ECO‐GEOMORPHOLOGICAL PROCESSES WITHIN GRASSLANDS, SHRUBLANDS AND BADLANDS IN THE SEMI‐ARID KAROO, SOUTH AFRICA

Vegetation type and cover play an important role in the operation of geomorphological processes by controlling runoff and sediment dynamics. In drylands, land degradation is particularly sensitive to these eco-geomorphic interactions. Although many geomorphological studies of land degradation focus on the change in hydrological response as a function of vegetation cover, few have investigated how the autogenic response of plants may influence the susceptibility of soil to erosion through a change of soil resources. This study investigates the hypothesis that shrub communities possess greater soil parameter heterogeneity compared with grasslands and assesses how these different scales of heterogeneity can influence the susceptibility of soil to erosion. Soil samples were taken from seven 60 m × 60 m plots within grasslands, shrublands and badlands situated in the Sneeuberg uplands of the central Karoo. One hundred and eight samples per plot were analysed for bulk density, organic matter, pH, conductivity and available sodium, calcium, magnesium, potassium and phosphorus. Geostatistical analyses determined that the grassland landscape was largely homogenous in its distribution of soil parameters, whereas shrublands demonstrated an increase in heterogeneity. Periodicity in the semi-variograms indicated that regular patterns across the landscape were evident for all parameters and thus likely to represent the differences between shrub and intershrub regions, areas of high and low erodibility. More pronounced patterns were identified in the badlands. This indicates that, if the conditions are right, changes in plant-soil interactions caused by soil parameter redistribution in shrubland landscapes can exacerbate erosion, leading to further degradation in the form of badlands. Copyright © 2012 John Wiley & Sons, Ltd.

[1]  R. Bryan.  Soil erodibility and processes of water erosion on hillslope , 2000 .

[2]  S. Rayburg,et al.  Nonlinear biofluvial responses to vegetation change in a semiarid environment , 2007 .

[3]  D. Dunkerley.  Ecogeomorphology in the Australian drylands and the role of biota in mediating the effects of climate change on landscape processes and evolution , 2010 .

[4]  Patch-level based vegetation change and environmental drivers in Tarim River drainage area of West China , 2010, Landscape Ecology.

[5]  S. Milton,et al.  Desertification in the semi-arid Karoo, South Africa: review and reassessment , 1995 .

[6]  B. D. Hahn,et al.  On the dynamics of grazing systems in the semi-arid succulent Karoo: The relevance of equilibrium and non-equilibrium concepts to the sustainability of semi-arid pastoral systems , 2005 .

[7]  J. Boardman,et al.  Evidence from field-based studies of rates of soil erosion on degraded land in the central Karoo, South Africa , 2009 .

[8]  D. Dunkerley,et al.  The influence of organic litter on the erosive effects of raindrops and of gravity drops released from desert shrubs , 1999 .

[9]  Debra P. C. Peters,et al.  Nonlinear dynamics in arid and semi-arid systems: Interactions among drivers and processes across scales , 2006 .

[10]  Mark A. Nearing,et al.  Detachment of soil by flowing water under turbulent and laminar conditions , 1994 .

[11]  A. Parsons,et al.  Depth distribution of interrill overland flow and the formation of rills , 2006 .

[12]  Rattan Lal,et al.  Towards sustainable land management in the drylands: Scientific connections in monitoring and assessing dryland degradation, climate change and biodiversity , 2011 .

[13]  J. B. Thornes,et al.  The Ecology of Erosion , 1985, Geography.

[14]  Costas Kosmas,et al.  The effect of land parameters on vegetation performance and degree of erosion under Mediterranean conditions , 2000 .

[15]  G. Levy,et al.  Wetting rate and sodicity effects on interrill erosion from semi-arid Israeli soils , 2002 .

[16]  J. Oades.  Soil organic matter and structural stability: mechanisms and implications for management , 1984 .

[17]  John Wainwright,et al.  Spatial variability of soil and nutrient characteristics of semi‐arid grasslands and shrublands, Jornada Basin, New Mexico , 2007 .

[18]  S. Milton,et al.  A Conceptual Model of Arid Rangeland Degradation , 1994 .

[19]  John Wainwright,et al.  Effects of vegetation change on interrill runoff and erosion, Walnut Gulch, southern Arizona , 1995 .

[20]  Bin Zhang,et al.  Shear strength of surface soil as affected by soil bulk density and soil water content , 2001 .

[21]  Plant-soil Interactions in Deserts , 1998 .

[22]  J. Boardman,et al.  Development of badlands and gullies in the Sneeuberg, Great Karoo, South Africa , 2003 .

