zyxwv zyxwv zyxwvu zyxwv zyxwvuts Nordic Journal of Botany A species-poortropical sedge community: Afrotrilepis pilosa mats on inselbergs in West Africa Stefan Porembski, Gary Brown and Wilhelm Barthlott Porembski, S., Brown, G. & Barthlott, W. 1996. A species-poor tropical sedge community: Afrotrilepis pilosa mats on inselbergs in West Africa. - Nord. J. Bot. 16: 239-245. Copenhagen. ISSN 0107-055X. The mat-forming poikilohydric sedge Afrotrilepis pilosa (Cyperaceae) occurs as the dominant member of characteristic stands on West African inselbergs. The species composition of this community has been examined in the Ivory Coast along a latitudinal gradient from the savanna to the rainforest region. Inselbergs consisting of Precambrian granites and gneisses, are prominent landscape elements characterized by large areas of exposed rock. Monocotyledonous mats are one of the most conspicuous communities of this ecosystem. Floristically, the mats are poor in higher plant species with a total of only 15 species (mostly therophytes) recorded in 110 relevks. Species diversity is likewise low, indicating extremely stressful environmental conditions. A major factor affecting plant colonization is the pronounced short-term variation in water-availability, even in the more humid regions in the south of the country. Afrotrilepis pilosa is highly competitive under these conditions, and the dense stands leave only little space for other species to become established. Longevity (attaining an age of several hundred years) and the ability, to spread radially to form almost monospecific stands are further characteristics which render this species a superior competitor. In marked contrast with most other tropical vegetation types, Afrotrilepis mats have therefore to be regarded as an example of a naturally occurring, extremely species-poor community, in which competitive displacement and the low degree of disturbance prevent higher diversity. zyxwv zyxwv Stefan Porembski, Gary Brown and Wilhelm Barthlott, Botanisches Institut der Universitat Bonn, Meckenheimer Allee 170, 0-53115 Bonn, Germany Introduction Across large areas of tropical Africa, inselbergs form characteristic landscape elements which rise abruptly from the surrounding plains. Both the small-sized shield-inselbergs and the large monolithic dome-shaped mountains, typically with steep curved sides of bare rock, consist mainly of Precambrian granites or gneisses. The geomorphology of these rock outcrops which are often older than 10 million years has been described in detail by Bremer & Jennings (1978). In respect of their biotic characteristics, tropical inAccepted 23-1 1-1995 0 NORDIC selbergs have rarely been the subject of detailed examination. According to Barthlott et al. (l993), inselbergs in general offer a range of varied habitats, and the following list circumscribes them physiognomically (forest communities omitted): 1. cryptogamic crusts on open rock surfaces (consisting of lichens and cyanobacteria, but destitute of vascular plants); 2. drainage channels; 3. seasonally water-filled rock pools; 4. flat depressions; 5 . monocotyledonous mats; 6. ephemeral flush vegetation; 7. wet flush vegetation. The vegetation of West African inselbergs has been dealt with by several authors (e.g. Adjanohoun 1964; Bonardi 1966; Hambler 1964; Porembski & Barthlott 1992, 1993; Porembski et al. 1994; zyxwvutsrq JOURNAL OF BOTANY Nord. 1. Bot. 16(3) 1996 239 Reitsma et al. 1992; Richards 1957; Villiers 1981). Although they are situated in a relatively mesic macroclimatic area, inselbergs are known to be edaphically and microclimatically dry (Phillips 1982). In particular, short-term fluctuations in water-availability can cause serious problems for plant growth. As a consequence, it is only to be expected that the harsh environmental conditions would have yielded a high percentage of plant species well-equiped with special morphological, anatomical and physiological properties. Of particular interest is the fact that poikilohydric vascular plant species occur in remarkably high numbers on inselbergs. The major component of monocotyledonous mats on West African inselbergs is the poikilohydric Cyperaceae Afrotrilepis pilosa (Boeck.) J. Raynal, a tufted perennial with roots possessing a velamen radicum like those of many