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Severely Degraded Dunes of the Southern Kalahari: Local Extinction, Persistence and Natural Reestablishment of Plants

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Severely Degraded Dunes of the Southern Kalahari: Local Extinction, Persistence and Natural Reestablishment of Plants Severely degraded dunes of the southern Kalahari: local extinction, persistence and natural re-establishment of plants Michael Charles Rutherford1,2* and Leslie Ward Powrie1 1Applied Biodiversity Research Division, Kirstenbosch Research Centre, South African National Biodiversity Institute, Private Bag X7 Claremont 7735, South Africa and 2Department of Botany and Zoology, Stellenbosch University, Private Bag X1 Matieland 7602, South Africa Une pression particulie`rement intense du paˆturage a ent- Abstract raıˆne´ un de´clin important du couvert de la canope´eetdu This study aimed to quantify and understand the impact of nombre d’espe`ces des formes vivantes annuelles et pe´r- severe land degradation on plant attributes and diversity ennes, monocotyle´dones et dicotyle´dones, rampantes et on dunes of the southern Kalahari. Heavy grazing pressure dresse´es, et des tiges feuillues, des touffes d’herbes et des in particular resulted in a significant decline of canopy architectures stolonife`res. Cependant, nous n’avons trouve´ cover and species number in annual and perennial life aucun changement significatif dans les arbustes et les forms; forb and graminoid growth forms; erect and pros- formes arbore´es qui survivaient sans nouveau recrutement trate habits; and leafy stem, tussock and stoloniferous visible. Dans ces conditions, la diversite´ des espe`ces de´cli- architectures. However, no significant change was found nait fortement et un certain nombre, surtout des gram- in shrub and tree forms which persisted without apparent ine´es, se sont apparemment e´teintes localement. L’arbuste new recruitment. Under these conditions, species diversity pe´renne Crotalaria cf. spartioides pre´sentait un sche´ma dropped sharply and a number of species, mainly grami- inverse avec une haute fre´quence de jeunes plants qui noids, became apparently locally extinct. The perennial s’e´tablissaient dans les dunes de´grade´es. Des changements shrub, Crotalaria cf. spartioides, showed the converse with a de dominance relative montrent que des traitements high frequency of establishing seedlings on the degraded extreˆmes favorisent les plantes pe´rennes par rapport aux dunes. Changes in relative dominance show that the annuelles, les plantes ligneuses par rapport aux gramine´es extreme treatment favours perennial over annual, woody et aux herbace´es a` fleurs, les plantes dresse´es par rapport over graminoid and forb, erect over prostrate and leafy aux rampantes et les tiges feuillues par rapport aux plantes stem over stoloniferous and tussock. Some of these results stolonife`res et aux touffes d’herbes. Certains re´sultats et and certain species and soil responses differ from those certaines re´ponses d’espe`ces et de sols diffe`rent de ceux qui reported from grazing studies elsewhere, and are possibly ont e´te´ enregistre´s lors d’autres e´tudes sur le paˆturage, no longer directly related to the impact of the primary re´alise´es ailleurs, et ils ne sont peut-eˆtre plus lie´s directe- grazing pressure but to the secondary effect of subsequent ment a` l’impact de la pression meˆme du paˆturage mais instability of the dunes. plutoˆta` l’effet secondaire qu’est l’instabilite´ des dunes qui en re´sulte. Key words: disturbance, extirpation, grazing, plant diver- sity, savanna, traits Re´sume´ Introduction Cette e´tude visait a` quantifier et a` comprendre l’impact This study contributes to a pilot research programme on d’une se´ve`re de´gradation de terrain sur les qualite´setla understanding the relationships between plant diversity diversite´ des plantes, dans les dunes du sud du Kalahari. and land degradation. The main cause of biodiversity loss in the arid savannas of southern Africa is land degradation *Correspondence: E-mail: [email protected] (Scholes & Biggs, 2005), which is also one of the major 930 Ó 2009 Blackwell Publishing Ltd, Afr. J. Ecol., 48, 930–938 Severely degraded dunes of the Kalahari 931 environmental issues in the subcontinent (Darkoh, 2009). This programme systematically targets areas of extreme degradation because of heavy grazing pressure within arid and semi-arid parts of South Africa. The focus in this study is on the southern Kalahari dunefield. A number of studies along degradation gradients in the southern Kalahari have indicated important compositional changes in the herba- ceous layer (Van Rooyen et al., 1990; Van Rooyen, Bre- denkamp & Theron, 1991a; Van Rooyen et al., 1994) and predicted woody plant encroachment (Jeltsch et al., 1997). This study aimed to assess which of these diversity-deter- mining results hold under particularly extreme levels of land degradation through overgrazing. Persistence was distinguished from re-establishment. The extent to which plant trait responses to grazing based on a global synthesis (Dı´az et al., 2007) apply under these extreme conditions was