Phytogeography of Passerina ( Thymelaeaceae )

Passerina L. comprises 20 species and four subspecies o f microphyllous. wind-pollinated shrubs. Once considered cos­ mopolitan, the genus as currently defined, is endemic to southern Africa. Endemism within the genus is highest in the Cape Floristic Region (CFR), where all members show morphological and anatomical adaptations to the winter rainfall and dry warm summers o f the Mediterranean or semi-Mediterranean climate o f the region. The Western Cape is the centre o f diversity for Passerina, from where certain species extend to the west, north and east. The outlier populations o f Passerina montana Thoday on the interior plateau o f South Africa and Zimbabwe, as well as the Auas Moutains in Namibia, most probably origi­ nated in the CFR and formed part o f a previously wider northern temperate Afromontane grassland-dominated vegetation during the Quartemary, o f which relicts remained in the high mountain areas. P. burchellii Thoday and P. paludosa Thoday have the most restricted distribution and are regarded as Vulnerable. All other species are either widespread or under no im­ mediate threat.


INTRODUCTION
redefined Passerina L. by clari fying the taxonomic position of 92 'species exclusae', thus changing the status of the genus from cosmopolitan to endemic in southern Africa, an opinion also reflected in the now outdated taxonomic revision of the group by Thoday (1924).A recent monograph of Passerina recog nized 20 species and four subspecies (Bredenkamp 2002;Bredenkamp & Van Wyk 2003); these are listed in Tables 1 and 2. All members of Passerina are woody, evergreen shrubs or subshrubs with microphyllous leaves and small, rather insignificant wind-pollinated flowers.It is the only exclusively anemophilous genus of the Thymelaeaceae and the plants usually grow gregariously.
Taking the most southerly distribution of Passerina montivaga Bredenkamp & A.E.van W'yk into consid eration, no less than 18 species of Passerina occur in the Cape Floristic Region (CFR).The CFR is also acknowledged as the smallest of the world's six floris tic kingdoms (Van Wyk & Smith 2001).Following a taxonomic revision of Lachnaea L. (Thymelaeaceae), Beyers (2001) reviewed the recognition of local centres of endemism within the CFR, from the initial descrip tions by Weimarck (1941) up to those of Goldblatt & Manning (2000).In this paper we follow the interpreta tion of Goldblatt & Manning (2000), which identifies the following six principal local centres of endemism: the Northwestern (NW), Southwestern (SW), Agulhas Plain (AP).Karoo Mountain (KM).Langeberg (LB) and Southeastern (SE) Centres.
Species of Passerina endemic to the CFR (Table 1) are morphologically and anatomically adapted to the winter rainfall and dry warm summers of the Mediterranean or semi-Mediterranean climate in the region (Bredenkamp & Van Wyk 1999, 2000. 2001).Most species in the CFR are associated with fynbos.a sclerophyllous vegetation type on oligotrophic soils derived mainly from quartzitic Cape Supergroup rocks.These species are adapted to a variety of habitats, e.g.high-mountain peaks above the snowline, where plants are often surrounded by mist (throughout the year) or covered by snow especially dur ing the winter months: forest and mountain fynbos; vleis and marshes; coastal limestone deposits and limestone hills; coastal fynbos.where the plants grow on sand dunes and in sandy areas.Many species are pioneers growing along roadsides and in disturbed places.
Species near-endemic to the CFR are more wide spread (Table 1).They have adapted to a wider amplitude of environmental conditions: where ranges extend north of the southern Cape mountain ranges, they are often adapted to arid karroid vegetation and summer rainfall; certain species are adapted to forest margins and others tolerate periodic falls of snow at high altitudes.
The few species of Passerina endemic to the northern Drakensberg or near-endemic to the Great Escarpment of southern Africa (Table 2) are associated with the high moisture levels prevalent on the eastern escarpment and conditions of summer rainfall.These plants are often found in the ecotonal belt between forest and grassland; they also grow along streams and riverbanks and on mountain slopes.
In this contribution we describe, for the first time in one paper, the patterns of geographical distribution shown by members of Passerina.Patterns are interpreted in terms of geology, climate, vegetation type and histori cal change, and the conservation status of threatened taxa is suggested.

