Species constancy depends on plot size - a problem for vegetation classification and how it can be solved.

Question: While it is well known that species richness depends on plot size, it is not generally recognised that the same must be true for constancy. Accordingly, many authors use varying plot sizes when classifying vegetation based on the comparison of constancies between groups of plots. We ask whether the constancy-area relationship follows a general rule, how strong the effect of plot sizes is on constancies, and if it is possible to correct constancies for area. Location: For empirical evaluation, we use data from plant communities in the Czech Republic, Sweden and Russia. Methods: To assess the potential influence of differences in plot size on constancies, we develop a mathematical model. Then, we use series of nested plot species richness data from a wide range of community types (herbaceous and forest) to determine the parameters of the derived function and to test how much the shape of the constancy-area relationship depends on taxa or vegetation types. Results: Generally, the constancy-area relationship can be described by C ( A)=1−(1− C₀)^{( A/ A0)^ d}, with C being constancy, A area, C₀ known constancy on a specific area A₀, and d a damping parameter accounting for spatial autocorrelation. As predicted by this function, constancies in plant communities always varied from values near 0% to near 100% if plot sizes were changed sufficiently. For the studied vegetation types, a two- to fourfold increase in plot size resulted in a change of conventional constancy classes, i.e. an increase of constancy by 20% or more. Conclusions: Vegetation classification, which largely relies on constancy values, irrespective of whether traditional or modern fidelity definitions are used, is strongly prone to distorting scale effects when relevés of different plot sizes are combined in studies. The constancy-area functions presented allow an approximate transformation of constancies to other plot sizes but are flawed by idiosyncrasies in taxa and vegetation types. Thus, we conclude that the best solution for future surveys is to apply uniform plot sizes within a few a priori delimited formations and to determine diagnostic species only within these formations. Finally, we suggest that more detailed analyses of constancy-area relationships can contribute to a better understanding of species-area relationships because the latter are the summation of the first for all species. [ABSTRACT FROM AUTHOR]