Soil erosion and desertification are recognised throughout the Mediterranean region as severe problems for land managers, the local community people, regional and national governments. These factors impact on the productivity of agricultural systems by reducing soil depth, increasing the amount of water running off fields, and thus decreasing water availability for crops. Off-site impacts include the siltation of irrigation and drinking water reservoirs, flash flooding and damage to irrigation water supply systems.

Current Mediterranean land-use is a response to climate, soil type and the economic environment. Consequently perennial crops such as olives and vines dominate the region. Growers in this semi arid area have traditionally been resistant to the use of inter-row vegetative cover to protect the soil because of fear of competition for water with the crop. Increased frost risk due to the evaporative cooling effect of ground cover in areas where low temperatures are experienced is an added concern.

Thus, adoption of vegetative cover as a soil protection measure in the Mediterranean continues to be variable despite promotion of its benefits. In this paper, trials to encourage soil conservation in Spanish olives and French vineyards are described . The work examines the impacts of vegetative cover and its management on soil erosion, crop yields and quality of production

Introduction

Desertification is recognized as an important problem by the countries of Mediterranean Europe (Beaufoy, 2001; European Commission, 2001; Battany & Grismer, 2000; Martinez-Casanovas & Sanchez-Bosch, 2000; Kertesz & Mika, 1999), with all signing up to the UN Convention to Combat Desertification. There is some evidence that the region is suffering from a progressive drying of the climate (Kertesz & Mika, 1999) and future projections (IPPC, 2001) suggest that the climate may become more severe. In response, a number of research projects have been conducted into desertification within this region - for example the MEDALUS projects funded by the European Union (see Brandt et al, 2002). However, much of this work has focused on understanding desertification of natural and semi-natural landscapes (e.g. Kosmos et al, 1997; Quinton et al, 1997). In contrast, agricultural land has received relatively little attention.

While various scenarios continually fuel the climate change debate, there is little doubt that agriculture within the region is changing at pace. Driven by social, political and economic factors, farmers mix traditional approaches to land husbandry with modern production methods to sustain and enhance their livelihoods in this harsh environment by optimizing their incomes from subsidy and market price (Martinez-Casanovas & Sanchez-Bosch, 2000; de Graaff & Eppink, 1999; Ramos & Porta, 1997).

This paper discusses the development of soil protection practices in olives and vines, two important crops grown in this environment, as a method of sustaining production.

Olives and vines in Europe

Between 1994 and 1997 the area of olives grown in the EU increased by 3% or 145 000 ha (Agra Europe, 2000). The area under vines grew by 202 000 ha to 3 533 000 ha between 1994 and the 1996/97 growing season (Agra Europe, 2000) - a 6% increase. The growth in olive production can be attributed to favourable subsidies (Beaufoy, 2001); the proliferation in vine acreage would seem to be driven by greater wine consumption throughout Europe.

Traditional land husbandry for olives integrated low density planting, 100 trees/ha or less (de Graaff & Eppink, 1999), with livestock - sheep or goats grazing an under-storey of annual vegetation. Such systems can still be found in the Mediterranean but now represent only a small proportion of regional production . New olive groves are planted at ever-increasing densities - up to 400 trees/ha - and are often irrigated (Beaufoy, 2000). In these more intensive plantings, and in less intensive older groves, ground between the crop rows (inter-row) is kept clear of weeds by a combination of regular tillage and residual herbicides. Farmers, seeking to maximise production and minimise the risk of a bad harvest, cite the competition for water and nutrients as the primary reason for this practice (Pastor & Castro, 1995).

The production of grapes for wine is another example of agriculture steeped in tradition. Again many farmers prevent competition for water and nutrients by keeping the inter-row clear of vegetation using a combination of regular tillage and residual herbicides. Management practices outlined by 'Appellation d'Origine Controlee' organisations may also be imposed on farmers (Jackson & Lombard, 1993; Winepros.org, 2002).

In both agronomic systems the use of traditional conservation farming practices, such as terracing and contour planting, have recently been ignored or removed due to maintenance costs and the mechanisation requirements of more intensive production methods (Beaufoy, 2001; Martinez-Casanovas & Sanchez-Bosch, 2000). This change in land management combined with clean weeding leaves bare soil vulnerable to raindrop impact, structural degradation and the formation of surface crusts. This reduces the potential for infiltration promoting runoff and erosion. From the one million hectares of olive plantations in Andalucia (Spain), is it estimated that as much as 80 million tonnes of soil are lost to erosion each year (Beaufoy, 2001). In comparison with other typical crops in the Mediterranean region, land under vines results in the highest soil losses (Martinez-Casanovas and Sanchez-Bosch, 2000).

Potential for soil protection

In both systems there is considerable potential to lower rates of soil erosion and conserve water by using some form of inter-row cover. The impact of cover on soil erosion is well documented (Morgan, 1986). A low, dense canopy and the litter produced from the vegetation reduce raindrop impact. The addition of plant litter to the soil can improve its structure, permeability and stability (Walsh & Voigt, 1977). Plant roots can increase soil strength (Tobias, 1994) and are thought to promote infiltration (Styczen & Morgan, 1994).
Schaller and Emde, cited in Walter (1991) reported a decrease in erosion with the use of green manures in the vineyards of southern Germany, on slopes between 10 and 32 %. Under green manure only the steepest slope (32 %) produced any sediment (3.1kg/ha), five orders of magnitude lower than the 96,000 kg/ha collected from the tilled plot. In olives Raglione et al (1999) compared ploughing with a cover crop of Festuca arundinacea, Lolium perenne and Poa pratensis. Mean soil losses were 2,524 kg/ha/event from the ploughed plots and 28 kg/ha/event from those with a cover crop. Castro and Pastor (1993) working on slopes between 8 and 10% showed that cover crops reduced erosion to less than 0.5 t/ha compared to 5 t/ha for zero tillage and 14 t/ha for tilled olive groves.