MATERIAL AND METHODS
All infrageneric taxa of Passerina w ere studied during extensive field work covering the complete geographi- cal range of the genus.Live material was collected, as far as possible, from at least five different localities for every taxon.Habitat and distribution data from 22 national and international herbaria were compiled in a Microsoft Access Database and integrated with the Pretoria Computerised Information System (PRECIS), from which distribution maps for all taxa were gener ated.For mapping purposes the degree square system was used (Edwards & Leistner 1971).Categories used to indicate Red List status of taxa are based on the criteria of the IUCN Species Survival Commission (2000).

OBSERVATIONS
The combined distribution of all species of Passerina is shown in Figure 1.The number of species per onedegree square is indicated in Figure 2. In Passerina the highest numbers of species per one-degree square are concentrated in a belt including those grids between 33° and 34°S and 18° to 27°E.The CFR (mainly Western Cape) is clearly the centre of diversity for Passerina, from where certain species extend to the west, north and east (see also Bredenkamp & Van Wyk 2001).The highest numbers of species occur in the grids 3321 (Ladismith), 3322 (Oudtshoom) and 3419 (Caledon).The highest diversity of species (six per half-degree square) occurs in the False Bay area, from Seekoeivlei.including the Cape Flats, to De Mond at the Palmiet River (3418B) (Bredenkamp 2002).Levyns (1938) was the first to show that the Caledon District is the centre of species richness in the CFR with a reduction in numbers to the north and east.Oliver et al. (1983) regard the quarter-degree square 3418BB as the richest area in the CFR.Beyers (2001), also working in the Thymelaeaceae, found that the highest number of Lachnaea L. species occurred in the quarter-degree square 3319AD (Worcester).
Species endemic or near-endemic to the CFR (Table 1) Passerina shows a high percentage of endemism within the CFR. as nine species out of 20 (45%), as well as the three subspecies, are endemic to this region.High percentages of species, 25%-65%, are also demonstrated in each of the local centres of endemism.P. esterhuyseniae (Northwestern Centre), P.paludosa (Southwestern Centre),/3.galpinii( Agulhas Plain Centre) and P. pendula (Southeastern Centre) are all endemic to one local centre of endemism only.
The Northwestern Centre has a relatively high con centration of Passerina species, as 40% of the species occur there.The occurrence of 65% of Passerina spe cies in the Southwestern Centre, confirms that the grids 3419 (Caledon) and 3418 (Simonstown) can be regarded as centres of total species richness [Levyns (1938) and Oliver et al. (1983)].Geology and soils play an impor tant role in the species composition of the Agulhas Plain Centre, where limestones extensively outcrop along the southern coast from the Agulhas Peninsula to Mossel Bay (Goldblatt & Manning 2000).Thirty percent of Passerina species occur in this centre.The percentages of Passerina species represented in the Karoo Mountain Centre (25%) and the Langeberg Centre (35%) are relatively low.and no Passerina species are endemic to either of these Centres.The Southeastern Centre has a relatively high concentration of Passerina species, with 40% of the species occurring there.
Species considered near-endemic to the CFR are Passerina comosa and P. nivicola.distributed from the CFR to the Northern Cape and P. falcifolia.P. rubra and P. truncata subsp.truncata distributed from the CFR to the Eastern Cape.Passerina nivicola.restricted mostly to mountainous areas, is possibly still under-collected.

Species endemic to the Northern, Western and Eastern
Cape and KwaZulu-Natal (Table I ) P. obtusifolia and P. corymbosa (= P. vulgaris) are socalled Cape ubiquists (Weimark 1941) as they are very common and adapted to a wide range of Cape habitats.Their distributions currently include all the Centres with in the CFR and both occur in three other South African provinces.Passerina rigida is confined to coastal areas from South Africa's western coast to the northeastern coast of KwaZulu-Natal.

