|Dengler, J: Zwischen Estland und Portugal - Gemeinsamkeiten und Unterschiede der Phytodiversitätsmuster europäischer Trockenrasen., Tuexenia, 25, 387-405 (2005)|
|Key words: Altitude, bryophytes, exponential function, Festuco-Brometea, Koelerio-Corynephoretea, latitude, lichens, relevé size, soil pH, species-area curve, species richness, vascular plants|
Dry grasslands (especially of the phytosociological classes Koelerio-Corynephoretea and Festuco- Brometea) are among the species-richest of plant communities. At the same time they are ideal model systems for the analysis of phytodiversity patterns and their causes. Here I give an overview of the spe- cies densities (means and maxima) of European dry grassland communities, considering relevé sizes bet- ween 1 mm2 and 100 m2. Generally speaking, Festuco-Brometea communities exhibit higher phytodiver- sity than Koelerio-Corynephoretea communities. However, the highest species densities are known from the Gypsophilo fastigiatae-Globularietum vulgaris (order Alysso alyssoidis-Sedetalia, Koelerio-Coryn- ephoretea), a basiphilous rock outcrop community of the Swedish island of Öland. Species densities in this community can be as high as 80 species per 4 m2, with a mean of 53.6. The lowest species densities within dry grasslands can be found in the Caricetum arenariae and the Corniculario-Corynephoretum (both belonging to the order Corynephoretalia canescentis, Koelerio-Corynephoretea), with less than 10 species on 4 m2. The species-area relations of dry grasslands can be conveniently described by an expo- nential function (S = c · A z) over all assessed relevé sizes. The z-values of different dry grassland types vary only marginally and show a mean of 0.21. On the one hand, this leads to the fact that the ranking of dry grassland communities according to their species richness remains more or less unaltered for very different relevé sizes. On the other hand it enables the extrapolation of species numbers for different relevé sizes. Subsequently, I discuss some major factors influencing the distribution of phytodiversity within dry grasslands. Of these, soil reaction seems to be most important. Many studies have shown a great increa- se of species densities with higher pH values, sometimes accompanied by a slight decrease above the neutral point. In at least two cases, the decrease of species densities per standard area with increasing altitude could be demonstrated. Finally, I present an explanation for the extraordinarily high species densities of dry grasslands. I point out research gaps and provide recommendations for future biodiversity studies as well as for phytosociological research in general. In my opinion, both the use of standardised relevé areas and the careful consideration of bryophytes and lichens in such studies are crucial.