Reconciling ecological and phytogeographical spatial boundaries to clarify the limits of the montane and alpine regions of sub-Sahelian Africa

South African Journal of Botany 98 (2015) 64–75

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Reconciling ecological and phytogeographical spatial boundaries to clarify the limits of the montane and alpine regions of sub-Sahelian Africa Clinton Carbutt a,⁎, Trevor J. Edwards b a b

Scientific Services, Ezemvelo KZN Wildlife, PO Box 13053, Cascades, 3202, South Africa Department of Botany, La Trobe University, Bundoora, Victoria, 3086, Australia

a r t i c l e

i n f o

Article history: Received 28 October 2014 Received in revised form 15 January 2015 Accepted 26 January 2015 Available online 19 February 2015 Edited by OM Grace Keywords: Afroalpine Afromontane Boundary reconciliation Cape element Great Escarpment High elevation floras High elevation vegetation Microrefugia Migration corridor Northern track Phytogeography Spatial scale-based Southern track Sub-Sahelian Africa Thermal belts Treeline

a b s t r a c t The Afromontane phytochorion is a region delineated on the basis of shared plant species distributions and centres of plant endemism occurring mainly in the high elevation regions of sub-Sahelian Africa. We have provided in this study a synthesis of the various contexts in which the Afromontane concept has been applied in the past, highlighting many complexities, nuances and shortcomings. A complicating factor is that the Afromontane region has both a phytogeographical and ecological context, operating at different spatial scales. We note that use of the Afromontane region as a broad floristic framework is problematic because it incorporates non-montane (alpine) regions above the treeline. Going back to first principles of phytogeography and ecology, we have developed a novel framework to resolve the aforementioned challenges. We argue that the best way to make sense of the Afromontane region is to reconcile the phytogeographical and ecological contexts to a common and unambiguous spatial boundary. We also question the recognition of the Afromontane phytochorion as a floristic region in future. Spanning some 48° of latitude, it is a rather ungainly frame of reference probably immune to detecting the nuances of floristic, physiognomic, elevational, climatic and topographic variability at finer scales, particularly in the species-rich grasslands and plateau margins. © 2015 SAAB. Published by Elsevier B.V. All rights reserved.

1. Introduction The ‘dark continent’ of Africa, second in size only to Asia (Ollier, 1996), is renowned for its vast natural landscapes and rich biodiversity (Burgess et al., 2004). Its mountain landscapes, in particular, occupy an area of 1.2 million km2 at the highest ruggedness threshold (Körner et al., 2011), and are grouped principally into (1) the ‘Saharan’ Mountains; (2) the Great Rift Mountains (e.g. Mount Kenya, Mount Kilimanjaro, Ethiopian Highlands); (3) the Albertine Rift Mountains (e.g. Rwenzori Mountains); (4) the highlands of West Africa; and (5) the Great Escarpment of southern Africa, the latter defined by the almost horseshoe-shaped eroded continental margin incorporating the high ground of Angola in the west, to Zimbabwe and Mozambique in the east. Africa's mountains are predominantly moist, but arid ⁎ Corresponding author. E-mail address: [email protected] (C. Carbutt).

http://dx.doi.org/10.1016/j.sajb.2015.01.014 0254-6299/© 2015 SAAB. Published by Elsevier B.V. All rights reserved.

exceptions include the Atlas Mountains (north-western Africa), Tibesti Mountains and Ennedi Plateau (northern and north-eastern Chad respectively), Jebel Marra (western Sudan), and the western Great Escarpment (Bussmann, 2006). The Great Rift Mountains combined with the eastern Great Escarpment, in particular, are a prominent signature feature discontinuously spanning two-thirds of the length of subSahelian Africa (Fig. 1; Table 1), akin to what the Rocky Mountains and Andean Cordillera are to North and South America respectively. Studies relating to the flora and vegetation of sub-Sahelian Africa's mountain regions have largely been viewed through the lens of White's (1978) Afromontane phytochorion. The Afromontane concept, published almost 40 years ago, applies to a large area spanning some 48° of latitude, and is further complicated by having both phytogeographical and ecological contexts that generally operate at different spatial scales. In this paper, we return to the first principles of phytogeography (essentially a description of centres of plant diversity and endemism and an understanding of the historical factors that may have

C. Carbutt, T.J. Edwards / South African Journal of Botany 98 (2015) 64–75

65

40º N

Sahara Sahel

Sahel

B

D F G

C E I

A

H

L

O

M

P

Q

R

N

60º E

25º W

K J

S T

V

U

W

N

X1 1000 km

Y

X2

X

35º S Fig. 1. The most prominent high elevation areas of the Afromontane region in sub-Sahelian Africa (A–X) as well as the location of the Cape Floristic Region (Y). Adapted from Hedberg (1994), Carbutt and Edwards (2001) and Wesche et al. (2008). The Great Rift is represented by B–N, Q–U; the Albertine Rift is represented by O and P; and the Great Escarpment is represented by V–X. Key to abbreviations: A, Mount Cameroon, Cameroon (4040 m); B–K, Ethiopian Highlands (north-east Africa, Ethiopia); B, Simen Mountains (4543 m); C, Mount Guna (4225 m); D, Mount Abuna Yosef (4260 m); E, Mount Choqa (4113 m); F, Mount Abuye Meda (4012 m); G, Mangestu Mountains (4072 m); H, Mount Bada (4036 m); I, Mount Kaka (4200 m); J, Bale Mountains (4377 m); K, Mount Guge (4203 m); L, Mount Elgon, Kenya/Uganda (4315 m); M, Mount Kenya, Kenya (5199 m); N, Aberdare Mountains, Kenya (3994 m); O, Rwenzori Mountains, Democratic Republic of Congo/Uganda (5109 m); P, Virunga Mountains, Democratic Republic of Congo/Rwanda/Uganda (4507 m); Q, Mount Kilimanjaro, Tanzania (5895 m); R, Mount Hanang, Tanzania (3417 m); S, Mount Meru, Tanzania (4565 m); T, Nyika Plateau, Malawi (2605 m); U, Mount Mulanje, Malawi (3002 m); V, Mount Nyangani, Zimbabwe (2592 m); W, Chimanimani Mountains, Mozambique/Zimbabwe (2440 m); X, Drakensberg Alpine Centre, Lesotho/South Africa (3482 m) bisecting the northeastern escarpment (X1) and the south-eastern escarpment (X2); Y, Cape Floristic Region.

shaped the distribution of a flora) and plant ecology (essentially detailing vegetation structure and how it may be partitioned in the landscape by the local abiotic environment). Our aims are to (1) produce a synthesis of the vast literature relating to the Afromontane phytochorion that spans almost four decades within a framework that refers to its various contexts, paying particular attention to its complexities, nuances and misapplied terminologies; (2) critically assess how well the Afromontane framework supports each context, making relevant amendments and suggestions; and (3) begin interrogating the relevance of the Afromontane phytochorion as a floristic region into the future. We therefore aim to extend the synthesis into a framework that clarifies and re-defines the Afromontane region by addressing the host of misapplications made in the past. This novel approach is simple and pragmatic in its execution — the underpinning principle is to ensure that the ecological and phytogeographical contexts are congruent and fully reconciled to a common and unambiguous spatial boundary. Other than a cursory reference to its outliers, we do not include an explicit review of the high elevation flora and vegetation of the Cape Fold Mountains (Linder et al., 1993) in the Cape Floristic Region. Its subalpine flora and vegetation are not sharply distinct from the lowland flora and vegetation due to the strong influence of nutrient-poor soils, summer drought, fire and southern temperate latitudes, hence the

pervasive occurrence of xeromorphic vegetation across elevational gradients (Marloth, 1902; Linder et al., 1992, 1993; McDonald et al., 1993). 2. Afromontane region: a broad framework of various contexts 2.1. Background and overview White (1978, 1981, 1983, 1993) mapped the broad floristic units of Africa into phytochoria and regional centres of endemism based on patterns of species richness and endemism. One of these phytochoria, the Afromontane region (Fig. 1), encompasses the disjunct highlands of sub-Sahelian Africa and comprised six units of vegetation: Afromontane forest, Afromontane bamboo, Afromontane evergreen bushland and thicket, Afromontane and Afroalpine shrubland, Afromontane and Afroalpine grassland, and mixed Afroalpine plant communities (White, 1983). Chapman and White (1970) had earlier applied the Afromontane concept, still under development at the time, to include the forests of the Cape region in South Africa, rendered ‘Afromontane’ by southern latitudes. This expanded Afromontane region, spanning a large latitudinal range of 48°, thereby incorporates a suite of floras and vegetation types at various spatial scales, viz. the montane and alpine floras and vegetation of tropical Africa (Fig. 2), south-central Africa, southern Africa (Fig. 3), and the Cape forests.

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Table 1 Maximum elevations and record of permanent snow deposits for the most prominent high elevation regions in sub-Sahelian Africa (read in conjunction with Fig. 1). Rows in bold denote mountains with a nival belt (after Barr and Chander, 2012). Abbreviations: DAC, Drakensberg Alpine Centre; DRC, Democratic Republic of Congo. Mountain/mountain range

Corresponding letter to Fig. 1

Country

Maximum elevation (m a.s.l.)

