Habitat diversity along a hydrological gradient in a complex wetland results in high plant species diversity

Plant communities and selected environmental parameters were studied at the intermittent Lake Cerknica. Seventeen plots (16 m2) were defined down a 600-m transect on a gentle sloping shore with an elevation decrease of 2.7 m, to investigate the different types of wetland vegetation characteristic of the different habitats. Vegetation was investigated using the standard Central European method. Soil samples were measured for pH, electrical conductivity, organic matter, total nitrogen, available phosphorous and potassium, cation exchange capacity, and texture. Large differences in floristic composition of the plant communities were found. Vegetation of mesic meadows was seen for the rarely flooded most elevated plots, with transition downwards to wet grassland communities from the alliance Molinion and from the alliance Deschampsion. The lower habitats with soils waterlogged until July were colonized by marsh communities, while community of emergent hydrophytes was at the lowest end of transect. Over 100 vascular plant species were recorded in the plots. Species richness increased with elevation and decreased with moisture, with >50 taxa in three most elevated plots, and 9-15 taxa per plot for the nine most frequently flooded plots. The hydrological gradient corresponded to the elevation gradient, and together with duration of flooding had a crucial role in shaping plant community composition, distribution and diversity. The flooding arose from Ca-rich lake water, while elevated habitats were rainwater fed, which the consequent enhanced nutrient leaching and lowered pH and electrical conductivity. This produced negative correlations between plant community diversity and species richness versus pH, Ca2+ and electrical conductivity, since these influences were overridden by hydrological gradient.