An analysis of the orchid flora of Mt Mulanje , Malawi

The composition of the orchid flora of Mt Mulanje, Malawi, is analysed. The altitudinal distribution of the orchids, the distribution of the terrestrial and epiphytic species and the extralimital distribution of the species is assessed for both genera and subfamilies (after the systematic concept of Dressier 1981). The altitudinal distribution of species endemic to Malawi is also assessed. The terrestrial species show a significant increase with altitude whereas the epiphytic species are more dominant al the lower levels. A similar increase is also found in the species endemic to Malawi. An analysis of the distributions shared with other African countries reveals that most species also occur in Zambia, Zimbabwe and East Africa, whereas significantly fewer species are shared with Angola, southern Africa, Mocambique, Zaire and West Africa. Most species shared with tropical African countries are found on the lower slopes of Mt Mulanje.

The Mulanje massif (Figure 1) is an isolated mountain block in the southeastern corner of Malawi.The massif is comparatively small, covering only about 640 km2 (Eastwood 1988; measured at the 800 m contour).From the surrounding plains at around 600-700 m the steep slopes rise abruptly to plateaux at 1 800-2 000 m, which are divided by ridges and peaks of up to 3 000 m.The slopes and the plateaux offer a wide variety of different habitats, ranging from lowland rain forests (Figure 2A) and savanna woodland (Figure 2B) to montane grassland (Figure 2C) and seepage areas overlying bare rock (Figure 2D).Almost the entire mountain massif is now a Forest Reserve, but very few of the lowland forests at the base of the mountain and on the lower slopes are left following the planting of tea and coffee.
The botanical exploration of Mt Mulanje began in 1891 (Chapman 1962) when Alexander Whyte made the first extensive collections.The earliest extensive botanical survey of the mountain was that of Chapman (1962)

RESULTS AND DISCUSSION
The orchid flora A total of 173 orchid species in 45 genera have been recorded on Mt Mulanje (Tables 1, 2).A recent enumera tion of orchids has also been provided for the Nyika Plateau in northern Malawi (Williamson 1979) 1, 3).This is in contrast to the Nyika Plateau where Habenaria as well as Disa and Satyrium outnumber Eulophia (Williamson 1979) Because of the occurrence of lowland forests at the base of the mountain and on the lower slopes, an abundance of epiphytes have been recorded in the past, although some species have been recorded before the expansion of the belonging to the tribe Vandeae clearly outnumber all other epiphytic genera, which, however, may partly also be the result of extensive taxonomic splitting in this tribe.
The systematic position of the genera found on Mt Mulanje is shown in Table 1.The classification follows Dressier (1981) throughout.All of the orchid subfamilies are represented except the non-African Cypripedioideae and Apostasioideae.However, the subfamily Spiranthoideae is very poorly represented (Table 4), and is confined to the base of the mountain (Figure 3).The three remaining subfamilies Orchidoideae, Epidendroideae and Vandoideae are rich in species, and all their African tribes and subtribes except Vanilleae and Epipogieae (both Epiden droideae) are represented.The percentage of Orchidoideae increases significantly with altitude, and all of the orchids found in zone 4 belong to this subfamily (Figure 3).Evidently the two subfamilies Epidendroideae and Vandoi deae are less dominant at higher levels than they are in zone 1.

Terrestrial and epiphytic species
Whereas the overall percentage of terrestrial and epiphytic species on the mountain is almost equal (51% and 49%, respectively), significant altitudinal differences exist (Table 2 and Figure 4).Only 30% of the species grow terrestrially at the Mulanje base, but the terrestrial species account for 44% in zone 2 and for 61% in zone 3. The orchids in zone 4 are exclusively terrestrial.Evidently, the percentage of epiphytic orchids decreases with altitude.