Nival zone

Glacier(s)

Mount Cameroon Simen Mountains Mount Guna Mount Abuna Yosef Mount Choqa Mount Abuye Meda Mangestu Mountains Mount Bada Mount Kaka Bale Mountains Mount Guge Mount Elgon Mount Kenya Aberdare Mountains Rwenzori Mountains Virunga Mountains Mount Kilimanjaro Mount Hanang Mount Meru Nyika Plateau Mount Mulanje Mount Nyangani Chimanimani Mountains Drakensberg Mountains, incl. DAC

A B C D E F G H I J K L M N O P Q R S T U V W X

Cameroon Ethiopia Ethiopia Ethiopia Ethiopia Ethiopia Ethiopia Ethiopia Ethiopia Ethiopia Ethiopia Kenya/Uganda Kenya Kenya DRC/Uganda DRC/Rwanda/Uganda Tanzania Tanzania Tanzania Malawi Malawi Zimbabwe Mozambique/Zimbabwe Lesotho/South Africa

4040 4543 4225 4260 4113 4012 4072 4036 4200 4377 4203 4315 5199 3994 5109 4507 5895 3417 4565 2605 3002 2592 2440 3482

No No No No No No No No No No No No Yes No Yes No Yes No No No No No No No

No No No No No No No No No No No No Yes No Yes No Yes No No No No No No No

White (1978, 1983) recognised the Afromontane region of subSahelian Africa as a centre of plant endemism with 3000 endemic species (75%) out of the total of 4000 species. Afromontane endemism at the higher taxonomic rank of genus and family is, however, extremely low (White, 1978, 1983). White (1981) and Beentje et al. (1994) regarded the ‘Afromontane Regional Centre of Endemism’ as the product of a complex evolutionary history with obscure origins. Not all scholars of African phytogeography recognize the Afromontane region as a centre of endemism (e.g. Clayton, 1983), whilst Burgoyne et al. (2005) consider the Afromontane region a biodiversity hotspot. Much of the Afromontane region, especially its southern limit, is characterised by a mosaic of forest patches resembling ‘islands’ in a ‘sea’ of grassland, with or without heathland (Killick, 1979; Meadows and Linder, 1989, 1993). Studies suggest that the grassland component may not be a recent, derived state — evidence suggests that grasslands were present during the Holocene (Meadows and Linder, 1993; Bond et al., 2003; Finch and Marchant, 2011; Neumann et al., 2014) and as far back as the last glacial (Scott et al., 1997; Scott, 1999). Of equal antiquity is the key abiotic grassland driver, namely fire (Scott, 2000; Bond et al., 2003). From a vegetation perspective, the Afromontane region is characterised by a common suite of herbaceous and shrubby taxa sharing remarkably homogeneous physiognomy (White, 1981; Meadows and Linder, 1993). Patchily distributed Afromontane forests are believed to have acted as ‘stepping stones’ for various species migrating along the southern track of migration (White, 1983; White, 1993). Despite the discontinuous nature of Africa's highlands, scattered populations of the same tree species (e.g. Afrocarpus gracilior; Hagenia abyssinica; Juniperus procera; Prunus africana) occur in montane forest patches throughout large parts of sub-Sahelian Africa (White, 1978, 1983; Kadu et al., 2011). Perhaps the most classic floristic indicators of the Afromontane region are grassland Kniphofia species (Ramdhani et al., 2008, 2009) and the forest species P. africana (Kadu et al., 2011). The long-term environmental stability of Afromontane forest regions is proposed as a mechanism for the accumulation and persistence of species during glacial periods, resulting in diverse species assemblages and centres of endemism (Finch et al., 2009). During the Last Glacial Maximum, vegetation now classed as montane was wider ranging in lower elevation habitats (Van Zinderen Bakker and Clarke, 1962). However, distinct changes in species ranges followed as a consequence of climate change (Livingstone, 1975; Castañeda et al., 2009), with

montane species retreating to disjunct high elevation refugia and evergreen forests expanding during the early part of the Holocene marked by warming (Livingstone, 1975; Maley, 1991). Cyclical climate change during the Pleistocene in particular is believed to be an important palaeo-driver that has had significant bearing on the phytogeographical composition of current Afromontane floras (Scott, 1989; Ramdhani et al., 2008; Clark et al., 2011). The mountain ranges inhabited by Afromontane species range age-wise from late Pliocene and Pleistocene (Hedberg, 1994) to Jurassic (McCarthy and Rubidge, 2005), although the Jurassic age flood basalts associated with the southern ranges such as the Drakensberg Alpine Centre were further subject to an intense period of uplift during the Neogene (c. 24–2.5 mya), accompanied by climatic cooling and drying and the flourishing of grasslands and herbaceous plants (Dimech, 2011). The diverse landscape physiography of these mountain environments and associated topoclimates has served as cryptic refugia, partially shielding plant species from changing climates (Dobrowski, 2011). Since its original delineation (White, 1978), scholars have further tested the Afromontane concept using mathematical analyses of distribution maps (Denys, 1980), large-scale spatial patterning of distributional data (Linder, 1998, 2001), and avian zoochoria of forest elements (Dowsett-Lemaire and Dowsett, 1998). Use of objective multivariate methods and large, species-level datasets has not retrieved the Afromontane region as a coherent phytogeographic region, most probably due to rapid species turnover, high levels of narrow endemism and the coarse spatial resolution of grid cells (Linder et al., 2005, 2012). However, more recent use of large datasets based on genera has retrieved the Afromontane region (Linder, 2014).

2.2. Inclusion of the Afroalpine region makes provision for including an alpine belt The global ‘alpine’ concept, pre-Indo-Germanic for ‘steep slopes’ or ‘high mountains’ (Körner et al., 2006), reached Africa by the 19th century (e.g. Engler, 1892; Engler, 1904), and in no way implied any systematic relationship with the vegetation of the European Alps (Killick, 1963; Herbst and Roberts, 1974). Hauman (1933, 1955) later added the prefix ‘afro’ to assert a distinctive African context when referring to the alpine flora and vegetation of tropical Africa, a tradition that was reinforced

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A

B

C

D Fig. 2. High elevation flora and vegetation of the Afrotropics. A. Lower alpine vegetation (Sanetti Plateau, Bale Highlands, Ethiopia); B. Lobelia rhynchopetalum Hemsl., endemic to the lower alpine belt of Ethiopia; C. L. gibberoa Hemsl., a forest-dwelling giant lobelia of the montane belt, restricted to tropical Africa; D. Montane forest (Harenna Forest, Bale Highlands, Ethiopia).

by Monod (1957) and Hedberg (1961, 1964, 1965, 1986) amidst criticism by others (e.g. Boughey, 1955). This Afroalpine region incorporates the highly disjunct enclaves of alpine flora and vegetation occurring in east and north-east equatorial African mountains between latitudes 14° N and 6° S, and longitudes 25° E and 40° E (Hauman, 1955; Hedberg, 1994; Wesche et al., 2008). The Afroalpine flora is depauperate, comprising no more than some 300 to 350 vascular plant species (Hedberg, 1994), some of which are characterised from a vegetation perspective by giant life-forms belonging to the genera Lobelia (Fig. 2A and B) and Dendrosenecio (the latter

often referred to as ‘giant groundsels’) (Hedberg, 1994). According to Hedberg (1994, 1997) and Wesche et al. (2008), the major types of Afroalpine vegetation are open Dendrosenecio woodland, Afroalpine scrub (previously Alchemilla and Helichrysum scrub), Carex and Scirpus-dominated bogs and tussock-forming temperate grassland dominated by Avena, Festuca, Koeleria and Poa. Perhaps seemingly in contradiction, by definition, of the alpine belt being treeless is the unorthodox occurrence of giant senecios and lobelias, but these are unique taxa adapted to unique environmental constraints that rarely aggregate in dense concentrations and are best regarded as giant herbs rather than

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A

B

C

D Fig. 3. The Drakensberg Alpine Centre, southern Africa, as previously recognised. A. upper montane grasslands above Sani Pass, southern KwaZulu-Natal Drakensberg; B. near-endemic shrub, Euryops evansii Schltr. subsp. evansii, of the upper montane belt; C. endemic forb, Moraea alticola Goldblatt, of the lower alpine belt; D. lower alpine grasslands of Lesotho.

trees. The sub-alpine (now upper montane) communities include ericaceous scrub dominated by Cliffortia, Erica, Hypericum and Stoebe, and grasslands dominated by Andropogon, Exotheca and Sporobolus (Hedberg, 1964; Wesche et al., 2008). Knapp (1973) showed a clear distinction between upper montane grasslands and alpine grasslands. Grimshaw (2001) proposed combining the alpine and ericaceous belts to form the ‘altimontane’ belt but part-use of the term ‘montane’ is contradictory given the inclusion of the alpine belt. The major vegetation types constituting the alpine and upper montane communities are subject to seral changes under different fire and grazing regimes, with grasslands usually replacing woody vegetation under high fire frequency (Wesche et al., 2000; Hemp, 2006a; Wesche, 2006; Abera and Kinahan, 2011). Wesche et al. (2008) gave an overview of detailed vegetation surveys of the larger northern Rift mountains spanning the last two decades, but noted knowledge gaps from the Albertine Rift and Ethiopia,

and from a vegetation type perspective, knowledge of the temperate grasslands above the treeline is limited. The majority of Ethiopian alpine vegetation studies have focussed on the Great Rift escarpment of northern Ethiopia (Aynekulu et al., 2012) and the Bale Mountains towards the south (Friis, 1986; Assefa et al., 2011; Tallents and Macdonald, 2011), the latter region containing the largest and most intact expanse of alpine vegetation in tropical Africa, on the Sanetti Plateau (Yalden, 1983; Brooks et al., 2004; Siebert and Ramdhani, 2004). The Afroalpine region was first recognised as a regional centre of plant endemism by Hedberg (1961) with c. 81% species endemism and four endemic genera (Hedberg, 1961, 1964). Its ‘Tropic-alpine’ flora is regarded as Africa's youngest (Linder, 2014). The evolution of endemics has been attributed to a steady adaptive response to the environment (Hedberg, 1964, 1969, 1986), characterised by glacial cycles that repeatedly stressed Afroalpine plant populations into speciation (Osmaston, 1998). The non-endemic element of the Afroalpine flora

