Research Articles
Soil Erosion Thresholds and Alternative States in Northeastern Patagonian Rangelands

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Abstract

In semiarid rangelands, continuous grazing may decrease vegetation cover, accelerating soil erosion and eventually causing a transition to an alternative, degraded state. State-and-transition models invoke process-based explanations of alternative states, but there are few examples that use empirical data on key factors and processes. We used rainfall simulation to determine 1) the relationships between soil surface characteristics and interrill erosion in 3 spatially related plant communities: stable grass with scattered shrubs (GS), degraded grass with scattered shrubs (DGS), and degraded shrub steppes (DSS), and 2) the site conservation threshold (SCT) of this rangeland. We also analyzed the effect of past erosion on soil and vegetation characteristics. In the GS, sediment production and sediment concentration were significantly lower (p < 0.05) than in the DGS and the DSS. The main soil protection factors in the GS and in the DGS were perennial grass and litter cover, while in the DSS, gravel cover became the main soil protection factor. The SCT, the point at which the rate of soil erosion increases markedly, corresponded to a plant-and-litter cover close to 90% and occurred within the DGS. Although this plant community may reverse back to the conserved GS, long-term accelerated erosion may result in enough soil loss to trigger irreversible changes and prompt the transition to the DSS. The threshold underlying this transition would be reached when the A horizon is severely reduced by erosion. Under these conditions, the soil hydrological properties are affected irreversibly, preventing perennial grass establishment. While the GS represents a resource conserving plant community, desirable for both forage production and soil protection, the DSS represents a dysfunctional state with a minimum forage value. The DGS represents an unstable and transitional community that, without management intervention to halt soil erosion, will likely change into the DSS.

Resumen

En los pastizales naturales semiáridos, el pastoreo continuo puede disminuir significativamente la cobertura de la vegetación, acelerar la erosión del suelo y eventualmente causar una transición hacia estados alternativos degradados. Los modelos de estados y transiciones generalmente explican estados alternativos en función de distintos procesos, sin embargo hay pocos ejemplos que usen datos empíricos. Nosotros empleamos lluvia simulada a los fines de determinar 1) las relaciones entre las características superficiales del suelo y la vegetación y la erosión laminar en 3 comunidades vegetales espacialmente relacionadas: una estepa herbácea con arbustos (EH), una estepa herbácea degradada con arbustos (EHD), y una estepa arbustiva degradada (EAD), y 2) el umbral de conservación del sitio (UCS) en éste pastizal. También analizamos el efecto de la erosión histórica sobre las características superficiales del suelo y la vegetación. En la EH la producción y la concentración de sedimentos fueron significativamente más bajos (p < 0.05) que en la EHD y la EAD. Los principales factores de protección del suelo en la EH y la EHD, fueron los pastos perennes y el mantillo, mientras que en la EAD, la cobertura de gravas fue el principal factor de protección del suelo. El UCS, definido como el punto donde la tasa de erosión de suelo aumenta notablemente, se correspondió con una cobertura de pastos perennes y mantillo cercana al 90% y ocurrió dentro de la EHD. Si bien ésta comunidad puede revertir hacia la EH, la erosión acelerada mantenida durante largo tiempo puede producir una pérdida de suelo tan importante como para provocar cambios irreversibles e iniciar la transición hacia la EAD. El umbral subyacente a la transición se alcanzaría cuando, como consecuencia de la pérdida de suelo, el horizonte A es severamente reducido. Bajo estas condiciones, las propiedades hidrológicas del suelo son afectadas irreversiblemente, limitando el establecimiento de pastos perennes. Mientras que la EH tiene una alta capacidad para conservar los recursos del suelo, representa la comunidad vegetal deseable tanto para la producción de forraje como para la protección del suelo, la EAD representa una comunidad con baja capacidad para conservar los recursos del suelo y bajo valor forrajero. La EHD es una comunidad inestable y transicional que, sin un plan de manejo tendiente a detener la erosión acelerada del suelo, cambiará irreversiblemente a una estepa arbustiva degradada.

Section snippets

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.

LITERATURE CITED (47)

  • BestelmeyerB.T. et al.

    Development and use of state-and-transition models for rangelands

    Journal of Range Management

    (2003)
  • BlakeG.R.

    Core method

  • CarterM.R.

    Soil quality for sustainable land management: organic matter and aggregation interactions that maintain soil functions

    Agronomy Journal

    (2002)
  • DavenportD.W. et al.

    Viewpoint: sustainability of piñon-juniper ecosystems – a unifying perspective of soil erosion threshold

    Journal of Range Management

    (1998)
  • DidhamR.K. et al.

    Are systems with strong underlying abiotic regimes more likely to exhibit alternative stable states?

    Oikos

    (2005)
  • ElissaldeN. et al.

    Inventario y evaluación de pastizales naturales de la zona árida y semiárida de la Patagonia

  • FriedelM.H.

    Range condition assessment and the concept of thresholds: a viewpoint

    Journal of Range Management

    (1991)
  • GileL.H. et al.

    The Desert Project soil monograph

    (1979)
  • GoodallD.W.

    Some considerations in the use of point quadrats for the analysis of vegetation

    Australian Journal of Science. Series B

    (1952)
  • HavstadK.M. et al.

    Long-term ecological monitoring

    Arid Land Research and Management

    (2003)
  • HudsonB.D.

    Soil organic matter and available water capacity

    Journal of Soil and Water Conservation

    (1994)
  • KettlerT.A. et al.

    Simplified method for soil particle-size determination to accompany soil-quality analyses

    Soil Science Society of America Journal

    (2001)
  • LalR.

    Monitoring soil erosion’s impact on crop productivity

  • Cited by (0)

    Research was funded by the Agencia Nacional de Promoción Científica y Tecnológica and CONICET of Argentina.

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