The response of soil erosion and sediment export to land-use change in four areas of Europe: The importance of landscape pattern
Introduction
Land-use change in Europe over the past decades has largely been driven by the introduction of new technologies (for a discussion see Ewert et al., 2005). The introduction of mechanized equipment, artificial fertilisers, herbicides, pesticides and new cultivars has led to an increase in productivity of between 400 and 500% (Hafner, 2003, Bakker et al., 2005a) depending on the crop type. Consequently, intensification has occurred in areas that were suitable for the implementation of these new technologies, and abandonment or de-intensification (i.e. decreasing agricultural production by reducing aggregate input levels per unit area) has occurred in areas that were less suitable (Mather et al., 1999, MacDonald et al., 2000, Baldock et al., 2002).
Whether or not an area is suitable or not for the application of new technologies is largely determined by accessibility, the potential of available soil resources, and the workability of the fields (i.e. slope, stoniness, etc.). Because of this, hilly and mountainous areas, especially those with degraded soils, have often experienced de-intensification whereas intensification has occurred on flatter areas with deep fertile soils.
De-intensification of land-use often implies the regeneration of natural land cover, or a conversion from a low protection cover type (e.g. arable land) to a higher protection cover type (e.g. grassland or forest). De-intensification of land-use may therefore be beneficial with respect to a reduction in onsite soil erosion and sediment export to rivers and lakes (Vanacker et al., 2005). Soil erosion and sediment export to rivers and lakes are commensurate with the cultivation of arable land and, depending on climate, on intensive grazing, especially in sloping areas. Soil erosion is to a large extent determined by the absence of protective land cover, whereas sediment export to rivers and lakes is determined by onsite sediment production and the connectivity of sediment sources and the river or lake. The latter factor is also a function of land-use, as the sediment transport capacity is different for different types of land-use (Van Rompaey et al., 2002).
Both soil erosion and sediment export to rivers and lakes are considered to be important environmental problems. Soil erosion is considered to deteriorate onsite soil quality in an irreversible way, which often results in a reduction of the production potential of the soil and may even be a driver of land-use change itself (Bakker et al., 2004, Bakker et al., 2005b). Sediment export to rivers and lakes is considered to be a problem for various reasons. Often the sediment is polluted with fertilisers, leading to eutrophication and the disturbance of fragile water ecosystems. Increased sediment supply to rivers will lead to excessive sedimentation in lakes and reservoirs, thereby threatening aquatic life and/or hydroelectric power production (Douglas, 1995, Vanacker et al., 2003).
Given that the drivers for intensification or de-intensification and erosion and sediment export are often similar (e.g. slope, soil type, distance to rivers) the response of erosion and sediment export to land-use changes resulting from intensification or de-intensification is likely to be non-linear. For example, the conversion of an erosion-prone land-use to a non-erosion-prone land-use (e.g. the conversion of arable land to shrublands, which is a land-use conversion that is typical of de-intensification) occurring on steep slopes would amplify the positive effects of the land-use change for soil conservation. With respect to the connectivity between sediment source and river or lake, the spatial configuration of a land-use change within the landscape is also important. For accessibility reasons, arable cultivation is generally maintained longer near to villages, which are also often close to rivers, while the arable fields located further away from the villages (and therefore from the rivers) are the first to become abandoned. This would lead to a relatively small reduction in sediment export to the rivers, even though the overall onsite sediment production would decrease significantly.
A reliable estimate of the impact of land-use change on erosion and sediment export is thus only possible by accounting for the non-random nature of land-use change within a landscape. In this paper we study the response of soil erosion and sediment export to past land-use change in both intensive and less intensive agricultural areas. We quantify the importance of land-use change with respect to soil erosion and sediment export as well as the role of the land-use change patterns.
Section snippets
Materials and methods
Land-use change was analysed over the past five to six decades in four case study areas of Europe (Fig. 1). Three of the study areas were subject to de-intensification (in Portugal, France, and Greece) whilst the fourth (in Belgium) was intensively cultivated. The selected areas represent typical European areas, as identified by MacDonald et al. (2000). Erosion and sediment export to lakes and rivers were modelled using the WaTEM/SEDEM model.
Land-use change
Fig. 2 shows the change in total area under arable land, grassland, forest, shrubland and other land-uses for all four areas. It can be seen that for the Amendoeira and Lautaret sites, the total area under arable land has decreased in favour of forest, shrubland and (Montado) grassland. In Lagadas, grassland decreased in favour of shrubland and forests. Over a longer time span, Hageland shows no clear land-use changes, except for the introduction of orchards. Table 3 shows the dominant land-use
Evolution of erosion and sediment export in de-intensified and intensified areas
De-intensification of marginal agricultural areas in Europe has resulted in a strong decrease in arable land in favour of less intensive land-uses (e.g. shrubland, forests and grasslands). These land-use conversions have led to a large decrease in soil erosion and sediment export to rivers in such areas. De-intensification generally leads to an increase in protective vegetation cover so that onsite sediment production decreases, whereas sediment transport capacity reduces resulting in further
Conclusions
A simulation of erosion response to land-use change in four typical European areas during the last 50 years, showed that de-intensification of land-use in marginal agricultural areas has strongly reduced erosion and sediment export to rivers. This reduction is often amplified by the conversion of an erosion-prone land-use to a less erosion-prone land-use (e.g. the conversion of arable land to forest) on steeper slopes. Arable land is often maintained the longest on suitable, but erodible
Acknowledgement
This research was carried out under the EU-funded VISTA project (Vulnerability of Ecosystem Services to land-use Change in Traditional Agricultural Landscapes), project nr. EVK2-CT-2002-00168.
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