epiphytic species of Orchidaceae and Araceae. Details on the morphology and anatomy of Afrotrilepis pilosa are given by Hambler (1961) and Porembski & Barthlott (1995). Even on steep rocky slopes, it forms stands of varying size which can be lifted up like a carpet. On account of the dominance of Afrotrilepis, these mats present a rather monotonous picture (Figs 1-3). Away from the inselbergs, the species occurs as a rockdwelling plant, and has never been recorded in nonrocky habitats. Its distributional range is restricted to West Africa, extending from Senegal southwards to Gabon. Afrotrilepis pilosa is a chamaephyte with an erect or ascending “stem” covered with the remains of old leaves and roots. It is a very polymorphic species and can display considerable variation in growth form and leaf indumentum between populations of different inselbergs. However, on a single inselberg, the morphological characteristics of the plants tend to remain constant (own unpubl. data). The objective of this study is to describe the diversity of vascular plants in the Afrorrilepis pilosa community and thereby to provide some insight into the factors which may regulate species richness and diversity of a highly-specialized tropical community. Afrotrilepis pilosa mats are a frequent occurrence on inselbergs in all climatic zones of the Ivory Coast. For this reason, it is possible to assess the influence of various environmental factors on diversity within this habitat, including precipitation, isolation and disturbance. April. Annual precipitation is lower than in the southern part of the country, and usually ranges from between 900 and 1600 mm. A detailed description of the climate has been given by Eldin (1971) and Anhuf (1994). In respect of the microclimate on inselbergs, measurements on exposed rock faces show that air temperature often exceeds 50°C, whilst relative air humidity frequently drops below 20 %, even on inselbergs in the rainforest zone (own unpubl. data). The inselbergs investigated in this study mainly consist of granites, less frequently gneisses (pers. comm. H. Sander, Cologne). As pedogenesis is often inhibited, exposed rock surfaces are a common feature of inselbergs. Afrotrilepis mats, however, are capable of forming and retaining their own substrate and are therefore sharply delimited from the surrounding bare rock. This substrate has a depth of between 5 and 15 cm and consists almost entirely of Afrotrilepis pilosa in various states of decay. In total, 110 relevCs were compiled on 1 I inselbergs in the Ivory Coast, following a climatic gradient from the savanna region southwards to the rainforest zone. The area surveyed on each inselberg was between 5000 and 10 000 m?. Fieldwork was carried out in September and October 1991. Care was taken to select clearly delimited Afrotrilepis mats (i.e. mats surrounded by bare rock and not affected by shade) that appeared to be identical in their principal characteristics (i.e. size, exposition). Mats showing traits of fire or anthropogenic influence were excluded. The Ajrotrilepis mats were circular in outline and covered an area of 5 m2 (+/- 5 %). The percentage cover of each vascular plant species was recorded. As a measure of diversity, the Shannon index was calculated for each relevC. zyxwv zyxwvutsr zyxwvu zyxwvutsrqpo Methods and study sites Situated between 4” 30’ N and 10” 30’N, the southern third of the Ivory Coast has a humid equatorial climate (“Af’ after the Koppen classification), receiving an annual rainfall of between 1800 and 2300 mm. The north of the country has a seasonal climate (“Aw”) with a dry period of up to six months, mainly between October and 240 zyxwvut Results Fifteen species of vascular plants from 10 families were found in the Afrotrilepis mats studied (Tab. I). The Cyperaceae (3) are the most speciose family, followed by the Orchidaceae ( 2 ) , Poaceae (2) and Rubiaceae (2). All other families are represented by a single species. The most frequently recorded associate in the Afrotrilepis mats was Cyanotis lanuta (in 64 % of the relevks), followed by Lindernia exilis (45 %), Fimbristylis dichotoma (28 %), Sporobolus festivus (23 %), Bulbostylis coleotricha (I8 %), Asplenium sruhlrnannii ( I 6 %) and Polystachya microbambusa (16 %), epiphytic on Afrotrilepis pilosa. The