Understanding the realised niche of an amphibious softwater plant, Eleocharis multicaulis

Abstract

The West European amphibious plant Eleocharis multicaulis, characteristic of softwater lakes, is rare and endangered in many regions. The present study aimed to evaluate several presumed niche features suggested in syntaxonomic studies, to reveal other important niche variables affecting the cover of this species and to quantify these responses. A dataset of 724 environmental and vegetation variables was built from a survey of 59 plots in The Netherlands and Belgium. Discriminant analysis, Pearson goodness-of-fit calculations, weighted averages and presence profiles were used. We found that Eleocharis multicaulis performs best in base-poor environments with an or- ganic top layer and an established vegetation. The species is apparently N limited and profits from N input as long as NO3 - is dominant over NH4 + . A high atmospheric N deposition is detrimental, because the resulting strong acidification leads to dominance of NH4 + over NO3 - . Intense acidification in agricultural areas has probably been an important extinction factor in the recent past. We hypothesise that current variability of soil Si, a very good predictor for the species' performance, might reflect these acidifi- cation processes. This hypothesis is supported by the fact that Si is correlated with trophic status, which is likely higher in agricultural regions. Nowadays, Eleocharis multicaulis suffers from agricultural P, K and alkalinity inputs as well as reduced oxy- gen supply (e. g. by reduced water-level dynamics), all leading to a lower redox state. These factors could bring about competitive suppression by other species, a high NH4 + /NO3 - ratio and P mobilisation.