Plant species as predictors of soil pH: Replacing expert judgement with measurements

Question: The use of expert-based indicator values to estimate abiotic conditions from vegetation is widespread. However, recent research has shown that expert judgement may contain considerable bias and thereby introduces a large amount of uncertainty. Could expert based indicator values be replaced by indicator values based on field measurements? Location: Europe. Methods: We developed a method to estimate species response based on measured physical data, and a method to predict abiotic conditions from the vegetation composition using these responses. This method was tested for soil pH. Results: We were able to estimate the pH response of 556 species of the Dutch flora. Ca. 20% of the responses were, at least, bimodal and many responses had a very wide range. The simplest method ('raw mean') yielded the best prediction of pH; the indicator value of a species is the mean of the soil pH values of the sites where it was observed. A list of all raw-mean estimates per species is given. The predicted pH of a new site is the mean of the indicator values of the present species. The estimated species responses were validated on independent Dutch and European data sets. Older successional stages seem to be predicted better than younger stages. Conclusions: Our method performed better than the popular Ellenberg indicator system for the Dutch data set, while being just as easy to use, because it only needs a single value per species. We foresee that, when more data become available, our method has the potential to replace the Ellenberg system.