remaining species occurred in fewer than 10 % of relevks. In respect of life form, therophytes clearly dominate (9 species), followed by the chamaephytes (3). Cryptophytes, hemicryptophytes and phanerophytes are each represented by a single species. Afrotrilepis pilosa is the dominant species in all sample plots. Accompanying species rarely attain cover val- zy N o d J . But. l h ( 3 ) 1996 zyxwvutsrqpon zyxwvut zyxwvutsrqpo zyxwvutsrqpo zyxwvutsrq zyxwvutsrqp Figs 1-3. - I : Inselbergs in the Ivory Coast consist of Precambrian granites and gneisses. Mats formed by the poikilohydric Cyperaceae AfrotriIppis / J ~ / O W are a characteristic element of their vegetation (photograph by Nadja Biedinger). - 2: Afrotri/rpi.s mats are sharply delimited communities forming vegetational islands surrounded by bare rock. They are generally species-poor and A. I J ~ ~ O Smay O attain 100 % cover (rainy season aspect). - 3: Environmental conditions on tropical inselbergs are very harsh and characterized by prolonged periods of drought. However, the poikilohydric Afrorrilrpis pilosci is highly competitive under these conditions (dry season aspect). 24 I 35 zyxwvut zyxwvutsrqponml 1 1 and Lindernia exilis do not occur solely on inselbergs, but also colonize other rocky habitats such as ferricretes and the slopes of mountain ranges. Only few species are found predominantly in savannas (e.g. Pennisetum polystachion) or disturbed places ( e g Borreria scabra). Phytogeographically, the greatest proportion of associates (9 species, e.g. Cyanotis lanata, Hymenodictyon jloribundum) belongs to a group which is widespread throughout tropical Africa or has its main centre of distribution in the Sudano-Zambezian-Region. Three species (including Afrotrilepis) occur in West Africa from Senegal across to Nigeria and southwards to Gabon. Two species (Fimbristylis dichotoma, Pennisetum polystachion) are pantropical and only Polystachya microbambusa is endemic to the Upper Guinea region. On average, each relevC contains 3.35 species, with a maximum of 6 recorded. Some plots are characterized by the presence of a single species, namely Afrotrilepis pilosa. Most frequently 3 (31 % of the plots examined) and 4 species (30 %) occur in the individual sampling plots, followed by 2 (19 %), 5 (13 %) and 6 species (3 %) (Fig. 4). The average Shannon index of diversity for all Afrotrilepis mats studied is relatively low (0.3) compared with other communities on inselbergs (Porembski et al. 1995). The most diverse Afrotrilepis plot has a Shannon index of 0.79, whereas communities in shallow depressions or ephemeral flush communities often attain values higher than 2.5. No correlation was found between the amount of precipitation along a latitudinal gradient and either species numbers (Fig. 5) or the Shannon index. With regard to species number and diversity, the mats on all l l inselbergs present a generally similar picture. Two inselbergs had slightly higher average species than most others (Tab. 2). The same applies to species diversity. Interestingly for instance, there is no relationship between the total area of each of the inselbergs and the number of species in Afrotrilepis mats (results not shown). r zyxwvutsrqponml 2 zyxwvutsrq species per plot zyxwvuts Fig. 4. Histogram recording number of species in percentage of sample plots. ues in excess of 5 %. In a few exceptional cases, cover values of up to 20 % were recorded for Borreria scabra, Cyanotis lanata, Hymenodictyon jloribundum and Polystachya microbambusa. The central part of Afrotrilepis mats is usually free of accompanying species. They are generally restricted to the sun-exposed fringes of the mats where they root in a substrate consisting of decaying roots and leaves of Afrotrilepis. Characteristically, small patches of mosses (in particular Bryum arachnoidewn and Brachymenium exile) and cyanobacteria (e.g. Schizothrix spp.) are also encountered around the edges of the mats. Most species found in Afrotrilepis mats are confined to rocky substrates. The following species are almost completely restricted to inselbergs: Asplenium stuhlmannii, Hibiscus scotellii and the epiphytic orchid Polystachya microbambusa (occurring only in mats of Afrotrilepis pilosa). Other species, such as Cyanotis lanata, Habenaria gabonensis, Hymenodictyon