| Shrublands and Soil Erosion. An State-of-the-Art
Arid and semiarid regions occupy two-fifth of the continents (Reynolds et al., 2007). These regions are characterized by dry climatic conditions, recurrent droughts and a scant rainfall pattern with a marked seasonality and a high inter-annual variability which makes water to be a scant resource and vegetation to follow a high variability spatial distribution pattern (Breshears et al., 1998; Cecchi et al., 2006; Dunkerley, 2008). These conditions make these areas more sensitive to climate change (Rowell, 2005) and to land use change as a consequence of land abandonment (Poyatos et al., 2003; Delgado et al., 2010; García-Ruiz, 2010), increasing the risk of desertification (Puigdefábregas and Mendizabal, 1998; Geeson et al., 2002), in such a way that 65-70% of arid and semiarid areas are vulnerable to this degradation process (UNEP, 1991). Soil Erosion and Land Degradation are closely related to the changes in the vegetation cover (Zhao et al., 2013). Although other factors such as rainfall intensity or slope (Ziadat and Taimeh, 2013) the plant cover is the main factor that controls the soil erosion, controlling the infiltration and runoff generation (Cerdà, 1998a; Kargar Chigani et al., 2012; Haregeweyn, 2013).
Soil erosion show non-sustainable rates under these regions, such as under Mediterranean conditions (Cerdà et al., 2010) and on agriculture land (Cerdà et al; 2007; 2009) due to climatic conditions, to parent material and to the roughed terrain (Romero Díaz et al., 2010). The traditional impact of grazing, of extremely intense fires, of ploughing and the widespread use of herbicides on agriculture, the increase of the road and railway embankments and the agricultural land abandonment cause vegetation removal. Canopy cover partitions rainfall reducing the amount of water reaching the soil and the kinetic energy of rainfall drops, protecting the soil against the impact of rainfall drops. Vegetation distribution controls the exposure of soils to rainfall drops affecting soil erosion (Cerdà, 1997a; Cammeraat et al., 2010; Kakembo et al., 2012). The lost of vegetation can trigger Desertification (Izzo et al., 2013) because soil erosion is highly dependent on the effective rainfall striking soil particles (Cerdà and Lasanta, 2005; Haile and Fetene; 2012; Miao et al., 2012, Prokop and Poręba, 2012).
Shrubs are the most characteristic vegetation type in semiarid and arid ecosystems all over the world (Tomaselli, 1981; Kummerrow, 1989), typical of intermediate stages of most vegetation succession series, being the first in terms of dominant vegetation coverage, occupying 24% of drylands, followed by crop vegetation with 20% (Reynolds et al., 2007). Moreover, shrub vegetation covers the soil permanently, being able to adapt to very unfavourable conditions like droughts, frosts, non-fertile soils,… improving the soil quality due to their capacity to activate organic matter cycles supplying greater amounts of litter (Alegre et al., 2004). Shrubs have complex root systems, inducing changes in soil properties and increasing soil macroporosity (indirect effects) that increase infiltration reducing runoff and the soil loss (Garcia-Estringana et al., 2010). Shrubs improve the infiltration capacity of soils (Cerdà, 1997), even in the most difficult conditions (Marques et al., 2005), the water retention capacity (Ruiz Sinoga et al., 2010) and the runoff and sediment redistribution.
Shrub vegetation has been seen as a key vegetation cover in semiarid lands to control the soil and water losses (Francis and Thornes, 1990; Barea et al., 1996; Romero Díaz, 2003; Cerdà and Doerr, 2007). But the majority of revegetation programmes in arid and semiarid regions still ignores the great potential of this type of vegetation. Romero Díaz et al. (2010) indicated that 99% of revegetation programmes carried out by public authorities in Spain used fast growing tree vegetation (Pinus sp. and Eucalyptus sp.) that grow faster in non-fertile soils resisting to isolation. But the introduction of these species is conducted using aggressive techniques like terracing, changing topography and making more vulnerable terrain to soil loss, with erosion rates one or two order of magnitude greater than other shrublands naturally recovered (Romero Díaz et al., 2010). In relation to tree vegetation shrubs cover the soil faster, being very efficient in reducing runoff and soil erosion (Kummerow, 1989; Haase et al., 2000), not being necessary aggressive techniques for revegetation operations. The land use is the key factor that determines the soil loss and the vegetation recovery which can contribute to reduce the soil and water losses. Land abandonment use to trigger an increase in soil erosion, but the vegetation recovery reduces the impact of the abandonment. The natural vegetation recovery is the most effective way to regenerate degraded soils although under arid and semiarid climatic conditions this process is delayed due to the water stress and soil degradation and revegetation programmes are carried out. A firm commitment for shrub vegetation is necessary for improving soil recovery in semiarid and arid lands.
Acknowledgements
The research projects 07 M/0077/1998, 07 M/0023/2000 and RTA01-078-C2- 2, GL2008-02879/BTE, LEDDRA243857 and RECARE FP7 project 603498 supported this research.
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