also examined. This synthesis has shown that grazing usually favours annual over perennial plants, short over tall plants, prostrate over erect plants and stoloniferous and rosette architecture over tussock architecture. Although our work considers the effects of extreme levels of degradation caused through heavy grazing pres- sure, the results should also relate to the anticipated effects of exposed and re-mobilized Kalahari dunes as a conse- Fig 1 Location of the study site within the southern Kalahari quence of climate warming in this century (Thomas, dunefield Knight & Wiggs, 2005). occurred under more arid, probably windier circum- Materials and methods stances, most recently between 17,000 and 10,000 years ago (Stokes, Thomas & Shaw, 1997). The aeolian sand of Study area the NW–SE trending parallel linear dunes varies from fairly The Kalahari dunefield in South Africa was systematically compact near the base of the dune to very loose on the scanned for major fence-line grazing contrasts using dune crests. The soil is classified as the Kalahari Family of satellite imagery (mainly SPOT5). There were many can- the Namib Form (Soil Classification Working Group, didate fence-line grazing contrasts, but the site that 1991). The less disturbed dunes lack slip faces. The study showed the greatest contrast was on the adjacent range- was limited to the dunes as they showed a more extreme land farms of Avonds Schijn and Ballater, and located contrast in vegetation than did the inter-dune areas. along a N–S fence near 26o56¢S21o08¢E in the Siyanda Height of dunes sampled varied between 7 and 21 m above District Municipality, Northern Cape Province. This site the inter-dune areas. The dunes are the only physical also complied with the sampling requirement that the features that break the otherwise flat landscape. Mean fence line cross at least 25 dunes to allow for adequate elevation was approximately 895 m amsl. randomized replication. Initial field inspection confirmed The site is situated in the Gordonia Duneveld vegetation that there was no indication of any pre-existing environ- type (SVkd 1) of the Savanna Biome (Mucina & Ruther- mental gradients across the fence. ford, 2006). Vegetation structure is an open, shrubby The site falls within the greater southern Kalahari grassland with occasional small trees. That the vegetation dunefield which is shared between Namibia, South Africa on the north- and south-facing slopes is very similar and Botswana (Fig. 1). These dunes were not formed un- (Leistner, 1967) probably relates to the NW–SE dune der current prevailing conditions and their emplacement orientation where solar incidence on the north slope is Ó 2009 Blackwell Publishing Ltd, Afr. J. Ecol., 48, 930–938 932 Michael Charles Rutherford and Leslie Ward Powrie compensated for by the exposure of the south (polar-fac- A total of 32 plots were paired between 25 and 45 m on ing) slope in the afternoons. both sides of the fence spread over a distance of 5.6 km. Mean annual rainfall is approximately 180 mm, falling Each plot was 50 m2 (belts of 20 · 2.5 m) and randomly mainly in summer and late summer from November to placed with the long axis at right angles to the linear dune. April and peaking in March. Summers are hot and severe As the width of dunes varied, randomization took into frost occurs in winter, particularly at ground level. Mean account width to ensure no bias against any section of a monthly maximum and minimum screen temperatures are dune. Minimum distance of plots to riverbeds or pans was 41.5°C and )4.0°C for December and July respectively, approximately 4 km. There were, however, a few indistinct according to a weather station in Gordonia Duneveld pan-like depressions (deflation basins – Van Rooyen, 1984) (Mucina & Rutherford, 2006). as close as 500 m to some of the plots. The farm Ballater had been stocked for about a decade Canopy cover of each vascular plant species was esti- with sheep at levels equivalent to approximately 6.5 ha mated using the same observer in all plots to reduce pos- per large stock unit (LSU), which is a few times more sible variation in these estimates between different intense than the recommended 24 ha per LSU (Jeltsch observers (Kercher, Frieswyk & Zedler, 2003). Species were et al., 1997) to 18 ha per LSU (Fourie, De Wet & Page, classed by life form (annual, perennial), growth form (forb, 1987) for the southern Kalahari dune region. The geophyte, graminoid, shrub and tree), habit (erect, pros- stocking level on the farm was relatively recently dropped trate) and architecture (leafy stem, rosette, stoloniferous to within the recommended range and comprised mainly and tussock). Canopy cover of all vegetation of each plot sheep and some springbok antelope. This part of the was estimated independently. Plant specimens were iden- study site is referred to as HU (High Utilization). Avonds tified by the National Herbarium of the South African Schijn’s stocking levels had remained below recom- National Biodiversity Institute. Persistence and seedling mended levels for years, also currently at approximately establishment were noted on HU.
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