Vegetation Ecology

Studies on the Flora and Vegetation

Apart from a few local studies, little was known up until recently about the vegetation of inselbergs. Our own research has gone a long way to rectifying this situation. On the basis of geological, geographical, microclimatic, floristic (cryptogams and vascular plants) and vegetation ecological studies, we have carried out a detailed analysis of the inselberg vegetation and the underlying abiotic features over a wide geographical area. The vegetation of inselbergs consists of a number of different fragmented habitats (e.g. monocotyledonous mats, rock crevices, seasonal rock pools, ephemeral flushes) that are physiognomically similar world-wide. Because of this, it has been possible to develop a uniform terminology to describe them. Floristically, there are striking differences between the tropical macroregions, particularly in respect of numbers of species and endemics. New species have been described or reported for the first time from various inselberg study areas.

Plant Adaptive Strategies

Concerning the adaptive strategies of plants on inselbergs, attention has focused on monocotyledonous caulescent rosette trees which, as mat-forming plants, are capable of forming extensive monospecific stands. In this context, two families play an important role, the Cyperaceae (especially Afrotrilepis, Microdracoides) and the Velloziaceae (Vellozia, Xerophyta). The "stems" are crowned by an apical tuft of leaves, and are in reality pseudostems. The actual stem is only a few millimetres in diameter and is enclosed in a massive sheath of adventitious roots and leaves. What is quite striking is the fact that most of the monocotyledonous caulescent rosette trees from inselbergs are poikilohydric plants ("resurrection plants") that are highly adapted to the rapid alternation of abundant water availability and drought which is a characteristic feature of certain habitats. Furthermore, we were able to show for the first time the existence of a velamen radicum within the Bromeliiflorae (Velloziaceae) and Commeliniflorae (Cyperaceae). As is the case with epiphytic orchids, the principal role of the velamen of the Cyperaceae and Velloziaceae found on inselbergs is probably to facilitate the rapid uptake of rainwater which is only available for brief periods. Experimental studies carried out on the carnivorous genus Genlisea (Lentibulariaceae) which occurs on both palaeotropic and neotropic inselbergs showed that plants had a hitherto unknown prey, namely protozoa. The plants possess subterranean traps, and our evidence suggests that the prey is attracted chemotactically. A detailed survey of the ciliate fauna of inselbergs in Brazil and the Ivory Coast is currently underway.

Biodiversity

In respect of the number of species, considerable differences exist between the various inselberg habitats. Habitats such as monocotyledonous mats are poor in species and display little fluctuation of species. They have low alpha and beta diversity and appear to be governed by deterministic processes. The opposite is the case with the extremely species-rich seasonal habitats such as flushes, where stochastic influences are an important factor. In this context, climatic variables, particularly the extremely variable spatial and temporal pattern of rainfall, play a major role in maintaining species-rich communities. Detailed studies of this aspect have been carried out in the Ivory Coast. A comparison of over 100 inselbergs revealed that the diversity of inselberg vegetation decreases along a latitudinal and climatic gradient from the savanna region of the north to the forested areas of the south. Interestingly, this decrease in diversity on inselbergs is in marked contrast to the vegetation of the surrounding areas, which increases along the same gradient. This inverse gradient of diversity characteristic of the inselberg flora is probably the result of pronounced metapopulation dynamics involving both local extinctions and recolonisation. Studies examining species turnover in fragmented habitats were carried out in permanent plots located in different habitat types on inselbergs. Frequent local extinction and recolonisation processes lead to highly dynamic community types. In particular, species-rich communities on inselbergs are subject to high rates of local extinction brought about by disturbance. The degree of disturbance is sufficient to prevent a climax stage being reached and thus contributes to maintaining more species-rich communities.