Evolution of species Stretch ing the flora The Cederberg-Tanqua tension zone by Barrie Low and Uschi Pond, Coastec Coastal and Environmental Consultants, Rondebosch The first article in this series The complex topography of the Witteberg sandstones and quartzites that provide (Veld &Flora September 2004) the geological buffer between the Cape Folded Mountains and Tanqua Karoo is as rugged as it is fascinating. It is logical to presuppose that such a varied environ­ painted a brief geological ment is likely to be reflected in the diversity of the flora and vegetation as one and palaeontological history travels along an imaginary gradient between fynbos and Karoo. of this special 'tension lone', On a regional scale the diversity of both the fynbos and succulent Karoo biomes is well documented, with various authors ascribing this to infertile soils, punt­ and now we examine the ing for fire or suggesting topographical gradients. Now researchers at the South exquisite flora in the ecosys­ African National Biodiversity Institute (SANBI) have claimed that major speciation tems encountered along the (the evolution of new species) patterns of both fynbos and succulent Karoo should be attributed to climatic oscillations during the Pleistocene (Le. over the past two marked ecological gradients million years). Our assertion is that no one factor would have dominated, however, between the Cederberg and fire would have been absent in the Karoo and much of this region has low topo­ the Tanqua Karoo graphic variation. High species turnover is no more evident than in the transition between mountain fynbos and adjacent Karoo and we suggest that whereas climate might have been important in the speciation process, the juxtaposition of differ­ ent substrates and varied topography in this Cederberg-Tanqua tension zone has ABOVE: Braunsia apiculata one of the many vygies dotting the rocky slab landscape in the tension zone. Photo: Barrie Low. accelerated the rate of appearance of stant shifting of the boundary between new species. fynbos and Karoo - a likely place for The work of Richard Lechmere-Oertel speciation! Evidence exists for sub­ in the Matjies River Nature Reserve stantial and relatively recent evolution explains the mosaic of succulent Karoo of species within the succulent Karoo and arid fynbos communities found flora and it is reasonable to expect that here. The Cederberg-Tanqua region not much of this speciation occurred within only represents a major change-over the Cederberg-Tanqua tension zone. in biomes (fynbos to Karoo) but also We would expect marked shifts in in vegetation types, with an impressive species patterns as we journey between range of seven vegetation types in less the Cederberg mountains and Tanqua than 20 km: from Cederberg Sandstone plains. And we are not to be disappoint­ Fynbos and Northern Shale Band ed! Thanks to collecting by Richard (25 %). Even adjacent sites within the Vegetation, through Agter-Sederberg Lechmere-Oertel in the Matjies River same vegetation type show small simi­ Shrubland, Swartruggens Quartzite Nature Reserve and Francine Rubin larities, with figures of 18.9% for the Fynbos, Swartruggens Quartzite Karoo, in the Tanqua Karoo National Park, arid fynbos of the Matjies River Nature to Tanqua Wash Riviere and the Tanqua and with major contributions from the Reserve and Zuurfontein Farm, and Karoo. This transition takes place along Cederberg Conservation Group's forays 33.4% for the succulent Karoo from the a marked rainfall gradient, with aridity into the region, we are able to build same sites. In fact the succulent flora increfu,ing as one travels eastwards into a reliable picture of species changes here share only a third of the combined the Tanqua region. between the arid fynbos of the south­ complement of 455 species! There is a dramatic drop in pre­ eastern Cederberg and the much drier Over great distances one would cipitation from Algeria (910 mm pal to Tanqua Karoo. expect the similarities to decline, and the Tanqua Karoo National Park that Our focus has been on the Matjies it does this rather dramatically between receives less than 100 mm pa. The River Nature Reserve and adjacent the central Cederberg and the Tanqua Matjies River Nature Reserve receives Zuurfontein Farm, where both arid fyn­ Karoo National Park, which share a on average 220 mm each year. bos and succulent Karoo types are mere seventeen species out of a com­ Now, with such a varied landscape found. Analysis of the two adjacent bined total of 1895, a paltry similarity and diversity in vegetation, one would floras provides not a little intrigue! of 3.6%! expect equal surprises from the flora. Comparing the floras of the two veg­ If we permit ourselves to stray about Fluctuations in climate during the etation