CONICET | Buscador de Institutos y Recursos Humanos

The challenge of rangeland degradation in a temperate semiarid region of Argentina: the Caldenal. Osvaldo A. Fernández and María Elena Gil. Em: ofernan@criba.edu.ar Centro de Recursos Naturales Renovables de la Zona Semiárida (CERZOS), Argentina Suggested conference topic area: Indirect drivers of desertification The phytogeographical region known as the Caldenal, comprises an area of approximately 10 million ha in the central part of Argentina, Southern South America (35-40º S, 62-66º W). It is a level to gently undulating plain covered with recent loessic sandy sediments of eolian origin. Dominant soils are Calciustolls with a petrocalcic hard pan horizon (caliche layer) at a depth of 0,5 to 1 m. The climate is temperate and semi-arid, characterized by an annual mean temperature of 15ºC, absolute maximum and minimum temperature have been 42.5ºC and -12.8ºC. There is an east-west precipitation gradient from 500 to 350 mm/yr concentrate in spring and fall. Annual potential evapotranspiration is 800 mm. This territory is characterized by the presence of the Caldén tree (Prosopis caldenia Burk.) which is the typical and almost exclusive tree, giving de name to the territory. Original physiognomy of the vegetation consisted in a gramineous steppe with isolated shrubs and trees. Rangeland deterioration was initiated at the beginning of the previous century with the settlement of colonizing Europeans ranchers. Stocking rates currently range from 5 to 7 cow/ha/year. Although the Caldenal have been always an almost empty land in human terms (0.5 to 1.3 person per square kilometer), the new settlers begun a livestock production industry based in natural grazing, with little knowledge and concern regarding ecosystem conservation. After few decades of inappropriate use, the Caldenal showed to be a fragile environment in which vegetation is easily damaged, slow to recuperate, and at times impracticable to recover. Major environmental impacts were the shift of plant community and structure and soil erosion, represented by: a) the replacement of the most abundant palatable bunchgrasses Poa ligularis, Stipa clarazii, S. tenuis, Piptochaetium napostaense, Digitaria californica, by the low nutritive unpalatable grasses such as Stipa gynerioides, S. tenuissima, S. ambigua S. bracychaeta and Elyonurus muticus; b) in extensive areas sites dominated by grasses which, have been converted to a scrubland, frequent species are: Condalia microphylla, Lycium chilense, Prosopis alpataco, Larrea divaricata, Chuquiraga erinacea; c) with a depleted plant cover, incipient but increasing soil erosion is present in extensive areas leading to desertification. The ecosystem may undergo irreversible desertification where the top soil has also been lost since many of the biotic and abiotic structural constituents are no longer present. With the purpose of reversing this situation, the remit of CERZOS in the Caldenal is to improve management conductive to a sustainable rangeland productivity while preserving its rich biodiversity. The initial approach initiated 20 years ago focused on implementing research to acquire the basic data needed for a better understanding of the ecosystem functioning in the presence of livestock. Research has been conducted to study major stresses (drought, herbivory, fire) on grass and shrub responses, as well as more basic studies on the ecophysiology of these species. We have gained knowledge which it extremely useful to initiate for the first time rangeland utilization based on scientific information. To date, devastation of the natural ecosystem has not reached levels of degradation as in other parts of the world were desertification is an irreversible consequence. Although degraded, the rangeland system and desirable perennial grasses still exists, we can anticipate increased sustainable rangeland productivity while preserving the natural resources.