Publication details

Phytosociological data give biased estimates of species richness

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Year of publication 2001
Type Article in Periodical
Magazine / Source Journal of Vegetation Science
MU Faculty or unit

Faculty of Science

Field Ecology
Description Large phytosociological datasets 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 datasets included altogether 12,975 relevés, sampled by different authors in different parts of the country between 1922-1999. It was shown that in the grassland datasets, the relevés sampled before the 1960s tended to have a larger plot size than the relevés made later. No temporal variation in plot sizes used was detected in forest relevés. 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 relevés in species-poor vegetation, many researchers probably tend to use larger plots in order to obtain species-richer relevés. They seem to do that because a higher number of species gives a higher probability of more diagnostic species being found in the relevé; in the Braun-Blanquet classification system such a relevé can be more easily classified. This behaviour of phytosociologists has at least two consequences: (1) phytosociological databases under-represent species-poor vegetation types or are artificially biased towards higher species richness; (2) the suitability of phytosociological data for species richness estimation is severely limited.
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