jloribundum Tab. 1. List of vascular plant species encountered in the Afrotrilepis mats and percentage occurrence. Family Species Cyperaceae Polypodiaceae Rubiaceae Cyperaceae Commelinaceae Cyperaceae Orchidaceae Malvaceae Rubiaceae Scrophulariaceae Poaceae Caryophyllaceae Orchidaceae Lamiaceae Poaceae Afrotrilepis pilosa 100.0 16.4 Asplenium stuhlrnannii 5.5 Borreria scabra 18.2 Bulbosvlis coleotricha Cyanotis lanata 63.6 Fimbristylis dichotoma 28.2 Habenaria gabonensis 0.9 Hibiscus scotellii 6.4 Hymenodicvon jloribundurn 5.5 44.5 Lindernia exilis Pennisetum polystachion 1.8 1.8 Polycarpaea eriantha Polystachya microbambusa 16.4 Solenostemon laterifolius 3.6 Sporobolus festivus 22.1 242 Frequency (%) Discussion The very modest number of species (1 5 in more than 100 relevCs) found in the Afrotrilepis mats gives a clear indication of the extremely stressful environmental conditions operating within these stands. The selective pressures exerted by seasonal drought have led to the domination of one species. As a consequence, competititon can be regarded as a prime determinant affecting community structure. No other habitat on inselbergs in which vascular plants predominate is as poor in species (own unpubl. data), and in respect of species diversity, Afrotrilepis mats are the least diverse community. Because of the dense growth of Afrotrilepis, little space re- zyxwvut Nord. J . Bot. 16(31 1996 zyxwvutsrq zyxwvu zyxw zyxw zy Tab. 2. Significantly (P < 0.05) different means between species numbers in Afrotrilepis mats on the 11 examined inselbergs (LSD test). The main type of vegetation surrounding the inselbergs is indicated (RF = rainforest, SAV = savanna, INT = intermediary type rainforest-savanna,IB = inselberg) zyxwvutsr Mean 2.40 2.70 2.80 2.90 3.10 3.10 3.30 3.90 3.90 4.30 4.50 Zone SAV INT RF RF SAV INT RF SAV INT SAV SAV IBI IBl IB2 IB3 IB4 IB5 IB6 IB7 IB 8 IB9 IBlO IBll * IB2 * * * * * * * * IB3 IB4 * * * * * * * mains for less competitive species to become established and thrive in the deep shade. Afrotrilepis pilosa is obviously a good colonist and shows typical signs of a K-strategist, i.e. slow growth and achieving considerable age. According to Bonardi (1966), stems lm high are about 200 years old. Phenomena that cause mortality in Afrotrilepis and are associated with disturbance (e.g. fire) result in higher species numbers of the mats (own unpubl. data). This is a direct consequence of the removal of the dominant species, allowing certain faster growing taxa to thrive temporarily. However, with the re-establishment of Afrotrilepis pilosa, growth of the other plants is suppressed and the number of species begins to decrease. On the basis of extensive field observations on inselbergs, it can be assumed that natural disturbance in Afrotrilepis mats caused by factors such as lightning is a relatively rare event. Most of the mats studied to date have certainly persisted without having been damaged for the last decades. Mats of Afrotrilepis pilosa have therefore to be considered as an example of a highly stable tropical community in which competitive displacement prevents higher species diversity (see Crawley 1986). The importance of disturbance in maintaining high species diversity has been emphasized by many authors (e.g. Connell 1978; Huston 1979; Sousa 1985). It also influences the floristic composition and diversity of other inselberg habitats. Ephemeral flush communities flourish best (i.e. are most diverse) under recurrent climatic oscillations between flooding during the rainy season and drought in the dry period. If flooding becomes less prevalent, our observations indicate that Afrotrilepis pilosa will invade these communities and species richness will approach values characteristic of Afrotrilepis mats. Studies in South East Africa have shown that physiNurd. J. But. 16(3) 1996 IB5 IB6 IB7 * * * * IB8 IB9 IBlO I B l l zy ognomically very similar mats formed by the likewise poikilohydric Cyperaceae Coleochloa setifera are also relatively poor in species (Porembski 1996). Monocotyledonous mats on South American inselbergs are more species-rich than their West African counterparts (own unpubl. data). In contrast to the monopolistic position of Afrotrilepis pilosa on West African inselbergs, in the Neotropics a variety of species belonging