Extralimital distribution of the species
The Africa-wide distributions of the species recorded on Mt Mulanje were arranged after zones as well as after    Most orchids shared with tropical, continental African countries occur in zone 1, and their number successively decreases with altitude on Mt Mulanje (Table 5).The sharp rise in zone 4 in the orchids shared with Madagascar is noteworthy although it may partly also be the result of a low overall number of species (n=10).This is to a lesser extent also found in the orchids shared with West Africa.It is also interesting that the tropically widespread spe cies are most common in zone 1, but they are missing in zone 4.
The Spiranthoideae are represented by widespread species only (Table 5).Rather surprisingly, the shared dis tributions are similar in the two subfamilies Orchidoideae and Vandoideae.The Epidendroideae stand out due to the high percentage of species shared with Madagascar, East and West Africa, and their low percentage of species shared with southern Africa.This is obviously the result of the distribution of Bulbophyllum to which the majority of the Epidendroideae on Mt Mulanje belong.The Epiden droideae are also significantly more frequently tropically widespread in their distribution than Orchidoideae and Vandoideae.

Endemic species
A list of endemic species is given in Table 6.Four species are endemic to Mt Mulanje, three of them being terrestrial.A further six species recorded on Mt Mulanje are endemic to the South Region, and two of the species are endemic to Malawi.It is interesting to note that the percentage of endemic species increases with altitude (Figure 5).This is, however, due to the increase of the S Malawi and the Mt Mulanje endemics, whereas the overall Malawi endemics decrease with altitude.
The number of endemics on Mt Mulanje is considerably lower than on the Nyika plateau where 18 'forms of life' (probably varieties, subspecies and species) are thought to be endemic including six species (Williamson 1979).Loedolff for the photographic work.The use of litera ture held in the Bolus Herbarium library is gratefully acknowledged.

APPENDIX: A PROVISIONAL CHECKLIST
The species recorded on Mt Mulanje are listed in alphabetical order.The nomenclature follows La Croix et. al. (1991) throughout.Most of the geographical names are indicated in the map provided (Figure 1).Habitats: e, epilithic or epiphytic; t, terrestrial; gr, grassland; If, lowland and mid-altitude forest; ma, marsh or bog; mf, montane or submontane forest; pi, pine plantation; ro, on or among rocks; sa, savanna woodland; se, seepage area.Altitudinal zones: 1, Mulanje base; 2, slopes; 3, plateaux; 4, peaks and ridges.Locality and literature references: 1, La Croix et al. (1983);2, La Croix et al. (1991);3, M orris (1970);4, Dowsett-Lemaire (1988); 5, Herbarium in Zomba (MAL); 6, Stewart (1980);7, Chap man (1962);8, Stewart & La Croix (1987);9, Cribb (1978);10, Linder (1985); 11, Cribb & Stewart (1985);12, Linder (1980);13, Wood (1982); 14, Cribb (1979);15, Summerhayes (1962) on the vegetation and the plant communities of Mt Mulanje, in which also climatological, geological and soil data were accumulated.Particularly the forest flora has since then attracted a great deal of attention, and has been the subject of several exhaustive publications (most notably Dowsett-Lemaire 1988).The orchid flora of Mt Mulanje is one of the richest in Malawi and is especially rich in terrestrial species.Although the orchid flora of the whole of Malawi has been written up already (La Croix et al. 1991; epiphytic orchids only; La Croix et al. 1983; Morris 1970), neither a local orchid flora of Mt Mulanje nor a complete checklist are available to date.However, some detailed information on the distribution of orchids on the massif was given in the orchid flora of Malawi (La Croix et al. 1991) as well as by Chapman (1962), Dowsett-Lemaire (1988) and in some taxonomic papers.As Mt Mulanje is relatively isolated and may be a halfway station between the eastern highlands of Zimbabwe and the large upland areas of Nyika and southern Tanza nia, an analysis of the composition of the orchid flora of Mt Mulanje is of interest.Consequently, special attention is paid to the altitudinal distribution of orchids, to the dis tribution of terrestrial and epiphytic species, to the distri bution of the species in other African countries, and to the distribution of species endemic to Malawi.It may be somewhat premature for an analysis of this kind as the data base may be incomplete, but it is unlikely that fur ther records will affect the main results of the study.The mountain massif has been divided into four altitudinal zones: 1, Mulanje base; this area comprises lowland rain forests in the adjacent parts of the surrounding plain as well as the foothills; it is now largely confined to forest patches in cultivated land; approximately up to 900 m; 2, slopes and gorges; savanna woodland and mid-altitude to afromontane forests; approximately 900-1 900 m; 3, plateaux; grassland and afromontane forest; approxi mately the southern African terrestrial orchid genera are represented on Mt Mulanje.Most of them reveal affinities to the orchid floras of South Africa and south central Africa where the largest concentration of species as well as most of the related species are found (Disa, Brownleea, Satyrium, Disperis, Schizochilus, Neobolusia, Stenoglottis).Disa, Disperis and Satyrium are rather poor in species numbers on Mt Mulanje when com pared to Nyika, which is probably partly due to the considerably larger grassland area on Nyika.As far as Disa is concerned, the watershed between the rivers Zaire and Zambesi which includes the Nyika Plateau is generally rich in Disinae and was considered an important centre of the subtribe (Linder 1983).As in most other African countries the large pantropical genera Habenaria and Eulophia are well represented on Mt Mulanje.Eulophia (20 species) is the largest terrestrial orchid genus, followed by Habenaria (14 species) and Satyrium (12 species; Tables