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appears more closely affiliated with the floras of the Northern Hemisphere than the Southern Hemisphere (Table 2), and is therefore regarded by Linder (2014) as a ‘northern temperate flora’. The strong focus on studies of vegetation zonation has had a major bearing on understanding the vegetation of the Afroalpine region. From a historic ecological perspective, the vegetation of the tropical African highlands was partitioned into the alpine (Fig. 2A and B), ericaceous and forest (Fig. 2C and D) belts (Hedberg, 1951, 1955, 1997). The stratification studies not only relate to the partitioning of distinctive vegetation belts within a landscape (e.g. Hedberg, 1951, 1955; Richards, 1963; Hall, 1973; Bussmann, 2006), but also relate to elevational zonation gradients within a single vegetation type such as forest (e.g. Chapman and White, 1970; Hamilton and Perrott, 1981; Hemp, 2006b). These vegetation belts are more sharply delineated than the high elevation vegetation of southern Africa (Hedberg, 1997), probably due to steeper elevational gradients in the Afrotropics. However, during the ice ages, much of Africa's high mountains were glaciated and vegetation belts descended to lower elevations by as much as 1000 m (Coetzee, 1964; Bonnefille and Riollet, 1988; Olago, 2001; Kiage and Liu, 2006; Schüler et al., 2012). Therefore, the Afroalpine region occupied a considerably larger area during the last glacial period, when the climate was cooler and drier (Ehrich et al., 2007). Strictly speaking, the Afroalpine flora and vegetation are restricted to the alpine belt, essentially the area above the upper treeline ecotone of the ericaceous belt, ranging from a lower limit of 3550 m–4100 m a.s.l. to an upper limit of 4500 m a.s.l. (Hedberg, 1997; Hemp, 2008; Wesche et al., 2008). Kiage and Liu (2006) erroneously placed the alpine belt below the ericaceous belt. The alpine belt in tropical Africa has an estimated total area of 3500 km2 (Hedberg, 1994). Finding a home for the placement of the Afroalpine region (and flora) in African phytogeography has been an ongoing dilemma. This is reflected in a change of Hedberg's (1986, 1994) thinking from it first being treated in isolation and viewed as a unique floristic region due to high species level endemism (c. 81%), to being combined as a sub-set of the broader Afromontane region. When referred to as part of the ‘endemic Afromontane element’, the levels of endemism changed significantly from 81% to 32% (Table 2), due to the incorporation of a larger number of true Afromontane species in a larger floristic region with disproportionally fewer endemics. The inclusion of the Afroalpine region as part of the broad Afromontane framework reflects the lack of alignment between the phytogeographical and ecological contexts of the phytochorion (see Weimarck, 1941; White, 1978; Hilliard and Burtt, 1987; Hedberg, 1997). The Afromontane region as a broad phytogeographical boundary is therefore a far larger area than that which should only reflect the distribution of montane flora and vegetation; the latter limited to a smaller area below the treeline and excluding the alpine belt. We concur that the alpine belt (with its associated flora and vegetation) should not form part of the Afromontane region given that the former is a distinct centre of endemism (Hedberg, 1961), home to a young ‘Tropic-alpine’ flora with unique extra-African

Table 2 Breakdown of the phytogeographical elements highlighting the complex derivation of the Afroalpine flora of tropical Africa. Adapted from Hedberg (1961, 1965, 1986, 1994, 1997) Element

No. of taxa

Percentage of total (%)

Pantemperate (sub-cosmopolitan) Endemic Afromontanea Northern Hemisphere (Boreal) Southern Africanb Mediterranean Himalayan (Siwalik) Southern Hemisphere ∑

87 82 34 25 18 8 6 260

33 32 13 10 7 3 2 100

a b

Endemic Afromontane element includes the endemic Afroalpine element. Southern African element comprises the South African and Cape elements.

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affinities (Linder, 2014). Furthermore, the alpine belt is characterised by a distinct assemblage of plant taxa both in terms of life forms and species composition, differing markedly from plant taxa of the lower montane elevations more closely related to lowland forests (Hedberg, 1994, 1997; Linder, 2014). 2.3. The Afromontane framework implicates Africa's northern and southern alpine regions as homologous The Afromontane framework includes the alpine belts of tropical and southern Africa, thereby surreptitiously creating a misconception that the northern and southern alpine regions are phytogeographically aligned. The Afroalpine region was initially viewed in isolation (e.g. Hedberg, 1955), most likely because its broader phytogeographic affinities were unknown at the time. However, it later became the focus of studies by Hedberg (1961, 1965, 1986, 1994) and Linder (1990, 1994). Hedberg (1961, 1965) recorded only 25 Afroalpine species as having connections to the mountains of southern Africa, and concluded that the Afroalpine flora of tropical Africa comprises a 10% southern African element (Table 2). Due to the high level of species endemism in the Afroalpine region (81%) and low percentage species overlap with other floras, scholars such as Edwards (1967), White (1978, 1983) and Hilliard and Burtt (1987) began to view the alpine floras of tropical and southern African as distinct at the species level, even though the overlap in genera is relatively large (Killick, 1978). The emerging recognition of divergent alpine regions was marked by the addition of the prefix ‘austro’ when referring to the ‘Afroalpine’ region of southern Africa (e.g. Coetzee, 1967; Van Zinderen Bakker and Werger, 1974; Werger, 1978). Killick's (1994) subsequent views recognised a distinct ‘Drakensberg Alpine Region’ because of its significant endemic element and stronger floristic connections with the Cape region, now confirmed by Van Wyk and Smith (2001) as one of southern Africa's official centres of plant endemism, and a major element of Africa's austro-temperate flora (Linder, 2014), namely the ‘Drakensberg Alpine Centre’ (Table 3). The ‘Afroalpine’ misconception in southern Africa has not been completely erased. Mucina and Rutherford (2006) refer to ‘Drakensberg Afroalpine Heathland’ instead of ‘Drakensberg Alpine Heathland’, and Steenkamp et al. (2005) and Grab et al. (2011) also perpetuate the ‘Afroalpine’ misnomer. Perhaps a further exacerbating factor to the continued use of ‘Afroalpine’ in the southern Africa context is the well accustomed use of ‘Afro’ in Afromontane terminology. Further differences between the two alpine regions in question can also be made on climato-ecological grounds (see Beentje et al., 1994). The climate experienced by alpine vegetation in southern Africa is seasonal. Winter temperatures drop below freezing, often resulting in frost. Snow is commonplace as the result of large cold frontal systems,

Table 3 Names applied in the past to the Drakensberg Alpine Centre, or loose equivalents, showing the conceptual shift among authors from it being considered part of the Afroalpine region to a discrete centre of plant endemism. Name

Authority

Drakensberg Centre (of the Cape element) Southern or austro-Afroalpine region

Weimarck (1941) Coetzee (1967), Van Zinderen Bakker and Werger (1974) Werger (1978) Hilliard and Burtt (1987) Hilliard and Burtt (1987) Killick (1994), Carbutt and Edwards (2001) Van Wyk and Smith (2001), Carbutt and Edwards (2004) Steenkamp et al. (2005)

Austral domain of the Afroalpine region Eastern Mountain Region South-eastern Mountain Regional Mosaic Drakensberg Alpine Region Drakensberg Alpine Centre Drakensberg Alpine Region of the Greater Afromontane region Drakensberg Grassland Bioregion of the Grassland Biome

Mucina and Rutherford (2006)