Species endemic or near-endemic to the Great Escarp ment of southern Africa
Endemism of Passerina drakensbei'gensis, P. mon tana and P. montivaga is indicated in Table 2.
Based on fossil pollen evidence.Scott et al. (1997) regard the dryer forest types of East Africa and Australia as the best apparent analogies for the palaeovegetation of southern Africa during the terminal Cretaceous to the early Tertiary.During the Neogene, plant communities in southern Africa evolved into equivalents of modem biomes of the subcontinent.Currently, temperate grass land is widespread on the interior plateau and includes fynbos-like vegetation in moist higher-altitude areas (O'Connor & Bredenkamp 1997).During the Quaternary, highveld grassland expanded at the expense of woody vegetation, coupled by a southward spread of relatively dry mountain fynbos elements.Evidence for the presence of such fynbos vegetation during the Holocene in the contemporary Grassland Biome has been found as far north as the Nyanga Mountains of Zimbabwe (Scott et al. 1997).A phylogenetic study by Bredenkamp (2002) indicates that Passerina filiformis and P. paludosa (both common in the Cape Peninsula) are probably the most primitive extant members of the genus, and P. truncata, P. montana and P. paleacea as the most advanced.We hypothesize that P. montana probably originated from an ancestor in the CFR and adapted to the environmental conditions of the high-mountain Afromontane grassland, which had a wider northerly distribution during the Quaternary.Because of environmental changes since the beginning of the Quaternary, the boundaries of the Grassland and Savanna Biomes changed, resulting in relicts of Afromontane grassland and fynbos elements in high-altitude areas such as Nyanga.the Huila Plateau and the Auas Mountains.In descriptions of the Afromontane Region.White (1981White ( , 1983) ) and Cowling & Hilton-Taylor (1997) mention the significant outliers of this phytochorion occurring on the high mountains of West Africa, the Eastern Zimbabwe Highlands and Angola (Huila Plateau).Although this broad pattern of distribu tion (Cape centre, eastern African extension with reduced species) is common to other genera as well.Passerina is unusual because it is represented in all the more pronounced Afromontane refuges in southern Africa, especially those isolated western outliers in Namibia and Angola.Rennie (1936) argued that the occurrence of certain species, including species of Passerina.on the Auas Mountains in Namibia could be interpreted as relicts of the CFR.suggesting that northward expansion of at least certain elements of that flora took place along the west coast into present-day Namibia.Unfortunately the Passerina specimens from the Auas Mountains avail able to him were sterile, resulting in their incorrect identification as P. truncata (= P. glomerata).As the most northerly know n distribution of P. truncata at the time was Steinkopf in Namaqualand.he concluded that P. truncata once ranged further north through Namibia to the Auas Mountains.However, the specimens from both Auas and Huila are unmistakeablv P. montana.a species l-s 8.
distributed mainly along the eastern Great Escarpment.The present distribution of P. montana renders Rennie's interpretation rather improbable.

CONCLUSIONS
In Passerina the highest numbers of species per onedegree square are concentrated in a belt between the 33° and 34°S and 18° to 27°E, occurring in the grids 3321 (Ladismith), 3322 (Oudtshoom) and 3419 (Caledon).The highest diversity of species occurs in the False Bay area, from Seekoeivlei to the Palmiet River (3418B).Hence the CFR (Western Cape) is the centre of diversity for Passerina.
Passerina demonstrates a high degree of regional endemism, with 45% of the species endemic to the CFR.Of these endemics, 20% are endemic to one of four centres of the CFR; 10% (P.montana and P. montivaga) are near-endemic to the Great Escarpment and 5% (P.drakensbergensis) is endemic to the Bergville District in the northern Drakensberg.
Passerina species that are near-endemic or endemic to the Great Escarpment probably originated in the CFR and adapted to conditions associated with the highmountain Afromontane grassland, a vegetation type which is hypothesized to have had a much wider dis tribution during the Quaternary.The widely disjunct distribution of P. montana is probably due to subsequent environmental changes.The boundaries of the Grassland and Savanna Biomes shifted, resulting in relicts of temperate grassland and fynbos elements (such as P. montana) in isolated high-altitude refuges such as the Waterberg Plateau in Limpopo (South Africa), Nyanga-Chimanimani Highlands in eastern Zimbabwe.Huila Plateau in Angola and the Auas Moutains in Namibia.

FIGURE 1 .
FIGURE 1.-Know'll geographical distribution o f the genus Passerina.Distribution in Angola on Huila Plateau near Lubango and Chela Mountains is shown in insert.

FIGURE 2 .
FIGURE 2.-Number o f species o f Passerina per one-degree grid square.Lines PQ and RS: boundaries between summer (A), intermediate (B) and winter (C) rainfall areas.Line XY shows northern boundary o f Cape Supergroup rock outcrops.Distribution on Huila Plateau and ( hela Mountains in Angola is shown in insert.

TABLE I Endemism of Passerina species in Cape Floristic Region (CFR). as well as i n various provinces of South Africa (cont.) Bothalia 36,2 (2006) 197
a: Z Ẑ Q