Biogeography of Prostrate-Leaved Geophytes in Semi-Arid South Africa: Hypotheses on Functionality

Plant Ecology 142: 105–120, 1999. 105 © 1999 Kluwer Academic Publishers. Printed in the Netherlands. Plant form and function Biogeography of prostrate-leaved geophytes in semi-arid South Africa: hypotheses on functionality Karen J. Esler1, Phillip W. Rundel2 &PietVorster1 1Department of Botany, University of Stellenbosch, Private Bag X1, Matieland, 7602 South Africa (e-mail: [email protected]); 2Department of Biology, University of California, Los Angeles, CA 90095, USA Received 2 November 1998; accepted in revised form 20 November 1998 Key words: Amaryllidaceae, Biogeography, Geophylly, Leaf Orientation, Succulent Karoo Abstract Nowhere is the species diversity of geophytes greater than in the five mediterranean-climate ecosystems of the world. Of these, the Cape mediterranean zone of South Africa is the most speciose. While the relative diversity and importance of geophytes of all of the other four mediterranean regions of the world drops off sharply as one moves into adjacent winter-rainfall desert regions, geophytes in the semi-arid to arid Succulent Karoo (including Namaqualand) remain a very important component of the flora, both in terms of abundance and diversity (com- prising 13 to 29% of the regional floras in this region). Apart from species richness, there are also a number of interesting geophyte growth forms in this region. One unusual growth form is geophytes with flattened leaves that lie prostrate on the soil surface. At least eight families (Amaryllidaceae, Colchicaceae, Eriospermaceae, Gerani- aceae, Hyacinthaceae, Iridaceae, Orchidaceae and Oxalidaceae) exhibit this growth form. While this growth form is relatively common in many geophyte lineages in the Succulent Karoo biome and the Cape mediterranean zone (Fynbos biome), and occurs infrequently through the summer-rainfall temperate regions of Africa, it is virtually absent in other regions worldwide. A null hypothesis is that the prostrate leaved trait is a neutral characteristic, however biogeographical data do not support this. A neutral trait would be unlikely to show such a clear pattern of distribution. Several alternative hypotheses on the adaptive significance of this growth form are discussed. These include: avoidance of herbivory, reduction in competition from neighbors, creation of a CO2 enriched environ- ment below the leaves, reduction of water loss around the roots, reduction of water loss through transpiration, precipitation of dew on the leaves and maintenance of optimal leaf temperatures for growth. Introduction but nowhere are they more diverse and abundant than in the five mediterranean-climate ecosystems of the Geophytes are plants that possess underground resting world (Doutt 1994; Rundel 1996). Of these, the Cape buds attached to storage organs such as rhizomes, tu- mediterranean zone of South Africa is generally the bers, bulbs or corms. Although some geophyte species most speciose (Goldblatt 1978), with geophytes com- are evergreen, many of them survive periods of en- prising up to 40% (Nieuwoudtville Wild Flower Re- vironmental stress such as summer drought or winter serve, Snijman & Perry 1987) of some regional floras cold by dying back to these underground storage or- that have been disturbed by heavy grazing. It should gans (Dafni et al. 1981). They then resprout new fo- be noted, however that Pate & Dixon (1982) have liage in the following growing season. Inflorescences recorded that percentages as high as these for some may be produced before, during or at the end of the regional floras in southwestern and western Australia. vegetative growing season, a phenology that is con- The evolutionary success of geophytes in the Cape stant for most species. Biogeographically, geophytes Mediterranean zone of South Africa extends well into are widespread around the world in many habitats, the arid parts of this region. Geophytes of the semi- 106 Figure 1. Large, broad, prostrate-leaves of (A) Massonia sp. (Hyacinthaceae Leaf size D 15 × 25 cm) and (B) Brunsvigia sp. (Amaryllidaceae Leaf size D 12 × 24 cm). arid to arid Succulent Karoo (a winter-rainfall desert the flora of the Goegap Nature Reserve near Springbok adjacent to the true mediterranean zone and incorpo- where geophytes make up 16% (and petaloid mono- rating Namaqualand) remain a very important compo- cots, 13%) of the flora (Van Rooyen et al. 1990). In the nent of the flora, both in terms of abundance and di- Karoo Garden Reserve near Worcester they comprise versity (Snijman 1984; Goldblatt 1986; Duncan 1988; approximately 29% of the flora (Perry et al. 1979). Hilton-Taylor 1996). The remarkable diversity of geo- In contrast, the relative diversity and importance of phytes in the Succulent Karoo can be seen clearly in geophytes in all of the other four