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and less frequently, mid-latitude cyclones. Soils of the alpine belt in the Drakensberg Alpine Centre therefore have frigid or cryic temperature regimes with mean annual temperatures ranging from 0 °C to 8 °C (Tyson et al., 1976; Schmitz and Rooyani, 1987). The absence of trees on the summit, and C3 dominated grasslands with a significant shrub and geophytic component, are partly symptomatic of the cold months of autumn and winter (O'Connor and Bredenkamp, 1997; Carbutt and Edwards, 2006), as is a non-flowering period of almost half a year (O'Connor and Bredenkamp, 1997; Carbutt et al., 2011). By contrast, the climate of the high elevation tropics is characterised by significant daily temperature ranges repeated throughout the year (Hedberg, 1964; Sarmiento, 1986). For example, in the Rwenzori Mountains, daily temperatures range between — 5 °C and 20 °C over a 24 hour period in the alpine and nival zones (Eggermont et al., 2009). The ecological constraints of the environment are reflected in the ‘giant’ life forms of Lobelia and Dendrosenecio in the Afroalpine region (Hedberg, 1964; Mabberley, 1986), convergent with Espeletia and Puya in the Andean Páramo (Meinzer and Goldstein, 1986), where similar conditions prevail. Pachycaulous architecture, incorporating large caulescent leaf rosettes that protect the developing shoot apex, and marcescent leaves that insulate the stem, is believed to limit cold injury given that night-time temperatures usually drop below freezing point (Hedberg, 1964; Mabberley, 1986; Meinzer and Goldstein, 1986; Hedberg, 1997). Warm daytime temperatures probably release adequate soil nutrients to sustain the large plants (Hedberg, 1964). The context of local environmental constraints therefore needs to be applied in the interpretation of adaptive syndromes. Therefore, use of the term ‘Afroalpine’ in a southern African context is misguided and is not supported on either floristic or ecological grounds. Another misapplied term is ‘tundra’ (e.g. Killick, 1997), a term that should not be applied to alpine environments (Körner, 2001; Körner, 2003), including those of southern Africa (Van Zinderen Bakker, 1981, 1983). Although there are various types of tundra, the term is most commonly associated with the Arctic and Antarctic regions characterised by permafrost. 2.4. High elevation treeline The correct partitioning of the Afroalpine and Afromontane regions (and associated flora and vegetation) hinges in part on the explicit understanding that the ‘treeline’ is a thermal boundary that separates the alpine and montane belts (Körner et al., 2011; Körner, 2012). The treeline (or treeline ecotone) in tropical Africa, grading into alpine scrub at higher elevations and montane forest at lower elevations, is the ericaceous belt (Hedberg, 1951; Wesche et al., 2000). This belt often comprises dense stands of trees and shrubs such as the arborescent Erica arborea (Hedberg, 1997), although small groves of E. arborea and Erica trimeria also occur in tropical Africa's alpine grasslands as outposts (Wesche et al., 2000). The most well developed and intact ericaceous belt is located in the Rwenzori Mountains, most likely because it is an exceptionally high rainfall region (Wesche et al., 2000). The ericaceaous belt is therefore the best placement for the treeline in tropical Africa, given that Körner's (2003, 2012) definition of the natural upper-climatic treeline includes the uppermost pockets of trees above the forest line. This treeline is not always easily discernable as it can grade seamlessly into montane forest, and is a dynamic boundary that will expand and contract depending on the prevailing environmental conditions. Therefore, the ‘climate-elevation’, or more recently the ‘isothermal’ control model of treelines (see Körner, 2003, 2012), must also be tempered with smaller-scale disturbance drivers in the Afrotropics such as regular dry season fire and grazing (Wesche et al., 2000; Fetene et al., 2006; Hemp, 2006a; Wesche, 2006), as well as climate change and associated warming and drying events (Hemp, 2005). The treeline is less useful as a boundary in south-central and southern Africa than in tropical Africa. In south-central Africa there is no alpine belt, and whilst such a belt does occur in southern Africa, its

occurrence is not distinguished as being the area above the treeline, but rather by physiography — the result of climatic conditions associated with high elevation induced by an escarpment of almost 900 m in vertical extent (Carbutt and Edwards, 2001). Although the treeline, like bioclimatic belts, is a ‘thermo-ecological’ term (see Körner et al., 2011) without a phytogeographic context, its correct delineation in sub-Sahelian Africa is essential to knowing the spatial boundaries of alpine and montane vegetation. This is central to our view of ensuring that ecological and phytogeographical boundaries are spatially explicit and congruent.

2.5. Afromontane region: a corridor central to the speciation of the high elevation flora of sub-Sahelian Africa Another dimension to the term ‘Afromontane’ is the framework it provides to discuss the more conceptual idea of a ‘corridor’ or ‘portal’ effect central to the speciation of sub-Sahelian Africa's high elevation floras. The high elevation floras of sub-Sahelian Africa contain elements derived from various sources, such as the Cape Floristic Region (Figs. 1 and 4), the Mediterranean, and from temperate regions in the Northern and Southern Hemispheres, in addition to some lowland tropical flora lineages (Hedberg, 1961, 1965; Gehrke and Linder, 2009; Eggermont et al., 2009). The disparate origins of these various elements mean that the Afromontane region, or Afromontane ‘track’ (Linder, 2014), has functioned as a corridor, facilitating the movement of taxa in multiple directions. Several Afroalpine species, mostly those occurring in the alpine and ericaceous belts, colonized Africa from Asia during the Pleistocene via the coastal mountain ranges of the Arabian Peninsula (Koch et al., 2006; Eggermont et al., 2009). One proposal is that the African mountains were almost void of vegetation above the treeline until land contact was established between the African and European plates in the Middle Miocene about 18 Ma BP, and again in 13 Ma BP, facilitating the migration of temperate plant genera into Africa's high elevation regions (Eggermont et al., 2009). A number of taxa accessed Africa via this northern track and then radiated, mostly post-Pliocene, on entering the Cape region at the southern end of the continent from mountain corridors to the north (Goldblatt and Manning, 2000), for example the Cape-centred Erica (McGuire and Kron, 2005). A number of other taxa, although not Cape-centred, also accessed Africa from the northern track. Examples such as Trifolium (Ellison et al., 2006), Arabis alpina (Koch et al., 2006), Cardamine (Carlsen et al., 2007), Lychnis (Popp et al., 2008), Alchemilla, Carex and Ranunculus (Gehrke and Linder, 2009), Dianthus (Valente et al., 2010) and Scabiosa (Carlson et al., 2012) are derived from Holarctic ancestors which accessed Africa from the north (Gehrke and Linder, 2009; Eggermont et al., 2009). Thus repetitive episodes of immigration from the Holarctic Region of the Northern Hemisphere, in combination with in situ radiation, have heavily influenced the composition of the high elevation floras of subSahelian Africa, making the Holarctic the most important source of lineage recruitment of African high elevation floras (Gehrke and Linder, 2009; Eggermont et al., 2009), as well as the most significant northern temperate source of emerging invasive alien plants in high elevation systems such as the Drakensberg Alpine Centre (Carbutt, 2012a). Reverse dispersal out of Africa seems to have been very rare (Gehrke and Linder, 2009). Typical Cape clades such as Disa, Irideae p.p., Moraea, Passerina, Pentaschistis, and Restionaceae have, by contrast to northern taxa, diversified over a longer time period from a Cape origin and have subsequently migrated northwards along mountain corridors into the mountains of tropical Africa. This southern track is thought to have facilitated at least 18 unidirectional radiation events (Linder, 2003; Galley and Linder, 2006; Bredenkamp and Van Wyk, 2006; Bytebier et al., 2007; Galley et al., 2007; Gehrke and Linder, 2009) (Fig. 4). Furthermore, a phylogeographic study of the Afromontane evergreen tree

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Fig. 4. Schematic representation of the centres and sub-centres of the Cape element in sub-Sahelian Africa (from Carbutt, 2004; Carbutt and Edwards, 2012; updated from Weimarck, 1941). The high elevation sub-centres in particular are often fragmented, hence the need for two hyphenated names when considering two dominant enclaves of close proximity. The most modern and geographically accurate names have been used. For example, ‘Abessinian Sub-centre’ is now ‘Ethiopian Sub-centre’; ‘Rungwe Sub-centre’ is now ‘Nyika-Rungwe Subcentre’; ‘Mitumba-Rwenzori Sub-centre’ is an amalgamation of Weimarck's (1941) ‘Katanga’ and ‘Kivu’ Sub-centres, named after the Mitumba Highlands, and the dominant Rwenzori Mountains further north. The double-sided arrows denote possible dual-direction migration events that may have resulted in reciprocal exchange of taxa between centres.

P. africana indicates a former east–west migration corridor between the highlands of East and West Africa (Kadu et al., 2011). Within the broader Afromontane corridor concept are nested smaller-scale corridors, or micro-corridors. For example, the southern portion of the Great Escarpment, in particular, is believed to have facilitated the movement of plant species between the Cape region and the Drakensberg Range (Weimarck, 1941; Levyns, 1964; Clark et al., 2009), with the Great Winterberg-Amatolas in particular serving as ‘stepping stones’ (Clark et al., 2011). Clark et al. (2011) showed support for both palaeo-connectivity and current connectivity between the Cape region and the southern Escarpment, with climate filtering mechanisms such as glacial–interglacial cycles and associated shifts in rainfall regime emerging as more important factors in floristic connectivity than geomorphological continuity. The Drakensberg Range sensu lato is a migration corridor that has facilitated the speciation, and influenced the distribution, of plant taxa in southern Africa (Carbutt and Edwards, 2001; Ramdhani et al., 2009; Uys and Cron, 2013; Bentley et al., 2014). It has functioned as a ‘portal’ (Carbutt, 2012b; Carbutt and Edwards, 2012) or a ‘stepping stone’ (Carlson et al., 2012) by selectively facilitating the movement of temperate plant taxa between southern and tropical Africa. Furthermore, Kurzweil et al. (1991), Carbutt (2004) and Galley et al. (2007) have suggested that the Drakensberg Range has served as a source of austral taxa also well represented in the Cape region (e.g. Pterygodium and Satyrium, Orchidaceae; Aloe and Kniphofia, Asphodelaceae), or alternatively as an important primary centre of plant diversity linked to more recent

radiations within the broader South Africa centre (e.g. Kniphofia; Ramdhani et al., 2009). Mountain corridors have therefore facilitated the expansion of the Cape flora into other regions (Fig. 4), resulting in floristic outliers of the Cape flora as ‘stations’ or ‘outposts’ (Hilliard and Burtt, 1987; Carbutt and Edwards, 2001, 2012). The Cape element extends northwards and westwards, in diminishing fashion, along the highlands of sub-Sahelian Africa (Adamson, 1947; Levyns, 1964; Rourke, 1998; Carbutt and Edwards, 2001; Fig. 4). The intervening areas devoid of the Cape element are referred to as intervals (Fig. 5). 3. The broad Afromontane framework: servant of all or master of none? We have from a literature synthesis associated the term ‘Afromontane’ with a range of contexts serving multiple purposes. It is clear that this range of contexts has created a distorted understanding of the term. Reference to the Afromontane region as a broad floristic framework is confusing because it does not reconcile the phytogeographical and ecological contexts to the same spatial boundaries, and includes the alpine belts. Furthermore, the broad framework's inclusion of all alpine belts has led to the northern and southern alpine regions being viewed as phytogeographically homologous. This is clearly problematic and has lead to much confusion. The broad framework also covers such a large range of latitude (48°) that we question its efficacy at displaying more finer-scale floristic and physiognomic variability, particularly in the