. However, the abundance of low-lying areas on Mt Mulanje partly accounts for the predominance of Eulophia. Only six species of the largely Madagascan genus Cynorlds occur on Mt Mulanje. Several other terrestrial orchid genera occur with a single or few species, including the palaeotropical genus Nervilia, the primarily Asian genus Calanthe, the cosmopolitan genera Liparis and Malaxis, and the southern African elements Herschelianthe and Monadenia. It is interesting that one largely terrestrial species belongs to the primarily epiphytic genus Polystachya. It is interesting to note that three terrestrial orchid genera, Bonatea, Centrostigma and Pterygodium, occur ring in southern and East Africa do not occur on Mt Mulanje. Pterygodium, although well represented in South Africa and also occurring in Tanzania, has not yet been collected in Malawi or central Africa. Corycium, closely related to Pterygodium and with a similar distribution (well represented in South Africa, also known in Tanzania; P. Cribb pers. comm.) has only recently been discovered in a rather restricted area on Mt Mulanje (Kurzweil, unpublished). Possibly the lack of Pterygodium in central tea and coffee plantations and may since have disappeared. The epiphytic orchids mainly belong to the genera Polystachya, Bulbophyllum, Aerangis, Angraecum, Cyr- torchis, Diaphananthe, Tridactyle and their allies, although epiphytic orchids are also known in some primarily terrestrial genera (Disa, Brownleea, Stenoglottis). Liparis is represented by terrestrial as well as epiphytic species. The largest epiphytic genera are the predominantly African genus Polystachya (20 species) and the large pantropical Bothalia 22,2 (1992) 2V FIGURE
2 .-Virioushabitats on Mt Mulanje.A, lowland forest; B, savanna woodland; C, plateau grassland, Satyrium trinerve in the foreground; D, seepage area on bare rock, here colonized by Satyrium rhynchantoides.

TABLE 1 .
Croix et al. 1991)ented on Mt Mulanje, their systematic po sition, worldwide number of species as well as number of species recorded on the mountain massif, and their overall distribution (classification after Dressier 1981, number of species after LaCroix et al. 1991).
genus Bulbophyllum (15 species; Tables1, 3).Several of the species have their southern-most and western-most limits of distribution in Malawi (see also Dowsett-Lemaire 1989).Comparatively few of the African epiphytic genera are not found on the mountain (e.g.Chamaeangis, Solenangis, Ancistrorrhynchus).The epiphytic genera

TABLE 2 .
-The number of orchid species recorded on Mt Mulanje, the number of terrestrial and epiphytic species and their altitudinal distribution.The altitudinal distribution and the distribution of terrestrial and epiphytic species is also summarized for the subfamilies and for the whole orchid family.An asterisk indicates taxa with species which may be epiphytic or terrestrial

TABLE 4 ,
Croix et al. (1991)chid genera and species on Mt Mulanje according to subfamilies.The classification follows Dressier(1981).Numbers of genera and species after LaCroix et al. (1991)