CONICET | Buscador de Institutos y Recursos Humanos

Under natural conditions, semi-arid rangelands in the transitional zone between southern Caldenal District in the Espinal fitogeographicProvince and the austral part of the Monte Province in Argentina (~38-40º S and ~62-64º W) are under grassland domain (Distel and Bóo 1996; Kröpfl 1999). They present a spatial homogeneous herbaceous cover scattered with shrubs and/or trees. Lama guanicoe is the main large herbivore, and fire recurs every five-to-ten years. Heavy continuous grazing by livestock relaxes grass competition and reduces fire frequency, moving the system to a shrubland domain (Distel and Bóo 1996). Presently, rangelands are in regular or poor condition. This is shown by the replacement of palatable grasses by unpalatable grasses and/or annual species, increases in woody species density and cover, formation of shrub patches interspersed in a low-cover matrix, soil denudation and soil erosion. As a consequence, biomass production is depressed at landscape level. Recovery strategies for degraded rangelands need to consider the spatial extensity and related restoration cost. From this perspective, fire emerges as a plausible alternative. The use of controlled fires could serve to reverse dominance of unpalatable grasses and woody species in favor of the dominance of palatable grasses (Bóo et al., 1996, 1997), and to redistribute resources (soil, seeds, organic matter) from vegetation patches to adjacent bare soil in combination with post-fire wind and water erosion (Ravi et al., 2009). The fire strategy needs in addition the control of grazing, to allow for successful recovery of desirable vegetation. Management strategies for the sustainable use of restored rangelands would require the appropriate control of stocking and grazing. Since drought occurs frequently and unpredictably, a sound strategy may be to adjust stocking rate of breeding stock to rangeland carrying capacity under ordinary drought conditions, and to use the accumulation of biomass in normal or wet conditions for recovering of plant vigor, in burning for shrub control, or in feeding young growing stocks destined to sell for fattening. Under extraordinary drought conditions, timely destocking should be implemented to attenuate or prevent rangeland deterioration. On the other hand, the grazing strategy should be based on the maintenance of plant vigor to allow expression of productive potential, reproductive capacity and competitive ability of the key species. The way to achieve this strategy is to keep a minimal residual biomass for plant and soil protection, and to allow appropriate rest periods for plant recovery after defoliation. This in turn requires the operation of an efficient rotational grazing system.