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Fig. 5. Schematic representation of the intervals between centres and sub-centres of the Cape element in sub-Sahelian Africa (from Carbutt, 2004; updated from Weimarck, 1941). Symbols: ▲, high elevation centres and sub-centres of the Cape element; ●, low elevation centre of the Cape element (Pondoland Centre). Most names are derived from the geographic regions in which the intervals occur.

species-rich grasslands and plateau margins. It cannot therefore serve multiple masters. We note the references to it functioning as a migration corridor but reiterate that this is a more conceptual palaeo-framework not explicit about elevation, floristic and vegetation spatial boundaries. 4. A pragmatic solution We have recognised the need to clarify the phytogeographical boundaries of sub-Sahelian Africa's montane and alpine regions at a conceptual level, particularly when viewed within the Afromontane framework. The latter framework's phytogeographical and ecological contexts, which often refer to ambiguous and incongruent boundaries (a fate also suffered in the delineation of ecoregions (Platts et al., 2011)), necessitate a simple and pragmatic application of terminology. Going back to first principles, we propose that the Afromontane region is simply the summed area covered by the montane flora and vegetation linked entirely to the montane belt and should exclude all higher and lower elevation areas. The Southern Afrotemperate Forests (Mucina and Rutherford, 2006) of the southern Cape, considered Afromontane due to the effect of southern (temperate) latitudes, are more appropriately referred to as ‘Afrotemperate’ than ‘Afromontane’ (Mucina and Rutherford, 2006). We concur with this sentiment that not all temperate forest vegetation should be considered ‘Afromontane’. Forests reaching sea level at temperate latitudes should therefore fall out of the ‘Afromontane’ framework as they do not occur at montane elevations. This simplified and clarified Afromontane region therefore occurs between ±1300 m and ± 4100 m a.s.l. in tropical Africa (the highest limit is recorded at Mt Kilimanjaro; Hemp, 2006c), and between ± 1300 m and ± 2800 m a.s.l. in southern Africa. The Afromontane

flora corresponds most closely to Linder's (2014) Tropic-montane flora although we show here, for pragmatic reasons, that it is not necessarily entirely tropical. Similarly, the Afroalpine region is the summed area covered by the alpine flora and vegetation of the alpine belt in the Afrotropics, occurring between ±3550 m and ±4100 m a.s.l., occasionally extending to 4500 m a.s.l. Its Tropic-alpine flora (Linder, 2014) is characterised by high endemism and, from a vegetation perspective, by large physiognomic forms subject to the ecological constraints of summer by day and winter by night. The tree and shrub-dominated ericaceous belt is the treeline ecotone separating the alpine and montane belts. In the spirit of consistency, we recognize the need to re-look at southern Africa's Drakensberg Alpine Centre, where we again note the ambiguity of a phytogeographic boundary not reconciled with its equivalent ecological boundary. This floristic centre of ‘alpine’ plant endemism is arbitrarily defined from the 1800 m contour up to its highest point, whereas its alpine belt is restricted to the summit area above ±2800 m a.s.l. (Killick, 1963, 1978). We are therefore confronted with the need to make one of two potential changes: either the Drakensberg Alpine Centre should refer exclusively to the alpine belt from ±2800 m to 3482 m as it is defined climato-ecologically by Killick (1963, 1978), or to the current delineation of 1800 m to 3482 m, renamed as the ‘Drakensberg Mountain Centre’ as not all of this area is strictly alpine. We prefer the former proposal with the Drakensberg Alpine Centre no longer including the upper montane (~ sub-alpine) belt, to be consistent with the classification of Körner et al. (2011). This re-clarified alpine centre (N ±2800 m a.s.l.) is neither Afroalpine nor Afromontane; therefore we regard it as Africa's austroalpine region (the sole alpine region of southern Africa). Although we note that the re-defined centre still

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supports a large proportion of the endemics recognised by Carbutt and Edwards (2006) for the former Drakensberg Alpine Centre, we recognize the need for a reassessment (Carbutt and Edwards, in preparation). The revised alpine centre will then incorporate plant species confined exclusively to the lower alpine belt, centred on an escarpment plateau delineated by physiography and not by the treeline, and subject to strong seasonal ecological constraints. The remaining upper and lower montane reaches of the former Drakensberg Alpine Centre are also worthy of separate attention and may qualify as a centre of endemism pending further studies. Linder's (1994) ‘Afrotemperate’ concept that consolidates the Cape and Afromontane floras as the two temperate floras of Africa should be extended to explicitly include the northern and southern alpine regions of Africa. We reiterate the need for all future ecological studies within the high elevation areas of sub-Sahelian Africa to be aligned with phytogeographical boundaries, and advocate the need for more ecological phytogeographical studies to determine the factors responsible for defining the current spatial distribution of plant species to complement historical approaches (Monge-Nájera, 2008). These studies should also extend to the Afromontane outliers of the Indian Ocean Islands (e.g. highlands of Madagascar and the Comoros). 5. End of the Afromontane era? Finally, notwithstanding the changes made here to clarify the Afromontane region, we begin to question the relevancy of the Afromontane region as a floristic concept in the longer term. We note that it has not been consistently retrieved as a distinct floristic unit in various large-scale numerical analyses (Linder, 2014). Furthermore, as a concept spanning almost four decades, the coarse Afromontane ‘template’ has not kept pace with vegetation-based finer-scale mapping techniques seeking to identify and define more discrete and meaningful spatial units such as ecoregions, biomes, bioregions and vegetation types. We conclude that such a broad-scale phytochorion, spanning a large latitudinal range of 48°, is a rather ungainly frame of reference probably immune to detecting the nuances of elevational, climatic, topographic, species-level floristic and physiognomic variability, particularly in the species-rich grasslands and plateau margins. Further largescale numerical analyses of plant taxa (preferably at the species level) in these habitats are essential to future studies testing the Afromontane floristic concept. Acknowledgements The authors thank the two anonymous reviewers for helpful comments and suggestions. References Abera, K., Kinahan, A., 2011. Factors affecting fire extent and frequency in the Bale Mountains National Park. In: Randall, D., Thirgood, S., Kinahan, A. (Eds.), Walia — Special Edition on the Bale Mountains. Frankfurt Zoological Society, Addis Ababa, pp. 146–157. Adamson, R.S., 1947. Some geographical aspects of the Cape flora. Transactions of the Royal Society of South Africa 31, 437–464. Assefa, Y., Wesche, K., Fetene, M., 2011. The status of the ericaceous vegetation on the southern slope of the Bale Mountains. In: Randall, D., Thirgood, S., Kinahan, A. (Eds.), Walia — Special Edition on the Bale Mountains. Frankfurt Zoological Society, Addis Ababa, pp. 158–170. Aynekulu, E., Aerts, R., Moonen, P., Denich, M., Gebrehiwot, K., Vågen, T.-G., Mekuria, W., Boehmer, H.J., 2012. Altitudinal variation and conservation priorities of vegetation along the Great Rift Valley escarpment, northern Ethiopia. Biodiversity Conservation http://dx.doi.org/10.1007/s10531-012-0328-9. Barr, J., Chander, A., 2012. Africa Without Ice and Snow. UNEP Global Environmental Alert Service, pp. 1–9. Beentje, H.J., Adams, B., Davis, S.D., 1994. Regional overview — Africa. In: Davis, S.D., Heywood, V.H. (Eds.), Centres of Plant Diversity. Oxford University Press, Oxford, pp. 101–148. Bentley, J., Verboom, G.A., Bergh, N.G., 2014. Erosive processes after tectonic uplift stimulate vicariant and adaptive speciation: evolution in an Afrotemperate-endemic

