Diversity and vegetation structure of tropical inselbergs

 

Inselbergs are monolithic rock outcrops or mountains that consist of Precambrian granites or gneisses. Organic substrates are largely absent. In terms of their geomorphology, inselbergs are very old structures, often millions of years old, and are found on all continents, especially in tropical regions. Up until recently, biotic characteristics of inselbergs had rarely been studied. As microclimatic and edaphic island-like habitats, they support a very different type of vegetation compared to the surrounding areas. Being ancient fragmented habitats, inselbergs offer a unique opportunity to study the role of deterministic and stochastic processes influencing species diversity in tropical plant communities.

Building on the results of preliminary studies (Barthlott, Bonn University), inselbergs have been the focus of a comprehensive study funded by the Deutsche Forschungsgemeinschaft since 1990. Fieldwork has concentrated on inselbergs in Africa (especially Benin, Ivory Coast, Guinea), and South America (e.g. Brazil, French Guiana).
	The island-like monolithic rocks contain a number of different habitats, each with its own characteristic type of vegetation, including monocotyledonous mats, flushes and rockpools.
Inselbergs are physiognomically identical throughout the world, but differ significantly in respect of their flora, alpha and beta diversity as well as degree of endemism. Large differences of species richness also occur among the different habitat types on individual inselbergs. Monocotyledonous mats are generally species-poor and are not subject to large fluctuations. They have low alpha and beta diversity and species composition is largely governed by deterministic processes.
	The opposite situation applies to highly seasonal habitats (e.g. ephemeral flushes) which are dominated by annuals. Here, very species-rich communities can be found, and stochastic processes are of primary importance in maintaining species diversity.

A particularly important factor promoting the formation of species-rich communities is rainfall which is subject to large fluctuations, both spatially and temporally. A detailed investigation of the vegetation of over 100 inselbergs in he Ivory Coast revealed that species diversity declined along a savanna-forest gradient, in marked contrast to the vegetation of the surrounding areas. This inverse gradient of diversity can probably be attributed to a high level of metapopulation dynamics, in which local extinctions and recolonisation are principal features.

Species turnover has been examined in permanent plots located on inselbergs. Rapid local extinction and recolonisation processes mean that the vegetation is subject to large fluctuations from one year to another. This in turn prevents a climax stage being reached, thus contributing to maintaining species-rich communities.

 

In addition, we have been able to shed light on some rather unusual adaptive strategies of inselberg plants. These include monocotyledonous "dwarf trees" which belong to the so-called "caulescent rosette trees" (especially Cyperaceae and Velloziaceae). Apart from pseudostems, they possess roots with a velamen radicum.

 

Most of the monocotyledonous "dwarf trees" are poikilohydric, and it can be generally stated that inselbergs are global centres of poikilohydric vascular plant diversity.

 

Genlisea is a little-known genus belonging to the Lentibulariaceae, and for the first time we were able to demonstrate carnivory in this genus. What is even more remarkable is the fact that the plants' prey are protozoa which are attracted by chemotaxis, the only known example of this phenomenon in the plant kingdom.