Fear of high places is the rule and such areas are
avoided by most humans and other animals. Cliff faces, in spite of their hazardous appearance, have nevertheless provided a stable refuge and safe haven for many plant and animal species. At times even man took refuge to cliffs -
coming to mind are the Jewish settlement at Masada (Israel) and many monasteries built above cliff faces which provided protection from attacks
by the Romans and other enemies. Certain bird species, such as vultures and eagles, build their nests on cliff faces and bees often have large colonies in inaccessible hollows and crevices. Most cliff-dwelling humans and animals benefit from cliffs for only a certain phase in their lives. However, there are certain life forms, especially plants, that are more or less strictly confined to the cliff habitat. These obligate cliff-dwelling plants have evolved special adaptations which enable them to thrive and reproduce in seemingly one of the most hostile terrestrial environments.
Because of their inaccessibility, sheer cliff faces are botanically among the most poorly explored of all terrestrial habitats on earth. Southern Africa is renowned for its succulent plant riches and during the past few decades many unique cremnophilous succulent species, often with very limited distribution, have been discovered in the region (Bruyns 1990; Van Jaarsveld 1988, 2000; Van Jaarsveld & Van Wyk 2000). This has led to this article, the first of a series on cliff-dwelling succulents in South Africa and Namibia. Information presented in these contributions emanate from a comprehensive project on cremnophytes, initiated by the senior author, and to be submitted for higher degree purposes at the University of Pretoria.
Succulent cremnophytes
The Greek word "cremnos" pertains to a cliff. Cliff plants are sometimes termed cremnads or cremnophytes (Jackson 1971). Cremnophytes are usually also chasmophytes (crevice plants), as they tend to grow in rock crevices. The term cremnophyte falls under the umbrella of chasmophyte, thus most cremnophytes are chasmophytes, but not vice versa, as there are many easily accessible rock outcrops where plants occur in crevices. Whereas many succulent plants are chasmophytes, occurring on rocky outcrops, only a relatively small number are obligate cremnophytes.
The cliff habitat
Cliffs are found throughout the mountainous regions of South Africa and Namibia, from just above sea level to altitudes of around 3000 m along the eastern parts of the Great Escarpment. Notable cliff areas in South Africa include the quartzitic sandstone of the Cape Fold Belt mountains in the south, the granites and quartzitic sandstone
mountains of the Richtersveld, Namaqualand and Bushmanland in the north-west and the shale, dolorite, basalt and sandstone cliff faces of the Great Escarpment which surrounds the high inland plateau. Other regions where cliffs are found, are along the escarpment mountains of Namibia, including the Baines Mountains of Kaokoland in the north (sandstone, dolomite and conglomerate) and the mountainous region bordering the Gariep River in the south. The Enon Conglomerate formation also has cliffs in the south-eastern part of South Africa. Physical features of the cliff face vary according to the situation and geological formation; some are small, others larger or broken. The local climate of the cliff face also varies considerably according to its proximity to the sea, precipitation (amount and season), altitude, aspect and colour of the rock formation.
A safe haven but environmentally unfriendly
On the one hand cliffs provide a stable habitat and protection from disturbances created by macro-herbivores and other animals, but on the other hand they have major obstacles to overcome. These include gravitation, high water runoff and exposure to the elements (solar radiation, wind). This has resulted in unique adaptations and other evolutionary adjustments, with plants allocating less resource to woodiness (resisting gravity) and defence strategies.
Succulent cremnophyte adaptations and life forms
In the case of a cremnophytic taxon, the degree of evolutionary change from relatives on even ground throws light on the evolutionary age of the taxon, and may also reveal features of the evolutionary adjustments which now enable it to colonise the cliff habitat.
Moreover, life on the cliff face also requires successful reproduction (gene exchange, seed dispersal, vegetative reproduction), which is another aspect that would be investigated. Tentative observations show that most cliff-dwelling succulent species have wind-dispersed seed, with water action also playing a role in others.
Cremnophytes vary in size from dwarf cluster-forming plants, through pendulous types to large trees (e.g. Ficus spp.). Pre-liminary observations show that most of the obligate succulent plant cremnophytes have a tight cluster life form, followed by the pendulous type (pendulous stems, leaves or both). The rest are erect shrublets.
Many plants (non-succulent and succulent) which normally grow on level ground are also found on cliff faces. These are so--called opportunistic cremnophytes. Succulent chasmophytes will occupy any crevice, whether on an accessible rock outcrop or on a vertical cliff. These include many Crassula spp., Cotyle-don orbiculata and other succulent plants with the cliff as part of their habitat range.
To qualify as an obligate cremnophyte, more than 80% of a species' population should be confined to cliffs and the plants should display one or more cliff specialisation features. Succu-lent cremnophytes specifically pertain to plants with succulent features (leaf and stem succulents) growing on cliffs and apart from the "typical" succulents, xeromorphic succulent bulbous types are also included in this category. Among the latter are species of Cyrtanthus, 0rnithogalum, Litanthus and Haemanthus. One of the aims of the present study will be to document adap-tational aspects of obligate cremnophytes. In addition, com-parative studies of life forms, reproduction strategies, distribu-tion patterns and representation in the various southern African biomes will also be included.
Representation and distribution in South Africa and Namibia
Tentative observations reveal that South Africa has a fairly rich obligate cremnophyte flora, many of which are endemic to the country. Of the 137 species hitherto identified, 70% are di-cotyledons (8 families, 22 genera) and 30% monocotyledons. The most prominent groups include the Mesembryanthemaceae with 39 species, Crassulaceae with 35 species and the Aloaceae with 28 species. The rest belong to families such as the Ascle-piadaceae, Lamiaceae, Asteraceae and Portulacaceae. About half the number of species (66 or 49%) occur in the Succulent Karoo and about a third (40 or 29%) in the thickets of the south-eastern region of South Africa. The remainder (31 or 22%) is found on cliffs in the fynbos, temperate montane grassland, bushveld and along the subtropical coast. Another noteworthy aspect is the size and weight of the plants. About 60% of the taxa have a dwarf habit 1-200 g (82 taxa) and are mostly confined to the Succulent Karoo and to a lesser extent the thicket regions. The rest of medium weight (200-500 g) and those that are larg-er (500 g and more) occur mainly in thicket and bushveld in the eastern and northern regions of the subcontinent.