73

paper daisy genus. BMC Evolutionary Biology 14, 27. http://dx.doi.org/10.1186/ 1471-2148-14-27. Bond, W.J., Midgley, G.F., Woodward, F.I., 2003. What controls South Africa vegetation — climate or fire? South African Journal of Botany 69 (1), 79–91. Bonnefille, R., Riollet, G., 1988. The Kashiru sequence (Burundi): palaeo-climatic implications for the last 40,000 yr BP in tropical Africa. Quaternary Research 30, 19–35. Boughey, A.S., 1955. The nomenclature of the vegetation zones of the mountains of tropical Africa. Webbia 11, 413–423. Bredenkamp, C.L., Van Wyk, A.E., 2006. Phytogeography of Passerina (Thymelaeaceae). Bothalia 36 (2), 191–199. Brooks, T., Hoffmann, M., Burgess, N., Plumptre, A., Williams, S., Gereau, R.E., Mittermeier, R.A., Stuart, S., 2004. Eastern Afromontane. In: Mittermeier, R.A., Gil, P.R., Hoffmann, M., Pilgrim, J., Brooks, T., Mittermeier, C.G., Lamoreux, J., Da Fonseca, G.A.B. (Eds.), Hotspots Revisited: Earth's Biologically Richest and Most Endangered Ecoregions. CEMEX, Mexico City, pp. 240–275. Burgess, N., Hales, J.D., Underwood, E., Dinerstein, E., Olson, D., Itoua, I., Schipper, J., Ricketts, T., Newman, K., 2004. Terrestrial Ecoregions of Africa and Madagascar: A Conservation Assessment. Island Press, Washington DC, pp. 1–501. Burgoyne, P.M., Van Wyk, A.E., Anderson, J.M., Schrire, B.D., 2005. Phanerozoic evolution of plants on the African Plate. Journal of African Earth Sciences 43, 13–52. Bussmann, R.W., 2006. Vegetation zonation and nomenclature of African Mountains — an overview. Lyonia 11 (1), 41–66. Bytebier, B., Bellstedt, D.U., Linder, H.P., 2007. A molecular phylogeny for the large African orchid genus Disa. Molecular Phylogenetics and Evolution 43, 75–90. Carbutt, C., 2004. Cape Elements on High-altitude Corridors and Edaphic Islands. PhD thesis. University of KwaZulu-Natal, Pietermaritzburg. Carbutt, C., 2012a. The emerging invasive alien plants of the Drakensberg Alpine Centre, southern Africa. Bothalia 42 (2), 71–85. Carbutt, C., 2012b. From crags to riches: why is the flora of the Drakensberg Alpine Centre so diverse? Veld & Flora 98 (1), 10–13. Carbutt, C., Edwards, T.J., 2001. Cape elements on high-altitude corridors and edaphic islands: historical aspects and preliminary phytogeography. Systematics and Geography of Plants 71, 1033–1061. Carbutt, C., Edwards, T.J., 2004. The flora of the Drakensberg Alpine Centre. Edinburgh Journal of Botany 60, 581–607. Carbutt, C., Edwards, T.J., 2006. The endemic and near-endemic angiosperms of the Drakensberg Alpine Centre. South African Journal of Botany 72, 105–132. Carbutt, C., Edwards, T.J., 2012. An Eden in exile: outliers of the Cape Floristic Region. PlantLife 41 & 42, 4–11. Carbutt, C., Tau, M., Stephens, A., Escott, B., 2011. The conservation status of temperate grasslands in southern Africa. Grassroots 11 (1), 17–23. Carlsen, T.P., Bleeker, W., Hurka, H., Elven, R., Brochmann, C., 2007. Biogeography and phylogeny of Cardamine (Brassicaceae). In: Carlsen, T.P. (Ed.), Are There Any Trees in The Arctic? Reconstruction of Evolutionary Histories in a Young Biome. Faculty of Mathematics and Natural Science, University of Oslo. Carlson, S.E., Linder, H.P., Donoghue, M.J., 2012. The historical biogeography of Scabiosa (Dipsacaceae): implications for Old World plant disjunctions. Journal of Biogeography 39, 1086–1100. Castañeda, I.S., Werne, J.P., Johnson, T.C., Filley, T.R., 2009. Late Quaternary vegetation history of southeast Africa: the molecular isotopic record from Lake Malawi. Palaeogeography, Palaeoclimatology, Palaeoecology 275, 100–112. Chapman, J.D., White, F., 1970. The Evergreen Forests of Malawi. Commonwealth Forestry Institute, Oxford. Clark, V.R., Barker, N.P., Mucina, L., 2009. The Sneeuberg: a new centre of floristic endemism on the Great Escarpment, South Africa. South African Journal of Botany 75, 196–238. Clark, V.R., Barker, N.P., Mucina, L., 2011. Taking the scenic route — the southern Great Escarpment (South Africa) as part of the Cape to Cairo floristic highway. Plant Ecology and Diversity 4 (4), 1–16. Clayton, W.D., 1983. Geographical distribution of present day Poaceae as evidence for the origin of African floras. Bothalia 14, 421–425. Coetzee, J.A., 1964. Evidence for a considerable depression of the vegetation belts during the upper Pleistocene on the East African mountains. Nature 204, 564–566. Coetzee, J.A., 1967. Pollen analytical studies in East and Southern Africa. In: Van Zinderen Bakker, E.M. (Ed.), Palaeoecology of Africa vol. 3. Balkema, Cape Town, pp. 1–146. Denys, E., 1980. A tentative phytogeographical division of tropical Africa based on a mathematical analysis of distribution maps. Bulletin du Jardin Botanique National de Belgique 50, 465–504. Dimech, A., 2011. Plant Evolution Tour. Plant Science Portal, Biosciences Research Division, Melbourne, Australia. Dobrowski, S.Z., 2011. A climatic basis for microrefugia: the influence of terrain on climate. Global Change Biology 17, 1022–1035. Dowsett-Lemaire, F., Dowsett, R.J., 1998. Parallels between F. White's phytochoria and avian zoochoria in tropical Africa: an analysis of the forest elements. In: Huxley, C.R., Lock, J.M., Cutler, D.F. (Eds.), Chorology, Taxonomy and Ecology of the Floras of Africa and Madagascar. Royal Botanic Garden, Kew, pp. 87–96. Edwards, D., 1967. A plant ecological survey of the Tugela River Basin. Memoirs of the Botanical Survey of South Africa 36, 181–196. Eggermont, H., Van Damme, K., Russell, J.M., 2009. Rwenzori Mountains (Mountains of the Moon): headwaters of the White Nile. In: Dumont, H.J. (Ed.), The Nile: Origin, Environments, Limnology and Human Use. Springer Science and Business Media B.V, pp. 243–261. Ehrich, D., Gaudeul, M., Assefa, A., Koch, M.A., Mummenhoff, K., Nemomissa, S., Intrabiodiv Consortium, Brochmann, C., 2007. Genetic consequences of Pleistocene range shifts: contrast between the Arctic, the Alps and the East African mountains. Molecular Ecology 16, 2542–2559.