Concerns about the use of vegetative cover

Torres et al (2000) identified the common concerns of Spanish olive farmers regarding the use of vegetative cover (Table 1); these are also likely to be key concerns for grape producers.

Of the issues given in Table 1, competition for water and increased frost damage have received greatest attention.

Water - yield v quality

It is clear that vegetative cover can have a positive impact on soil erosion and runoff in both vines and olives, however farmers retain resistance to their adoption. For growers in dry environments, perhaps the greatest concern is large crop losses, caused by competition for water, similar to those recorded in trials by Civantos & Torres in 1981 and Pastor in 1990 (Pastor & Castro, 1995). However, with careful selection and management of the cover such problems can be avoided.
Working in German vineyards Bamberger (1991) showed that while initially soil moisture was depleted more rapidly in a vegetated plot, there was subsequently greater rainfall infiltration . Work by Schaller and Emde (in Walter 1991) corroborates this. They found that green manures limited runoff coefficients to less than 0.004 compared to between 0.12 and 0.16 for tilled soils. Oliveira (2001), Klik et al(1998) and Gril et al(1989) have suggested that a degree of water stress can even benefit the quality of the vine crop.
In olives, Raglione et al (1999) found similar results. Runoff coefficients were reduced from 12.8% in ploughed plots to 3.5% in plots with cover crops. The cover crop benefited olive yield, producing 10.2 kg/tree compared to 6.1 kg/tree obtained from the ploughed treatment. In contrast, Castro and Pastor (1993) found that at the end of the dry season the soil profile contained more water when the soil had been tilled than when cover crops had been grown. Destroying the cover crop by spraying with glyphosate prevented the uptake of water and left a protective residue of plant material on the soil surface. This increased the amount of water available to the olives, when compared with mechanical weed control and zero tillage.

Risk of frost

While it is understood that the use of vegetative cover can, in some circumstances, escalate the risk of frost damage - particularly to vines (Agroecology.org, 2002; Ingles, 2002; Gril et al, 1989) - careful selection of cover type and management practices can over come this risk. When discussing the benefit of permanent grass and mulching on runoff and erosion limitation in vineyards, Gril et al (1989) stated that 'the erodible areas (slopes) rarely coincide with the freezing zones (flats).' As a precautionary measure the cover can be mowed or sprayed off just before bud break although mowing may reduce some of the cover's benefits (Agroecology.org, 2002). Research by Pradel & Pieri (2000), into the effects of grass layers on soil temperature in vines, found that the grass layer kept soil temperatures steadier and, therefore, kept thermal amplitudes lower when compared to bare soil. This can have a positive influence on berry quality at harvest.

This project

Trials established in vineyards near Orange and Carcassonne, France and in olive groves near Seville, Spain are being used to develop vegetative cover management practices, which have minimal impact on crop yield, and quality of production, whilst protecting the soil from erosion. A holistic approach is being adopted, encompassing agronomy, climate, soil, legislation / regulation and economic viability.
In this first year, grass mixtures appropriate for local conditions were planted for inter-row cover. In the future, mixtures with legumes or the natural weed flora will be investigated. Currently, the contact herbicide Gramoxone® is the tool of choice to manage the cover crop in the spring before competition for water (and nutrients) becomes limiting for the main crop. One key objective is therefore to determine the appropriate timing and rate of application, in order to maximize the production of the crop, yet maintain sufficient cover during the summer

Promoting soil protection

Although the principles of using vegetative cover to protect the soil has been recognised for many years and are widely promoted in France and Spain, the majority of farmers appear reluctant to adopt the practice. In part this can be attributed to possible risks to the production of their crop. Potential problems associated with cover crop use in olives and vines are outweighed by benefits to the farmer and the environment. Energy costs are reduced as the number of tillage operations decline dramatically. There is the prospect for improved returns by better marketing products under the conservation agriculture banner. The reduction of runoff and erosion may lead to improved water availability, reduced incidental losses of fertilisers and other agrochemicals applied to the soil surface, as well as maintaining the productivity of the soil. By maintaining inter-row biomass, soil structure is likely to be improved and there may be additional benefits as more organic matter is continually returned to the soil.
The Spanish government has legislated for the use of vegetative cover under olives by offering an additional subsidy to farmers. It was not possible to find official figures detailing uptake of the offer, but observations suggest that at present, adoption is less than 25%. In France, vineyard owners are being encouraged to join a scheme promoting inter-row cover. Uptake has been more frequent in areas where effective rainfall is greatest. In the drier areas of the South-east, few farmers have elected to join.

Conclusions

It is clear that without some form of soil protection the cultivation of vines and olives in many areas of the southern Mediterranean is not sustainable. Farmers are an integral part of the process to develop locally appropriate conservation systems but require help in understanding the benefits of adopting erosion control practices, of which cover crops are one, if their farms are to continue to produce grapes and olives for future generations.

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