The influence of taxonomic revisions on species distribution assessment: the case of three Asplenium species on Tuscan ultramafic soils

Herbarium data can be useful to deduce geographical information (e.g. distribution, rarity) about plant species. The rarefaction method can be used to compare the distribution of two or more species based on collections containing different numbers of specimens. Herbarium collections are however continually subject to taxonomic revisions, which cause variation in the number of specimens attributed to each species and, as a consequence, in knowledge of the species distribution. This paper investigated the effects of taxonomic revisions on species distribution, using the rarefaction technique. Herbarium data for three Asplenium species of ultramafic soils of Tuscany before and after recent taxonomic revision were analysed with the use of individual-based rarefaction curves. Rarefaction curves for the data set prepared before and after taxonomic revision appear different. The starting data set showed a great number of specimens determined as Asplenium cuneifolium. The rarefaction curves of each of the three taxa show non-saturation of locations. After taxonomic revision, Asplenium onopteris and Asplenium adiantum-nigrum exhibit a higher number of samples and locations than before revision but their curves do not reach the asymptote. Asplenium cuneifolium remains restricted to few locations but its almost horizontal curve suggests that very few new locations can be expected for this species. The study highlights the importance of taxonomic accuracy in the use of herbarium data especially, for making inferences about species abundance and distribution.