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1 February 2001 Marginal Lands and Erosion in Terraced Fields in the Mediterranean Mountains
Teodoro Lasanta, José Arnáez, Marco Oserín, Luis M. Ortigosa
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Abstract

Since the 1950s, the Spanish Mediterranean mountains have become a marginal territory, with few inhabitants and limited economic activity. As a consequence, significant land use changes (farmland abandonment, development of extensive cattle rearing, and reforestation) have taken place, resulting in landscape degradation and new hydromorphological processes. In this article, the process and impact of farmland abandonment in the Camero Viejo (northwestern Iberian System) are described. The authors also studied the geomorphological evolution of terraced fields after cultivation was given up. Runoff rates and sediment yield in abandoned and grazed terraces were measured using a rainfall simulator. Results show that the speed and intensity of the hydrological and erosional response increased if plots were grazed intensively. The study demonstrates that new land management systems in the Camero Viejo, in particular extensive cattle grazing, have generated additional source areas of sediments.

Introduction

For centuries, the Mediterranean mountains provided a variety of resources for a relatively high number of inhabitants, whose livelihoods were secured by a dynamic economy (livestock rearing, textile industry, forestry, and diversified agriculture) (Lasanta 1990; MacNeill 1992). However, as of the 1960s, the economic significance of these regions diminished drastically due to their limited possibilities of reacting to market demands and the lack of population. Moreover, environmental conditions in these mountain areas make them unsuitable for new economic activities. Snow does not accumulate enough to enable the development of winter tourism (ski resorts). The landscape, marred by reduced forest cover and extended erosion processes, is not attractive for summer tourism. The Mediterranean mountains have become marginal territories with few inhabitants; cultivation has been almost completely abandoned and livestock numbers have decreased. This evolution has resulted in radical modification of the landscape (García-Ruiz and Lasanta 1993; Douglas et al 1994).

Such processes were monitored by the authors in the Camero Viejo, a massif in the northwestern part of the Iberian System (La Rioja, Spain). Important land management changes due to depopulation and the influence of nearby cities were assessed. This assessment showed that agricultural areas in this mountainous region have been abandoned and recolonized by scrub. Some abandoned fields and hillslopes have been either reforested or are being used for extensive cattle grazing. In short, the physical environment and social conditions in the area have been profoundly modified, reflecting a widespread development throughout the mountain regions in Spain and other Mediterranean countries (Rodriguez-Aizpeolea and Lasanta 1992).

This paper presents information on the type of erosion caused by recent land use changes in the Camero Viejo. The case study shows how the environmental situation in the Spanish Mediterranean mountains has evolved in recent decades. Particular attention was given to erosion processes on abandoned terraces in order to gain some insight into the future development of this form of agricultural landscape.

The study area

The study site, known as the Camero Viejo, includes the high valleys of the Leza and the Jubera, both of which are tributaries of the River Ebro. It covers 392 km2 and is located in the province of La Rioja (Figure 1). The Camero Viejo is a region classified as mountainous with a moderate elevation, with altitudes ranging between 600 and 1700 m. The dominant lithological types are quartz sandstones, sandstones, and limestones of the Lower Cretaceous (Weald facies). The relief is characterized by very gentle summits, moderately steep slopes, and narrow valley bottoms.

According to García-Ruiz and Martín-Ranz (1992), the climate type is Mediterranean with mountain influences. The annual mean temperature is 10 to 11°C and average precipitation per year ranges between 600 and 800 mm. The highest precipitation values are recorded mainly in spring and autumn. The summer is a very dry season.

Abandoned fields and submediterranean scrub prevail in the study area. On calcareous soils, Genista scorpius, Thymus vulgaris, Rosmarinus officinalis, and Buxus sempervirens are the most abundant scrub species, while on siliceous soils, Cistus laurifolius is most common. Remnants of the original forest exist only in remote places with limited access: small stands of Fagus sylvatica and Quercus pyrenaica cover the steepest and most inaccessible slopes, particularly on shady sites.

Methods

Several methods were used to obtain information on recent land use changes and resulting erosion processes. Aerial photographs taken in 1956, 1977, and 1991 were used to draw land use maps in order to determine how the landscape evolved during these 35 years. Three types of abandoned fields (terraces, sloping fields, and flat fields) and forest areas were given particular attention in this mapping process. The data were digitalized by means of a Geographical Information System (GIS). It was thus possible to reconstruct the process of farmland abandonment and reforestation in a relatively comprehensive manner.

A total of 86 terraced fields with various topographical and geoecological features (different altitudes, lithological types, soils, aspects, slope angles, and slope shapes) were selected for a detailed study. Topographical, botanical, and geomorphological data were collected from each field. Two types of erosion process were studied: slides, resulting in collapsed terrace walls and massive soil movement, and sheet wash erosion.

The length, width, and depth of each slide were measured. Outfall volume was calculated as a function of these factors (Haigh 1984). The data were correlated with different topographical variables (slope gradient and aspect) and the terrace features (plot length, plot width, terrace wall height, and terrace wall length).

Runoff and erosion due to low frequency/high magnitude events were studied with the help of rainfall simulation tests. The authors conducted 26 experiments on abandoned terraces with different types of vegetation cover and degrees of grazing intensity. The median rainfall intensity used in our experiments was 75 mm/h (standard deviation 14.8). In each experiment, rainfall was simulated for a period of 45 minutes on a plot of 1385 c