Habitat ecology of southern African quartz fields: studies on the thermal properties near the ground Ute Schmiedel* and Norbert Jürgens Botanical Institute and Botanical Garden, University of Hamburg, Ohnhorststraße 18, Hamburg, D-22609, Germany; *Author for correspondence (e-mail: Uschmiedel@botanik.uni-hamburg.de) Received 16 January 2001; accepted in revised form 30 August 2002 Key words: Desert pavement, Dwarf growth forms, Leaf-surface temperature, Soil-surface temperature, Succu- lent Karoo, Winter-rainfall desert Abstract Desert pavements of white quartz stones (quartz fields) represent azonal habitats in several arid regions of south- ern Africa. The vegetation of these quartz fields is characterised by dwarf and highly succulent growth forms which contrast strongly with the shrubby vegetation of the surroundings. Incoming and reflected global solar radiation, air temperature near the ground, soil-surface temperatures and leaf-surface temperatures of dwarf plants were determined under natural habitat conditions inside and outside of the quartz fields. Surface temperatures of quartz and shale stones were compared. The study was conducted in the Knersvlakte and the Little Karoo (Suc- culent-Karoo Biome), South Africa. The daily maximum temperatures of the air near the ground of quartz fields was several degrees lower than the air near the ground of neighbouring soils without quartz cover. Maximum soil-surface temperatures of the quartz fields, however, were only lower in summer. Accordingly, the leave sur- faces of dwarf plants (Aizoaceae) growing inside quartz fields were up to 3 K cooler than identical plants outside the quartz fields. The lower maximum temperatures is associated with an about 5% higher reflection of quartz fields compared to neighbouring soils without quartz cover. At night, the minimum temperatures of the air near the ground and the soil surface temperatures on quartz fields were above those of soils without quartz cover. The nocturnal surface temperatures of quartz stones did not differ considerably from that of shale and often ranged above the temperatures and dew-point of the ambient air. Consequently, the relatively cooler quartz fields seem to provide less adverse growing conditions for plants near the ground compared to surrounding soils without quartz cover. Introduction The quartz fields Quartz fields in the arid part of southern Africa repre- sent a unique, azonal habitat type. They are charac- terised by a dense layer of white, angular quartz stones on the soil surface. The vegetation of these quartz fields differs considerably from its surround- ings by a particular species composition and the dom- inance of dwarf and ground level growth forms (Fig- ure 1) (Schmiedel and Jürgens 1999, 2002) with low height and low cover values (Figure 2). Quartz fields are clustered into six regions in arid to semi-arid southern Africa (Schmiedel 2002) (Fig- ure 3). The Knersvlakte, Riethuis-Wallekraal area, Richtersveld and Little Karoo fall within the Succu- lent-Karoo Region sensu Jürgens (1991) of the Greater Cape Flora, which is botanically defined by a high number of species of Aizoaceae sensu Bittrich and Hartmann (1988) and a strong dominance of leaf- succulent growth forms (Milton et al. 1997). The Re- gion is ecologically characterised by a relatively mild temperature regime and highly predictable winter rainfall (Hoffman and Cowling 1987; Cowling et al. 1999). The Warmbad and Pofadder region are part of the Nama-Karoo Region of the Palaeotropis (Jürgens 153 © 2004 Kluwer Academic Publishers. Printed in the Netherlands. Plant Ecology 170: 153–166, 2004.