Research ArticlesSoil Erosion Thresholds and Alternative States in Northeastern Patagonian Rangelands
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INTRODUCTION
Soil erosion is a natural process but the quantity and rate of surface runoff and sediment production may be accelerated by different types of land use or different management practices (Weltz et al. 1998). Soil erosion affects the characteristics and the dynamic of many semi-arid plant communities and ultimately may limit the capacity of the land to produce different goods and services. In the long term, erosion may create a mosaic of land surfaces that represents the various state of soil
Study Area
The study area is located in the northeastern of the Chubut province, in the Patagonian region of Argentina, centered at approximately lat 43°00′S, long 64°30′W. The climate is arid and temperate. Mean annual precipitation is 258 mm (1995–2004). Although most of the rainfall occurs during the cold season (from April to September), heavy rainfall events are more common during the warm season. The mean annual temperature is 12.5°C and the mean annual wind velocity at 10 m above ground level is 4.6
Surface Characteristics
Plant and litter cover were significantly higher (P < 0.05) in the grass and the degraded grass steppes than in the shrub steppe (Fig. 1). Gravel cover showed an opposite trend with greater values in the DSS. Plant and litter cover were positively correlated (r = 0.42), while both were negatively correlated with gravel cover (r = -0.74 and -0.75, respectively). In the DSS where erosion has been more intense than in the other plant communities, we found that the surface gravel concentration
Soil Surface Characteristics
In the Punta Ninfas rangelands we recognized a two-phase mosaic, a GS with scattered shrubs and DSS that represent the extremes of the degradation gradient. Soil surface characteristics differ greatly among these plant communities. While in the conserved GS and the DGS the soil protection factors are represented mainly by biotic or biotic-derived factors, plant and litter cover, in the degraded shrub-interspaces areas of the DSS, these protection factors are in part replaced by gravel cover (
MANAGEMENT IMPLICATIONS
Perennial grasses are the main forage resources for domestic and wild herbivores and constitute, along with the litter they produce, the main soil protection factor in the original grass with scattered shrub steppes of the Punta Ninfas rangelands. This plant community is not resilient to continuous grazing (i.e. vegetation changes are not continuous and reversible), and in those areas where perennial grasses cover were reduced, soil erosion has increased markedly. The erosion process has
CONCLUSIONS AND IMPLICATIONS
Grazing by removing perennial grasses and pulverizing the surface soil can have a major impact on soil erosion. The primary importance to apply the site conservation threshold concept is to depict the circumstances surrounding vegetation and soil changes on rangeland ecosystems and to use this information to anticipate and intercept persistent transitions in the future (SRM Task Group 1995).
In Punta Ninfas rangelands the increase in soil erosion has been closely associated with the decrease in
ACKNOWLEDGMENTS
We thank Lina Videla and Alicia Toyos for their assistance in fieldwork. We acknowledge the constructive comments provided by an anonymous reviewer. We are especially grateful to Brandon T. Bestelmeyer for many valuable suggestions that contributed to improve the manuscript.
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Research was funded by the Agencia Nacional de Promoción Científica y Tecnológica and CONICET of Argentina.