A model-based integrated assessment of land degradation by water erosion in a valuable Spanish rangeland

Highlights

• Integrated assessment model of degradation by water erosion in dehesa rangelands.

• System dynamics approach to face the relative scarcity of data.

• Calibration for an average dehesa defined over 10 representative farms.

• 85% of soil was lost within 360 years, on average, affected by positive feedbacks.

• Impacts of climatic factors much greater than those of the economic ones.

Abstract

This paper presents an integrated assessment model aimed at evaluating land degradation by water erosion in dehesa rangelands in the Iberian Peninsula. The model is built following the system dynamics approach. The degradation risk is likened to the probability of losing a certain amount of soil within a number of years, as estimated over a great number of stochastic simulations. Complementary indicators are the average times needed to lose different amounts of soil over the simulations. A group of exogenous factors are ranked in order of importance. These factors are mainly climatic and economic and potentially affect soil erosion. Calibration is carried out for a typical dehesa defined over 22 working units selected from 10 representative farms distributed throughout the Spanish region of Extremadura. The degradation risk turns out to be moderate. The importance of climatic factors on soil erosion considerably exceeds that of those linked to human activities.

Introduction

Rangelands cover approximately 90,000 km² in the central and south-western Iberian Peninsula (Gea-Izquierdo et al., 2006). These rangelands were created from former oak forests, mainly composed by holm oak and cork oak (Quercus ilex rotundifolia and Quercus suber) as the dominant tree species. By forest thinning, clear-cutting of shrubs, livestock grazing and cultivation, a dynamic mosaic of cover types was created in the form of open or wooded pasturelands and scrublands of variable tree densities. This land use is called dehesa in Spain and montado in Portugal. These systems, most of them held on private ownership (Plieninger et al., 2004), have evolved as an adaptation to poor soils and adverse rainfall conditions that cannot support intensive agricultural use. Their dominant use at present is livestock rearing (sheep, cattle, pigs and goats) and forestry (cork, wood and charcoal). Cultivation is of minor importance and restricted to limited areas with good soil conditions. Similar agro-silvo-pastoral systems can be found in other Mediterranean countries as well (Papanastasis and Mansat, 1996, Pardini, 2007).

Dehesa landscapes are highly interesting from a cultural, economic and environmental point of view (Campos Palacín, 1993, Díaz et al., 1997). However, these valuable rangelands have been suffering localized environmental problems mainly because of overgrazing, undergrazing and/or the lack of tree regeneration (Campos Palacín, 1983, Marañón, 1988, Pinto Correia, 1993, Montero et al., 2000, Pulido et al., 2001, Plieninger et al., 2003, Moreno and Pulido, 2009, Pulido and Picardo, 2010).

Soil degradation by water erosion, the process which is assessed here, is associated with overgrazing. This was favoured by the abandonment of transhumance and the resulting predominance of fencing and permanent grazing over restricted areas. Subsidies paid by the EU's Common Agricultural Policy have also been cited as one of the causes leading to the increase in the number of animals in dehesas (Donázar et al., 1997).

Excessive livestock causes soil degradation, usually understood as a deterioration of the physical, chemical or biological properties of soil that results in the reduction of its potential productive capacity (Imeson, 1988). Indeed, trampling decreases soil porosity thereby reducing water retention capacity and increasing runoff (Gamougoun and Smith, 1984, Mulholland and Fullen, 1991). Also, grazing reduces biomass and thus the protection it provides against erosion, “the sign ‘par excellence’ of degradation” (van der Leeuw, 1998, p.4).

Concerns about land degradation and desertification in the Mediterranean are evidenced by the number of research projects that have dealt with the matter over the last few decades. As an illustration, Baartman et al. (2007) report 39 projects specifically focussed on Mediterranean regions. Consequently, scientific publications are also plentiful (e.g. Brandt and Thornes, 1996, Wainwright and Thornes, 2004, Boardman and Poesen, 2006).

In the particular case of dehesas, a soil degradation survey carried out in a large number of farms in the region of Extremadura (SW Spain) evidenced that approximately 23% of them suffered high risk of soil degradation, including soil erosion, while approximately 60% of the region showed high sensibility to degradation processes (Schnabel et al., 2006, Lavado et al., 2009). Sheet erosion is particularly observed on hillsides, gullying takes place at the bottom of small upland valleys and rill erosion occurs mainly in the cultivated areas (Schnabel, 1997, Schnabel et al., 1999).

The Spanish National Action Programme to combat Desertification (SNAPD) (MAGRAMA, 2008) includes dehesas among the vulnerable socio-ecological systems needing integrated assessments of land degradation. In order to make such integration effective, the SNAPD has adopted an assessment methodology based on multidisciplinary models of representative areas, hereafter the SNAPD models. The methodology has been applied so far to five socio-ecological systems. This paper presents the model corresponding to one of these applications and the assessment procedure based on it.