Biogeography of prostrate-leaved geophytes in semi-arid South Africa: hypotheses on functionality

Nowhere is the species diversity of geophytes greater than in the five medi terranean-climate ecosystems of the world. Of these, the Cape mediterranean zone of South Africa is the most speciose. While the relative diversity an d importance of geophytes of all of the other four mediterranean regions of the world drops off sharply as one moves into adjacent winter-rainfall des ert 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 (comprising 13 to 29% of the regional floras in this region). Apart from species richness, there are also a number of inter esting geophyte growth forms in this region. One unusual growth form is geo phytes with flattened leaves that lie prostrate on the soil surface. At lea st eight families (Amaryllidaceae, Colchicaceae, Eriospermaceae, Geraniacea e, Hyacinthaceae, Iridaceae, Orchidaceae and Oxalidaceae) exhibit this grow th form. While this growth form is relatively common in many geophyte linea ges in the Succulent Karoo biome and the Cape mediterranean zone (Fynbos bi ome), and occurs infrequently through the summer-rainfall temperate regions of Africa, it is virtually absent in other regions worldwide. A null hypot hesis is that the prostrate leaved trait is a neutral characteristic, howev er biogeographical data do not support this. A neutral trait would be unlik ely to show such a clear pattern of distribution. Several alternative hypot heses 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 environment below the leaves, reduction of water loss around the roots, reduction of water loss through transpiration, prec ipitation of dew on the leaves and maintenance of optimal leaf temperatures for growth.