Reserve paddock as an agroforestry technique in the arid lands of Mendoza, Argentina/El potrero de reserva como tecnica agroforestal en la zona arida de Mendoza, Argentina.

• Autor: Carretero, Eduardo Martinez

INTRODUCTION

The eastern plains of Mendoza extend over approximately 56,000 [km.sup.2]. These plains are constituted by alluvial material, showing water and aeolian erosion. Sand dune systems are Holocenic (Masotta & Berra, 1994). Elevation ranges between 450 and 750 m, and average annual precipitation between 110 and 250 mm (Table 1). Aridity increases northwards, reaching hyperarid conditions in the northeastern most localities (Roig et al., 1988). The scarce monthly precipitation and high monthly evapotranspiration --80 mm in July and 193 mm in January (Berra & Ciancaglini, 1979)--determine an annual hydric deficit of 1,312.5 mm in the entire area (Mendez et al., 1993).

The plains lie on sloping sandy and sandy-loamy soils with dunes up to 20 m high to the north. The landscape is dominated by three units: sand dunes, interdune depressions, and saline flat lands. In interdune depressions soils are mainly deep and sandy, showing 6.4

The dominant vegetation consists of perennial shrub species, particularly Larrea divaricata and Bulnesia retama (Zygophyllaceae), accompanied by Capparis atamisquea, Tricomaria usillo, Atriplex lampa, among others, and associated with open forests of Prosopis flexuosa. The study area is located at 32[degrees]00'- 33[degrees]10' S and 67[degrees]15'68[degrees]30' W, and stretches over nearly 15,000 [km.sup.2].

The drylands in the east of Mendoza (Figure 1) are unsuitable for agriculture, so effort must be put into managing native forage species.

The area shows only a few scattered human settlements. Their main source of income is livestock breeding (mostly goats). In addition, people use the natural resources available to meet their basic needs, as is common in thinly populated semiarid regions (Nair, 1994).

The number of goats recorded for the department of Lavalle (northeastern Mendoza) was 75,000 in 1996 (Guevara et al., 1999), which represented 16% of the goats in Mendoza, managed primarily by small-scale farmers.

In these zones, where natural resources are limited, one of the inhabitants' survival strategies is the diversified use of the various ecosystem components. Peasants ensure their basic income by using the different existing resources: goat herds, fauna, flora, fuelwood, etc. Handicrafts, and temporary jobs in nearby oases are additional sources of income. The use of a resource is necessarily based on an integrated production system, where every component participates in the total revenue. This use, whereby specialization gives way to diversification of the existing resources, is a common practice, as well as a genuine adaptive economic strategy, in arid environments. Thus, diversification and control over the entire production process are two key elements in rural organization (Regalsky, 1986).

Any policy designed to improve production systems should take into account the farmers' knowledge about diversified resource use, and therefore, it should be validated and accepted by the community involved. Nevertheless, in the particular case of the drylands of Mendoza, there is little intervention of the local population in improving the forage supply, as a passive and extractive attitude still prevails.

The most relevant livestock activity in the northeastern plain of Mendoza is goat breeding, where the current productivity of livestock ranches is low. According to Guevara et al. (1991) disorderly livestock management and seasonal forage deficiencies are the major factors affecting productivity. The internal rate of return for a few models of livestock production efficiency has been evaluated by Piacentini et al. (1975) and Nahman (1986). Revenue from livestock production in arid lands is low (Guevara et al., 1981, 1985) on account of commercialization problems, lack of investment on infrastructure, and overgrazing, among other factors.

Recently implemented municipal programs have encouraged the community to participate in integrating goat breeding with tree nursery (plantules of forage species for multiple use) in order to reverse a strictly livestock-oriented view.

Prosopis species are relevant in dryland agroforestry systems. Prosopis flexuosa is a multiple-purpose tree: aside from being a source of timber, it is an efficient dune fixer, a provider of shade, fuelwood and fodder, a producer of honey, and a nitrogen fixer. Being scarce in arid lands, Prosopis forests require a special appraisal as system stabilizers, mainly by promoting non-extractive uses to prevent tree loss.

Current management

Currently, small-scale farmers make a multiple use of resources; however, this use is merely extractive. The major source of income is goat commercialization (Table 2), when the animals have reached a weight of 8-10 kg.

Livestock grazing is continuous, which results in both overuse of the areas next to watering places, and little use of the rest of the field. The overgrazing problem gets worse in communal lands, on which herds of different farmers exert constant pressure.

In the NE of Mendoza, the current mean forage biomass is 150 kg of dry matter per ha, dominated by shrubs, which represents a carrying capacity of 3.65 ha/animal unit. The carrying capacity assessed using the non-destructive modified Point Quadrat method (Passera et al., 1983) was 2.12 ha/animal unit at sites in good conditions, and 7.5 ha/animal unit at sites in poor conditions (saline areas).

The current management shows high kid mortality (40-45%) after parturition in the autumn-winter period (May-June), owing to inadequate infrastructure and malnourishment of mother goats. Enclosures protected from southern winds...