Changes in species richness and species composition of vascular plants and bryophytes along a moisture gradient.

We focused on the gradient in moisture along transects of from 8 to 30min length from spring fen to semi-dry grassland vegetation. We selected an area in the calcium-rich part of the Western Carpathian flysch zone (Czech and Slovak Republics) where small spring-fed fens occur in close contact with semi-dry grasslands. Altogether 126 vegetation plots of 75 × 75 cm were sampled along 15 transects (one per locality) that each extended from the wettest part of a spring-fed fen into the surrounding semi-dry grassland. In addition, samples of standing plant biomass and soils were analysed for nutrients (N, P, K, C, Ca) and the upper-soil moisture measured. Using this study system and these sampling methods, we can test directly the effects of moisture and so avoid the confounding effects of different background environmental conditions that occur when data from many distinct sites is used. Data were processed using linear mixed-effect models and other statistical techniques. The trend in the number of species of vascular plants was unimodal with the optimum skewed towards lower moisture values. This response was not caused by an edge effect alone as replacing the moisture gradient with the positions of plots on transects resulted in a much weaker unimodal relationship and there was a group of species that occurred mainly in the species-richest moderately moist plots but did not occur in fens or the driest grasslands. The correlations between species richness and productivity (positive) and soil calcium (negative) differed from most of those reported in the literature, which suggests that the observed relationship between species richness and moisture was probably not greatly confounded by these factors. Species richness correlated negatively and the percentage of endangered species positively with the N:P biomass ratio, which is in accordance with other published results, but their correlations with moisture were stronger. For bryophytes, species richness linearly decreased towards the dry plots and did not correlate with any of the nutrients measured. Most of the species of vascular plants and bryophytes in the wettest patches were fen specialists, while more generalists made up the species richness in grasslands, including the species-richest patches. For bryophytes, the percentage of specialists was very high in fen plots. Although Mantel tests showed that bryophyte and vascular plant species turnovers were similar, we recorded substantial differences in their species richness patterns. Comparison with the results of a previous study on molluscs revealed a greater similarity between bryophytes and molluscs than between molluscs and vascular plants in terms of correlations between species richness and environmental variables. We argue that soil moisture should be taken into account when explaining current patterns in diversity in extremely rich temperate grasslands.