to different life forms (succulents, poikilohydrics) are competitive under the harsh growth conditions. For example, various members of xerophytic Bromeliaceae and poikilohydric Velloziaceae are particularly well-adapted and can be frequently found coexisting in a single mat. Consequently, it is much more difficult to make predictions about species composition of monocotyledonous mats in the Neotropics. Although some differences were detected between the different inselbergs, no clear pattern of species rich- 7 5 . . I . .I I. 1 0 350 I I. 1 400 450 500 550 600 latitude Fig. 5. Species number in Afrorrilepis mats along a latitudinal gradient (in minutes). Due to the considerable overlap of values, not all points are visible. 243 zyxw zyxwvu zy zyxwvutsr ness or diversity emerges along a latitudinal gradient. This is in contrast with the results of Porembski et al. ( 1995). Here, species diversity of shallow depressions on inselbergs in the Ivory Coast decreased from north to south, and was negatively correlated with annual precipitation. Furthermore, the degree of isolation of the inselbergs does not appear to have any influence on the number of species found in the Afrotrilepis mats. Isolation is much more pronounced in the rainforest zone as there are fewer, more scattered inselbergs and other comparable habitats than in the north of the country. In addition, large tracts of dense forest rather open savanna separate the inselbergs in the south. Among the most frequently recorded accompanying species in Afrotrilepis mats, “inselberg-specialists’’ (e.g. Cyanotis lanata, Lindernia exilis) clearly dominate. However, within this group only Polystachya microbambusa is strictly confined to the mats. Other species such as Cyanotis lanata and Sporobolus festivus are also encountered in other habitats (rock pools, shallow depressions, crevices). The presence of these species could possibly be attributed to the so-called “mass effect”, i.e. the flow of diaspores from core habitats in the vicinity (Shmida & Wilson 1985). Vagrants generally play a minor role in Afrotrilepis mats although one or two species like Fimbristylis dichotoma do regularly occur. Phytogeographically Afrotrilepis mats contain fairly widespread elements with most species having been recorded from inselbergs all over tropical Africa. Endemics which have a particularly limited distribution and are confined to very few inselbergs are completely lacking. This is in marked contrast with the situation on inselbergs in other parts of the world, for example South America (Alves & Kolbeck 1994). A species deserving particular mention for a number of reasons is the epiphytic orchid Polystachya microbambusa, whose distributional range is at least restricted to Upper Guinea. For suitable growth conditions, it is dependent on Afrotrilepis pilosa growing in dense mats. Why it has not extended its range to coincide with that of its specific “phorophyte” remains unclear. In other parts of West Africa, several other species of Polystachya ( P . dolichophylla, P. odorata var. trilepidis, P. pseudo-disa) are also found in Afrotrilepis mats. These mats on inselbergs appear to present a very specialized habitat which have enabled speciation processes within the genus Polystachya to take place. Other vascular epiphytes are not known in mats from the study area. However, similar relationships do exist between the major constituents of monocotyledonous mats and epiphytic orchids on inselbergs in other tropical regions (e.g. Polystachya johnsronii restricted to stems of the Velloziaceae Xerophyta splendens in Malawi). 244 Acknowledgements - Financial support by the Deutsche Forschungsgemeinschaft (Ba 605/4-2) is gratefully acknowledged. The authorities of the CBte d’Ivoire are thanked for the permission to conduct research. Furthermore we would like to express our thanks to L. Ake Assi (Abidjan), N. Biedinger (Bonn), S. Dorrstock (Bonn), K. E. Linsenmair (Wiirzburg), H. Sander (Cologne), R. Seine (Bonn) and J. Szarzynski (Bonn) for support during fieldwork and valuable comments on the manuscript. References zyxw Adjanohoun, E. 1964. Vegetation des savanes et des rochers dtcouverts en CBte d’lvoire centrale. - Mtmoires ORSTOM 7. Alves, R. J. V. & Kolheck, J. 1994. 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