mediterranean re- 107 gions of the world drops off sharply as one moves Methods into winter-rainfall desert regions (Rundel 1996). Geo- phytes comprise only about 1% of the floras of the The relative and absolute species diversity of na- winter rainfall zone of the Mojave and the Sonoran tive geophytes within the five mediterranean floras of Desert in North America, and similarly small frac- the world was extracted from a variety of published tions of the winter-rainfall Atacama Desert of Chile, data sources (Table 1). Within the southern African northern Sahara Desert, and desert areas of Western flora, an analysis of the biogeographical patterns of Australia (Rundel 1996). geophytes with prostrate leaves was conducted on Not only is the species diversity of geophytes in species in the families Amaryllidaceae, Colchicaceae, the Succulent Karoo remarkable, but so is the growth Eriospermaceae, Hyacinthaceae, Iridaceae and Orchi- form diversity. In addition to typical monocot geo- daceae. Species were considered to be prostrate-leaved phyte growth forms with upright rosettes of basal if they possessed broad leaves adpressed (or more-or- leaves, there is an unusual growth form that is rela- less adpressed) to the soil surface. In addition to per- tively rare elsewhere. These are geophytes with leaves sonal communications from A. Le Roux & C. Boucher that lie fully pressed against the soil surface (‘geo- and P. Goldblatt & J. Manning, the following source phylly’ (sensu Eller & Grobbelaar 1982), referred to material was referenced: Adamson & Salter 1950; as ‘prostrate-leaved geophytes’ in this paper). This Bond & Goldblatt 1984; Snijman 1984; Le Roux growth form is distinct from the basal rosettes com- & Schelpe 1988; Du Plessis & Duncan 1989; Jeppe monly found in North American desert annuals (Mul- 1989; Arnold & De Wet 1993 and Shearing & Van roy & Rundel 1977). Prostrate geophyte leaves are Heerden 1994. Species described as prostrate or more- frequently large and broad, in contrast to North Amer- or-less prostrate in at least part of their distribution ican basal rosettes of desert annuals that have multiple were included in the lists (Appendix 1). The prostrate small and thin leaves. The prostrate-leaf growth form, leaf character, although often highly consistent within which often includes one or two broad leaves ori- a species (e.g., in Ammocharis coranica), can also be ented 180◦ from each other (although there can be variable in some species. Species were included in up to five leaves arranged in a basal rosette), can be the analysis if they were prostrate in at least part of found in at least eight Succulent Karoo families. These their distribution. A complete study was conducted on include species from petaloid monocotyledonous fam- the Amaryllidaceae, however for other families, the ilies but remarkably this characteristic is also noted lists provided should not be considered exhaustive. in dicotyledonous geophytes, including species from Nomenclature was obtained from the TURBOVEG the Oxalidaceae and Geraniaceae (not included in this 9.42 program (South African Version). analysis). Commonly encountered prostrate-leaved For each of the above-mentioned families, locali- geophytes in the Succulent Karoo include Brunsvigia ties, habitat descriptions and leafing times were com- and Haemanthus (Amaryllidaceae); Lachenalia, Mas- piled and summarized. Numbers of species falling into sonia and Whiteheadia (Hyacinthaceae); Eriosper- the winter-, summer- or all-year rainfall regions were mum (Eriospermaceae); and Satyrium and Holothrix determined (Figure 2). If a species fell into two or (Orchidaceae). more rainfall regions, it was scored for both regions. In this paper, we provide a biogeographical analy- Specific emphasis was placed on the family sis of the southern African prostrate-leaved geophytes. Amaryllidaceae that has the highest number of species We ask the questions: Why are there so many geo- of flat-leaved geophytes of the eight families under phytes in the Succulent Karoo compared to other investigation. For this family, detailed habitat descrip- winter-rainfall deserts? What selective pressures have tions were compiled, as well as flowering and leafing resulted in the high incidence of the prostrate leaf times. For each genus in the family Amaryllidaceae, form among geophytes in the winter-rainfall region of the distribution of species with and without prostrate southern Africa? Despite their charismatic appearance leaves was mapped at a 1-degree grid scale (100 km × (Figure 1), very little research has been conducted on 100 km). these plants, although a variety of hypotheses relating to the functional significance of this unusual growth form exist. These are reviewed and discussed. 108 Figure 2. Selected climate data for stations

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