types, similarities between arid 50 km further south to the arid fynbos Pleistocene would have ensured con- fynbos and succulent Karoo are low of the Swartruggens Range (Katbakkies September 2005 Veld&Flora 119 Pass), then the similarity is of the order (Worcester) and Thesium capitulijlorum of 23.3 %- again a major difference, but (Cape Peninsula). this time over a much longer distance. Many species are restricted to the Chief floristic casualties as one moves Bokkeveld-Karoopoort region, with eastwards along our rainfall gradient several localized endemics such as are typical fynbos families (proteas, Agathosma dentata (Vulnerable), ericas and other ericoid-Ieaved shrubs, Sederberghaarpuis Euryops wageneri, restios and sedges) with concomitant Helichrysum aureofolium, Oncosiphon increases in succulents (mesembs, intermedium, Phylica fruticosa (Rare), euphorbs, aloes) and the Aizoaceae­ P. levynsiae (Insufficiently Known), Molluginaceae groups. Geophytes also knikknopsuikerbos Protea pendula and display an interesting pattern, with Swartruggens vexator Vexatorella amoe­ the Hyacinthaceae increasing towards na. The drier quartzite slabs boast sev­ the Karoo at the expense of the more eral habitat endemics, typical of which typical Cape lridaceae and Orchidaceae are the low, pungent vygies Braunsia families. apiculata, Sederberg dekriet Cannomois And what gems await the expect­ taylorii, the aptly named klipheide Erica ant botanist? The study has produced maximiliani, klipharpuisbos Euryops numerous new records for both the othonnoides, veterbos Passerina trun­ region as well as for the Cape Flora. cata, named for its useful stringy bark, Major range extensions include (near­ Phiambolia mentiens and the striking est record, then Red Data status in kaaingsuikerbos Protea glabra, one of brackets) Agathosma elata (Gifberg, the few species in the area to reach Vulnerable), Aloe microstigma, Oxalis more than 3 m. convexula (Ceres), Antimima per­ Another interesting habitat endemic sistens, bobbejaanarm Cadaba aphylla, is the turflelie Crinum variabile that Pteronia membranacea, Ruschia amico­ occurs along the dry streambeds of rum (Montagu, Rare), Antimimapaucifo­ Namaqualand, the western Karoo and lia (Bokkeveld Mountains), geelkwassie the Cederberg-Tanqua tension zone. Our Bulbine annua (Saldanha), Lampranthus sampling in the Doring River extends brevistamineus (Matroosberg), Othonna this species' known distribution south­ retrofracta, skaapbos Tripteris aghillana wards from the Biedouw Valley. LEFT TOP: Braunsia apiculata. LEFT CENTRE: Sederberghaarpuis Euryops wageneri is a classic rocky sandstone and plateau species, thriving in both arid fynbos and on the edge of the succulent Karoo. A local endemic, it is found only between here and the Biedouw Valley. ABOVE: The prominence of the Iridaceae tends to decline as one enters the Tanqua Karoo. Kabong Lapeirousia fabricii is an exception and this particular specimen was found at the top of one of the Doring River 'side kloofs' on Zuurfontein Farm. LEFT: Kaaingsuikerbos Protea glabra is a standard-bearer for arid fynbos on rocky slabs. Confined to sandstone and quartzite 'pavements', this distinctive species is a regional endemic, with a distribution from the Bokkeveld Plateau to the Kouebokkeveld Mountains. Photos: Barrie Low. 120 September 2005 Veld&Flora And not to be forgotten, several Again low similarities in the order of new taxa have been discovered in the between 4% and 6% are found for the Cederberg-Tanqua tension zone - two southern Langeberg-central Karoo and Asteraceae (Senecio and Othonna), two Tsitsikamma-eastern Karoo. Cyperaceae (both Ficinia) and a new It follows that these areas would lxia. Most of the species in the new each sport their own 'tension zone' vygie genus, Phiambolia, have either with concomitant high species num­ been collected from this area or in bers and endemism. The eastern Karoo, a band stretching southwards to the although not implicated in the SANBI Ceres Karoo. climatic work, would nevertheless have But what about other Karoo-fynbos exhibited a similar tension zone, with a interfaces? Some seventy years ago, gradient from fynbos to Nama (and not Prof. Compton wrote about the Witteberg Succulent) Karoo. one of the south-western Karoo moun­ Clearly these tension zones represent tains (fynbos), and adjacent Whitehill as much a past as a current opportu­ Karoo Garden (Karoo) floras, where only nity for speciation, except that climate four out of a total of 650 species were change is likely to cause unnatural shared! Again the sharp line between acceleration at the hands of global fynbos and Karoo floras is clear. Abrupt warming. Ecotonal boundaries such flora transitions are also strongly evi­ as these must be incorporated into