74

C. Carbutt, T.J. Edwards / South African Journal of Botany 98 (2015) 64–75

Ellison, N.W., Liston, A., Steiner, J.J., Williams, W.M., Taylor, N.L., 2006. Molecular phylogenetics of the clover genus (Trifolium—Leguminosae). Molecular Phylogenetics and Evolution 39, 688–705. Engler, A., 1892. Über die Hochgebirgsflora des tropischen Afrika. Verlag der Königl. Akademie der Wissenschaften, Berlin. Engler, A., 1904. Plants of the northern temperate zone in their transition to the high mountains of tropical Africa. Annals of Botany 18, 523–540. Fetene, M., Assefa, Y., Gashaw, M., Woldu, Z., Beck, E., 2006. Diversity of Afroalpine vegetation and ecology of treeline species in the Bale Mountains, Ethiopia, and the influence of fire. In: Spehn, E.M., Liberman, M., Körner, C. (Eds.), Land Use Change and Mountain Biodiversity. CRC Press, Boca Raton, Florida, pp. 25–38. Finch, J., Marchant, R., 2011. A palaeoecological investigation into the role of fire and human activity in the development of montane grasslands in East Africa. Vegetation History and Archaeobotany 20, 109–124. Finch, J., Leng, M.J., Marchant, R., 2009. Late Quaternary vegetation dynamics in a biodiversity hotspot, the Uluguru Mountains of Tanzania. Quaternary Research 72, 111–122. Friis, I., 1986. Zonation of forest vegetation on the south slope of the Bale Mountains, South Ethiopia. Ethiopian Journal of Science 9, 29–44. Galley, C., Linder, H.P., 2006. Geographical affinities of the Cape flora, South Africa. Journal of Biogeography 33, 236–250. Galley, C., Bytebier, B., Bellstedt, D.U., Linder, H.P., 2007. The Cape element in the Afrotemperate flora: from Cape to Cairo? Proceedings of the Royal Society 274, 535–543. Gehrke, B., Linder, H.P., 2009. The scramble for Africa: pan-temperate elements on the African high mountains. Proceedings of the Royal Society Britain 276, 2657–2665. Goldblatt, P., Manning, J.C., 2000. Cape Plants: A Conspectus of the Cape Flora. Strelitzia 9. National Botanical Institute and the Missouri Botanical Garden. Grab, S.W., Goudie, A.S., Viles, H.A., Webb, N., 2011. Sandstone geomorphology of the Golden Gate Highlands National Park, South Africa, in a global context. Koedoe 53 (1) (Art. #985, 14 pages). Grimshaw, J.M., 2001. What do we really know about the Afromontane Archipelago? Systematics and Geography of Plants 71, 949–957. Hall, J.B., 1973. Vegetational zones on the southern slopes of Mount Cameroon. Vegetatio 27, 49–69. Hamilton, A.C., Perrott, R.A., 1981. A study of altitudinal zonation in the montane forest belt of Mt. Elgon, Kenya/Uganda. Vegetatio 45, 107–125. Hauman, L., 1933. Esquisse de la végétation des hautes altitudes sur le Ruwenzori. Bulletin de l'Academie Royale de Belgique 5, 602–616 (702–717, 900–917). Hauman, L., 1955. La “région afroalpine” en phytogeographie Centro-Africaine. Webbia 11, 467–469. Hedberg, O., 1951. Vegetation belts of the East African mountains. Svensk Botanisk Tidskrift 45 (1), 141–196. Hedberg, O., 1955. Altitudinal zonation of the vegetation on the East African mountains. Proceedings of the Linnean Society London 165, 134–136. Hedberg, O., 1961. The phytogeographical position of the Afroalpine flora. Recent Advances in Botany 1, 914–919. Hedberg, O., 1964. Features of Afroalpine plant ecology. Acta Phytogeographica Suecica 49, 1–144. Hedberg, O., 1965. Afroalpine flora elements. Webbia 19, 519–529. Hedberg, O., 1969. Evolution and speciation in a tropical high mountain flora. Biological Journal of the Linnean Society 1, 135–148. Hedberg, O., 1986. Origins of the Afroalpine flora. In: Vuilleumier, F., Monasterio, M. (Eds.), High Altitude Tropical Biogeography. Oxford University Press, Oxford, pp. 443–468. Hedberg, O., 1994. Afroalpine Region: east and north-east tropical Africa. In: Davis, S.D., Heywood, V.H. (Eds.), Centres of Plant Diversity. Oxford University Press, Oxford, pp. 253–256. Hedberg, O., 1997. High-mountain areas of tropical Africa. In: Wielgolaski, F.E. (Ed.), Polar and Alpine Tundra. Elsevier Scientific, Amsterdam, pp. 185–197. Hemp, A., 2005. Climate change driven forest fires marginalize the impact of ice cap wasting on Kilimanjaro. Global Change Biology 11 (7), 1013–1023. Hemp, A., 2006a. The impact of fire on diversity, structure, and composition of the vegetation of Mt. Kilimanjaro. In: Spehn, E.M., Liberman, M., Körner, C. (Eds.), Land Use Change and Mountain Biodiversity. CRC Press, Boca Raton, Florida, pp. 51–68. Hemp, A., 2006b. Continuum or zonation? Altitudinal gradients in the forest vegetation of Mt. Kilimanjaro. Plant Ecology 184 (1), 27–42. Hemp, A., 2006c. Vegetation of Kilimanjaro: hidden endemics and missing bamboo. African Journal of Ecology 44, 305–328. Hemp, A., 2008. Introduced plants on Kilimanjaro: tourism and its impact. Plant Ecology 197, 17–29. Herbst, S.N., Roberts, B.R., 1974. The alpine vegetation of the Lesotho Drakensberg: a study in quantitative floristics at Oxbow. Journal of South African Botany 40, 257–267. Hilliard, O.M., Burtt, B.L., 1987. The Botany of the Southern Natal Drakensberg. National Botanic Gardens, Cape Town. Kadu, C.A.C., Schueler, S., Konrad, H., Muluvi, G.M.M., Eyog-Matig, O., Muchugi, A., Williams, V.L., Ramamonjisoa, L., Kapinga, C., Foahom, B., Katsvanga, C., Hafashimana, D., Obama, C., Geburek, T., 2011. Phylogeography of the Afromontane Prunus africana reveals a former migration corridor between East and West African highlands. Molecular Ecology 20, 165–178. Kiage, L.M., Liu, K.-B., 2006. Late Quaternary palaeo-environmental changes in East Africa: a review of multiproxy evidence from palynology, lake sediments, and associated records. Progress in Physical Geography 30 (5), 633–658. Killick, D.J.B., 1963. An account of the plant ecology of the Cathedral Peak area of the Natal Drakensberg. Memoirs of the Botanical Survey of South Africa 34, 1–178.

Killick, D.J.B., 1978. The Afroalpine Region. In: Werger, M.J.A. (Ed.), Biogeography and Ecology of Southern Africa. W. Junk, The Hague, pp. 515–542. Killick, D.J.B., 1979. African Mountain Heathlands. In: Specht, R.L. (Ed.), Elsevier Scientific, Amsterdam, pp. 97–116. Killick, D.J.B., 1994. Drakensberg Alpine Region — Lesotho and South Africa. In: Davis, S.D., Heywood, V.H. (Eds.), Centres of Plant Diversity. Oxford University Press, Oxford, pp. 257–260. Killick, D.J.B., 1997. Alpine tundra of southern Africa. In: Wielgolaski, F.E. (Ed.), Polar and Alpine Tundra. Elsevier Scientific, Amsterdam, pp. 199–209. Knapp, R., 1973. Die vegetation von Afrika unter Berücksichtigung von Umwelt, Entwicklung, Wirtschaft, Agrar- und Forstgeographie. G. Fisher, Jena. Koch, M.A., Kiefer, C., Ehrlich, D., Vogel, J., Brochmann, C., Mummenhoff, K., 2006. Three times out of Asia Minor: the phylogeography of Arabis alpina L. (Brassicaceae). Molecular Ecology 15, 825–839. Körner, C., 2001. Alpine ecosystems. In: Levin, S.A. (Ed.), Encyclopedia of Biodiversity. Academic Press, San Diego, pp. 133–144. Körner, C., 2003. Alpine Plant Life: Functional Plant Ecology of High Mountain Systems. 2nd ed. Springer, Heidelberg. Körner, C., 2012. Alpine Treelines: Functional Ecology of the Global High Elevation Tree Limits. Springer Basel, Basel. Körner, C., Nakhutsrishvili, G., Spehn, E., 2006. High-elevation land use, biodiversity, and ecosystem functioning. In: Spehn, E.M., Liberman, M., Körner, C. (Eds.), Land Use Change and Mountain Biodiversity. CRC Press, Boca Raton, Florida, pp. 3–21. Körner, C., Paulsen, J., Spehn, E.M., 2011. A definition of mountains and their bioclimatic belts for global comparisons of biodiversity data. Alpine Botany 121 (2), 73–78. Kurzweil, H., Linder, H.P., Chesselet, P., 1991. The phylogeny and evolution of the Pterygodium–Corycium complex (Coryciinae, Orchidaceae). Plant Systematics and Evolution 175, 161–223. Levyns, M.R., 1964. Migrations and origin of the Cape flora. Transactions of the Royal Society of South Africa 37, 85–107. Linder, H.P., 1990. On the relationship between the vegetation and floras of the Afromontane and the Cape regions of Africa. Mitteilungen aus dem Institut für Allgemeine Botanik Hamburg 23, 777–790. Linder, H.P., 1994. Afrotemperate phytogeography: implications of cladistic biogeographical analyses. In: Seyani, J.H., Chikuni, A.C. (Eds.), Proceedings of the XIIIth Plenary Meeting AETFAT, Malawi. National Herbarium and Botanic Gardens, Zomba, pp. 913–930. Linder, H.P., 1998. Numerical analyses of African plant distribution patterns. In: Huxley, C.R., Lock, J.M., Cutler, D.F. (Eds.), Chorology, Taxonomy and Ecology of the Floras of Africa and Madagascar. Royal Botanic Garden, Kew, pp. 67–86. Linder, H.P., 2001. Plant diversity and endemism in sub-Saharan tropical Africa. Journal of Biogeography 28, 169–182. Linder, H.P., 2003. The radiation of the Cape flora, southern Africa. Biological Review 78, 597–638. Linder, H.P., 2014. The evolution of African plant diversity. Frontiers in Ecology and Evolution 2 (38), 1–14. Linder, H.P., Meadows, M.E., Cowling, R.M., 1992. History of the Cape flora. In: Cowling, R.M. (Ed.), The Ecology of Fynbos: Nutrients, Fire and Diversity. Oxford University Press, Oxford, pp. 113–134. Linder, H.P., Vlok, J.H., McDonald, D.J., Oliver, E.G.H., Boucher, C., Van Wyk, B.-E., Schutte, A., 1993. The high altitude flora and vegetation of the Cape Floristic Region, South Africa. Opera Botanica 121, 247–261. Linder, H.P., Lovett, J., Mutke, J.M., Barthlott, W., Jürgens, N., Rebelo, T., Küper, W., 2005. A numerical re-evaluation of the sub-Saharan phytochoria of mainland Africa. Biologiske Skrifter 55, 229–252. Linder, H.P., de Klerk, H.M., Born, J., Burgess, N.D., Fjeldså, J., Rahbek, C., 2012. The partitioning of Africa: statistically defined biogeographical regions in sub-Saharan Africa. Journal of Biogeography 39, 1189–1205. Livingstone, D.A., 1975. Late Quaternary climate change in Africa. Annual Review of Ecology and Systematics 6, 249–280. Mabberley, D.J., 1986. Adaptive syndromes of the Afroalpine species of Dendrosenecio. In: Vuilleumier, F., Monasterio, M. (Eds.), High Altitude Tropical Biogeography. Oxford University Press, New York, pp. 81–102. Maley, J., 1991. The African Rain Forest vegetation and palaeoenvironments during late Quaternary. Climatic Change 19, 79–98. Marloth, R., 1902. Notes on the occurrence of alpine types in the vegetation of the higher peaks of the south-western region of the Cape. Transactions of the South African Philosophy Society 11, 161–168. McCarthy, T., Rubidge, B., 2005. The Story of Earth and Life. Struik Publishing, Cape Town. McDonald, D.J., Oliver, E.G.H., Linder, H.P., Boucher, C., 1993. Is there alpine vegetation on the mountains of the south-western Cape? Veld and Flora 79, 17–19. McGuire, A.F., Kron, K.A., 2005. Phylogenetic relationships of European and African ericas. International Journal of Plant Sciences 166, 311–318. Meadows, M.E., Linder, H.P., 1989. A reassessment of the biogeography and vegetation history of the southern Afromontane region. In: Geldenhuys, C.J. (Ed.), Biogeography of the Mixed Evergreen Forests of Southern Africa. Foundation for Research Development, Pretoria, pp. 15–29. Meadows, M.E., Linder, H.P., 1993. A palaeoecological perspective on the origin of Afromontane grasslands. Journal of Biogeography 20, 345–355. Meinzer, F., Goldstein, G., 1986. Adaptations for water and thermal balance in Andean giant rosette plants. In: Vuilleumier, F., Monasterio, M. (Eds.), High Altitude Tropical Biogeography. Oxford University Press, New York, pp. 381–411. Monge-Nájera, J., 2008. Ecological biogeography: a review with emphasis on conservation and the neutral model. Gayana 72 (1), 102–112. Monod, T., 1986. Les grandes divisions chorologiques de l'Afrique. CCTA/CSA publication no. 24, London, pp. 1–147.