[23]  J F Reynolds,et al.  Biological Feedbacks in Global Desertification , 1990, Science.

[24]  Jean Poesen,et al.  The importance of plant root characteristics in controlling concentrated flow erosion rates , 2003 .

[25]  J. Thornes.  Coupling erosion, vegetation and grazing , 2005 .

[26]  Jean Poesen,et al.  Effects of grass roots on the erodibility of topsoils during concentrated flow , 2006 .

[27]  N. West,et al.  Plant-Induced Soil Chemical Patterns in Some Shrub-Dominated Semi-Desert Ecosystems of Utah , 1975 .

[28]  William E. Dietrich,et al.  Flow resistance and sediment transport by concentrated overland flow in a grassland valley , 1995 .

[29]  I. Burke,et al.  Heterogeneity of soil and plant N and C associated with individual plants and openings in North American shortgrass steppe , 1991, Plant and Soil.

[30]  Richard Teeuw,et al.  Variations in soil dispersivity across a gully head displaying shallow sub‐surface pipes, and the role of shallow pipes in rill initiation , 2004 .

[31]  Artemi Cerdà,et al.  The effect of patchy distribution ofStipa tenacissimaL. on runoff and erosion , 1997 .

[32]  William H. Schlesinger,et al.  ON THE SPATIAL PATTERN OF SOIL NUTRIENTS IN DESERT ECOSYSTEMS , 1995 .

[33]  S. Milton,et al.  Disturbance, drought and dynamics of desert dune grassland, South Africa , 2000, Plant Ecology.

[34]  S. Milton,et al.  The application of state‐and‐transition models to rangeland research and management in arid succulent and semi‐arid grassy Karoo, South Africa , 1994 .

[35]  D. Tongway,et al.  Small-scale resource heterogeneity in semi-arid landscapes , 1994 .

[36]  R. Nowak,et al.  Soil resource heterogeneity in the Mojave Desert , 2002 .

[37]  Mark A. Nearing,et al.  Soil Detachment by Shallow Flow at Low Slopes , 1991 .

[38]  David S. G. Thomas,et al.  Soil water movement and nutrient cycling in semi-arid rangeland : vegetation change and system resilience , 1998 .

[39]  K. Havstad,et al.  Long-term influences of shrub removal and lagomorph exclusion on Chihuahuan Desert vegetation dynamics , 1999 .

[40]  H. Olff,et al.  Interactions between above‐ and belowground biota: importance for small‐scale vegetation mosaics in a grassland ecosystem , 2000 .

[41]  R. Cowling,et al.  Vegetation dynamics in the semi-arid eastern Karoo, South Africa: the effect of seasonal rainfall and competition on grass and shrub basal cover. , 1990 .

[42]  John Wainwright,et al.  A conceptual framework for understanding semi‐arid land degradation: ecohydrological interactions across multiple‐space and time scales , 2008 .

[43]  M. Meadows,et al.  The nature, extent and causes of land degradation in South Africa: legacy of the past, lessons for the future? , 2002 .

[44]  Fernando T. Maestre,et al.  Spatial patterns of surface soil properties and vegetation in a Mediterranean semi-arid steppe , 2002, Plant and Soil.

[45]  Lalit Kumar,et al.  Fine-scale spatial distribution of plants and resources on a sandy soil in the Sahel , 2002, Plant and Soil.

[46]  John Wainwright,et al.  Plot-scale studies of vegetation, overland flow and erosion interactions: case studies from Arizona and New Mexico , 2000 .

[47]  Jean Poesen,et al.  Impact of plant roots on the resistance of soils to erosion by water: a review , 2005 .

[48]  A. D. Abrahams,et al.  Vegetation influences on water yields from grassland and shrubland ecosystems in the Chihuahuan Desert , 2002 .

[49]  John Wainwright,et al.  Responses of interrill runoff and erosion rates to vegetation change in southern Arizona , 1996 .

[50]  J. Poesen,et al.  Modified topsoil islands within patchy Mediterranean vegetation in SE Spain , 1999 .

[51]  W. Bond,et al.  Desertification of the eastern Karoo, South Africa: Conflicting paleoecological, historical, and soil isotopic evidence , 1995, Environmental monitoring and assessment.

[52]  R. Bryan,et al.  The use of rainfall simulation tests to assess the influence of vegetation density on soil loss on degraded rangelands in the Baringo District, Kenya , 1995 .

[53]  Hong S. He,et al.  Periodicity in spatial data and geostatistical models: autocorrelation between patches , 2000 .