (figure 13.1). As a result, these previously cultivated areas are now being colonized by natural vegetation, with important implica-tions for processes such as water balance (Bellot et al. 2001), wildfire regime (Pausas 2004) and carbon sequestration (DeGryze et al. 2004). Meanwhile, landscapes on the southern (African) rim of the Mediterranean basin still suf-fer from overexploitation, especially overgrazing (after clearing), increasing cultivation on pronounced slopes, and associated degradation problems (Le Houérou 1993; Taiqui 1997; Redjali 2004). These contrasting patterns are re-lated to the different socioeconomic, demographic, and political trends oc-curring between the northern and southern Mediterranean (Puigdefábregas and Mendizabal 1998) Thus there is some association between land abandonment in mesic Mediterranean ecosystems and overexploitation in drier Mediterranean eco-systems. However, there are also areas in southern Europe (e.g., southeast Spain) that are considered semiarid and are undergoing the process of land abandonment (Bonet et al. 2004; Bonet et al. 2006). In this chapter we will fo-cus on such areas, where studies of vegetation dynamics have been developed only recently (Bonet et al. 2001; Bonet 2004; Bonet and Pausas 2004; Pausas et 247 ch13:IP_Cramer 6/4/07 al. 2006; Pugnaire et al. 2006). Vegetation dynamics under semiarid condi-tions differ from that observed in more mesic areas, due to the reduced plant cover that characterizes the former, the differences in the relative importance of interspecific interactions such as facilitation and competition (Bertness and Callaway 1994), and the role that abiotic factors play in the dynamics of plant populations (Escudero et al. 1999). Many studies on secondary succession in both tropical and temperate regions have been carried out in abandoned fields after agricultural use of previously forested lands. Succession in these areas usually leads to the development of forest, although the composition may change from the original forests (Foster et al. 1998; Grau et al. 2003). In the Mediterranean Basin, some studies also indicate a forest development with succession following land abandonment (Housard et al. 1980; Tatoni and Roche 1994; Debussche et al. 1996, 2001; Debussche and Lepart 1992; Maz-zoleni et al. 2004), but under semiarid conditions the forest may not be repre-sentative of late stages of succession and is usually not present in remnant veg-etation, due to both low water resources and intensive human pressure since the Neolithic period (Badal et al. 1994). In semiarid Spain there is some un-certainty about which vegetation type was present prior to agricultural prac-tices, but many authors suggest that the late-successional communities in these territories are composed of shrublands with a variable tree cover (Bolòs 1967; Rivas-Martínez 1987; Ruiz de la Torre 1990). For practical reasons, we use a chronosequence approach (i.e., syn-chronic approach or space-for-time substitution) to infer successional dy-namics. Although we recognize that there are some potential problems with using this approach (Pickett 1989), most predictions made with this approach 248 case studies from around the world