Phytosociological data give biased estimates of species richness

Large phytosociological data sets of three types of grassland and three types of forest vegetation from the Czech Republic were analysed with a focus on plot size used in phytosociological sampling and on the species-area relationship. The data sets included 12 975 releves, sampled by different authors in different parts of the country between 1922 and 1999. It was shown that in the grassland data sets, the releves sampled before the 1960s tended to have a larger plot size than the releves made later on. No temporal variation in plot sizes used was detected in forest releves. Species-area curves fitted to the data showed unnatural shapes, with levelling-off or even decrease in plot sizes higher than average. This distortion is explained by the subjective, preferential method of field sampling used in phytosociology. When making releves in species-poor vegetation, researchers probably tend to use larger plots in order to include more species. The reason for this may be that a higher number of species gives a higher probability of including presumed diagnostic species, so that the releve can be more easily classified in the Braun-Blanquet classification system. This attitude of phytosociologists has at least two consequences: (1) in phytosociological data bases species-poor vegetation types are underrepresented or releves are artificially biased towards higher species richness; (2) the suitability of phytosociological data for species richness estimation is severely limited.