C. Carbutt, T.J. Edwards / South African Journal of Botany 98 (2015) 64–75 Mucina, L., Rutherford, M.C., 2006. The vegetation of South Africa, Lesotho and Swaziland. Strelitzia 19. SANBI, Pretoria. Neumann, F.H., Botha, G.A., Scott, L., 2014. 18,000 years of grassland evolution in the summer rainfall region of South Africa: evidence from Mahwaqa Mountain, KwaZulu-Natal. Vegetation History and Archaeobotany http://dx.doi.org/10.1007/ s00334-014-0445-3. O'Connor, T.G., Bredenkamp, G.J., 1997. Grassland. In: Cowling, R.M., Richardson, D.M., Pierce, S.M. (Eds.), Vegetation of Southern Africa. Cambridge University Press, Cambridge, pp. 215–257. Olago, D.O., 2001. Vegetation changes over palaeo-time scales in Africa. Climate Research 17, 105–121. Ollier, C.D., 1996. Planet Earth. In: Douglas, I. (Ed.), Companion Encyclopedia of Geography: The Environment and Humankind. Routledge, London, p. 30. Osmaston, H.A., 1998. The influence of the Quaternary history and glaciations of the Rwenzori on the present landscape and ecology. In: Osmaston, H., Tukahirwa, J., Basalirwa, C., Nyakaana, J. (Eds.), The Rwenzori Mountains National Park, Uganda. Proceedings of the Rwenzori Conference. Department of Geography, Makerere University, Kampala, Uganda, pp. 49–65. Platts, P.J., Burgess, N.D., Gereau, R.E., Lovett, J.C., Marshall, A.R., McClean, C.J., Pellikka, P.E., Swetnam, R.D., Marchant, R., 2011. Delimiting tropical mountain ecoregions for conservation. Environmental Conservation 38 (3), 312–324. Popp, M., Gizaw, A., Nemomissa, S., Suda, J., Brochmann, C., 2008. Colonization and diversification in the African ‘sky islands’ by Eurasian Lychnis L. (Caryophyllaceae). Journal of Biogeography 35, 1016–1029. Ramdhani, S., Barker, N.P., Baijnath, H., 2008. Exploring the Afromontane centre of endemism: Kniphofia Moench (Asphodelaceae) as a floristic indicator. Journal of Biogeography 35, 2258–2273. Ramdhani, S., Barker, N.P., Baijnath, H., 2009. Rampant non-monophyly of species in Kniphofia Moench (Asphodelaceae) suggests a recent Afromontane radiation. Taxon 58 (4), 1141–1152. Richards, P.W., 1963. Ecological notes on West African vegetation. III. The upland forests of Cameroons Mountain. Journal of Ecology 51, 529–554. Rourke, J.P., 1998. A review of the systematics and phylogeny of the African Proteaceae. Australian Systematic Botany 11, 267–285. Sarmiento, G., 1986. Ecological features of climate in high tropical mountains. In: Vuilleumier, F., Monasterio, M. (Eds.), High Altitude Tropical Biogeography. Oxford University Press, New York, pp. 11–45. Schmitz, G., Rooyani, F., 1987. Lesotho: Geology, Geomorphology, Soils. National University of Lesotho, Roma. Schüler, L., Hemp, A., Zech, W., Behling, H., 2012. Vegetation, climate and fire-dynamics in East Africa inferred from the Maundi Crater pollen record from Mt Kilimanjaro during the last glacial–interglacial cycle. Quaternary Science Reviews 39, 1–13. Scott, L., 1989. Climatic conditions in southern Africa since the last glacial maximum, inferred from pollen analysis. Palaeogeography, Palaeoclimatology, Palaeoecology 70, 345–353. Scott, L., 1999. The vegetation history and climate in the Savanna Biome, South Africa, since 190 000 KA: a comparison of pollen data from the Tswaing Crater (the Pretoria Saltpan) and Wonderkrater. Quaternary International 57&58, 215–223. Scott, A.C., 2000. The Pre-Quaternary history of fire. Palaeogeography, Palaeoclimatology, Palaeoecology 164, 297–345. Scott, L., Anderson, H.M., Anderson, J.M., 1997. Vegetation history. In: Cowling, R.M., Richardson, D. (Eds.), Vegetation of Southern Africa. Cambridge University Press, Cambridge, pp. 62–90.

75

Siebert, S., Ramdhani, S., 2004. The Bale Mountains of Ethiopia. Veld & Flora 54–59 (June 2004). Steenkamp, Y., Van Wyk, A.E., Smith, G.F., Steyn, H., 2005. Floristic endemism in southern Africa: a numerical classification at generic level. Biologiske Skrifter 55, 253–271. Tallents, L.A., Macdonald, D.W., 2011. Mapping high-altitude vegetation in the Bale Mountains, Ethiopia. In: Randall, D., Thirgood, S., Kinahan, A. (Eds.), Walia — Special Edition on the Bale Mountains. Frankfurt Zoological Society, Addis Ababa, pp. 97–117. Tyson, P.D., Preston-Whyte, R.A., Schulze, R.E., 1976. The Climate of the Drakensberg. The Town and Regional Planning Commission, Pietermaritzburg. Uys, E., Cron, G.V., 2013. Relationships and evolution in the Drakensberg near-endemic genus, Craterocapsa (Campanulaceae). South African Journal of Botany 86, 79–91. Valente, L.M., Savolainen, V., Vargas, P., 2010. Unparalleled rates of species diversification in Europe. Proceedings of the Royal Society B: Biological Sciences 277, 1489–1496. Van Wyk, A.E., Smith, G.F., 2001. Regions of Floristic Endemism in Southern Africa. Umdaus Press, Pretoria. Van Zinderen Bakker, E.M., 1981. The high mountains of Lesotho — a botanical paradise. Veld and Flora 67, 106–108. Van Zinderen Bakker, E.M., 1983. The late quaternary history of climate and vegetation in east and southern Africa. Bothalia 14, 369–375. Van Zinderen Bakker, E.M., Clarke, J.D., 1962. Pleistocene climates and cultures in North Eastern Angola. Nature 196, 639–642. Van Zinderen Bakker, E.M., Werger, M.J.A., 1974. Environment, vegetation and phytogeography of the high- altitude bogs of Lesotho. Vegetatio 29, 37–49. Weimarck, H., 1941. Phytogeographical groups, centres and intervals within the Cape flora: a contribution to the history of the Cape element seen against climate changes. Lunds Universitets Årsskrift 37, 1–143. Werger, M.J.A., 1978. Biogeographical divisions of southern Africa. In: Werger, M.J.A. (Ed.), Biogeography and Ecology of Southern Africa. W. Junk, The Hague, pp. 145–170. Wesche, K., 2006. Is afroalpine plant biodiversity negatively affected by high-altitude fires? In: Spehn, E.M., Liberman, M., Körner, C. (Eds.), Land Use Change and Mountain Biodiversity. CRC Press, Boca Raton, Florida, pp. 39–49 Wesche, K., Miehe, G., Kaeppeli, M., 2000. The significance of fire for Afroalpine ericaceous vegetation. Mountain Research and Development 20 (4), 340–347. Wesche, K., Assefa, Y., Von Wehrden, H., 2008. Temperate grassland region: equatorial Africa (high altitude). Compendium of regional templates on the status of temperate grasslands conservation and protection. Report Prepared for the World Temperate Grasslands Conservation Initiative Workshop: ‘Life in a Working Landscape: Towards a Conservation Strategy for the World's Temperate Grasslands’. Hohhot, Inner Mongolia (China). 28th & 29th June 2008. Temperate Grasslands Conservation Initiative, Vancouver, Canada, pp. 3–21. White, F., 1978. The Afromontane region. In: Werger, M.J.A. (Ed.), Biogeography and Ecology of Southern Africa. W. Junk, The Hague, pp. 463–513. White, F., 1981. The history of the Afromontane archipelago and the scientific need for its conservation. African Journal of Ecology 19, 33–54. White, F., 1983. Long-distance dispersal and the origins of the Afromontane flora. Sonderbände des Naturwissenschaftlichen Vereins in Hamburg 7, 87–116. White, F., 1993. Refuge theory, ice-age aridity and the history of tropical biota: an essay in plant geography. Fragmenta Floristica et Geobotanica Supplement 2, 385–409. Yalden, D.W., 1983. The extent of high ground in Ethiopia compared to the rest of Africa. Ethiopian Journal of Science 6, 35–38.