A simulation model for a shrub ecosystem in the semiarid Karoo, South Africa

Ecology, Oct, 1995 by Thorsten Wiegand, Suzanne J. Milton, Christian Wissel

In this paper we respond to Walker's (1993) call for new rangeland models and we show that dynamic automata models can be applied successfully to semiarid ecosystems.

THE MODEL

Aim of the model

We present a simulation model, based on long-term field-investigations, for a typical semiarid ecosystem in the southern Karoo, South Africa. The aim of the model is to capture the main events and mechanisms that determine the temporal and spatial dynamics of common plant species on a large temporal scale. Detailed information about interannual variation in rainfall (which has an important impact on the dynamics of semiarid ecosystems), and on rainfall-dependent plant attributes (including seed production, germination, recruitment, and mortality factors), are included in the model. We employ the dynamic automata method to simulate the temporal and spatial dynamics of the ecosystem in annual time steps.

Site description

The 100-ha study site. Tierberg Karoo Research Centre (TKRC), lies at the southern edge of the semiarid Great Karoo, South Africa (33 [degrees] 10' S latitude, 22 [degrees] 17' E longitude, 800) m above sea level), 26 km west of Prince Albert. Ambient temperatures ranged from -3 [degrees] C (winter minimum) to 36 [degrees] C (summer maximum). The Universities of Cape Town and Natal carried out intensive field investigations and experiments at this site over 6 yr (1988-1993). Rainfall has been recorded by an automatic weather station at TKRC since 1988, and has been manually recorded at Prince Albert jail (26 km east of the study site) since 1878. Average ([ or -] 1 SE) annual rainfall (93 complete data sets) at Prince Albert is 167 [ or -] 7 mm with a range of 50-400 mm (Milton et al. 1992), and can occur at any season, although monthly averages peak in autumn (Fig. 1). Winter rain is associated with cyclonic fronts and is frequent though seldom heavy. Summer rain falls during thunderstorms (Cowling 1986), often in the form of brief cloudbursts, which result in rapid run-off. The even land surface of the study site is broken by three types of topographic feature: minor drainage lines (washes), drainage lines, and heuweltjies. Neither type of drainage line carries water for more than a few hours during rain storms. Heuweltjies are low mounds of [proximate] 15 m in diameter, formed by termites (Microhodotermes viator) and maintained by the activities of various other animals (Lovegrove and Siegfried 1989). Heuweltjies are evenly scattered over the plains (Milton et al. 1992).

[Figure 1 ILLUSTRATION OMITTED]

Vegetation, comprising perennial succulent and nonsucculent shrubs 0.2-0.4 m in height, covers 15-20% of the soil surface (Milton et al. 1992). Grasses and annuals are restricted to drainage lines and appear to play little part in the dynamics of this vegetation (Milton 1994) where succulent and nonsucculent shrubs replace one another cyclically (Yeaton and Esler 1990). The most extensive habitats are the run-off sites (plains), which cover 78% of the TKRC study site, and where plant species diversity is low (8-18 species per 25 [m.sup.2] quadrat) (Milton et al. 1992). Plant cover in plains shrubland averages 18% (range 11-13%) at the study site and is fairly evenly distributed between succulent and nonsucculent shrubs. The plains shrubland is characterized by isolated shrubs or by small, mixed-species clumps of shrubs interspersed with bare ground. Shrub density ranges from three to seven plants per square metre. Mean shrub crown diameter ranges between 200 and 500 mm, mean height from 150 to 600 mm, depending on the species. The structure and the dynamics of the vegetation of the plains are isolated from the other habitats, which have more plant-available water or nutrients (Milton et al. 1992). The vegetation at TKRC was moderately grazed by sheep from 1850 to 1987, after which time it was fenced to exclude domestic livestock. All biological data included in the model were collected in currently ungrazed vegetation.