Performance of four Dactylorhiza species over a complex trophic gradient

Spontananeous distribution and survival in experimental plots of four marsh orchids ( Dactylorhiza spp.) in a hay-meadow complex were related to mineral composition of groundwater, soil nutrient availability and species composition of the vegetation. Differences in Ca2+ contents of the groundwater pointed to the discharge of various groundwater flows in the meadow. Soil fertility and extent of N limitation, measured with the phytometer Cirsium palustre, varied with position on the gradient. Total N content of the soil and levels of exchangable K+ and NH4+ were correlated to organic matter content. Highest total P levels were found in areas with a pronounced discharge of mineral-rich groundwater. Concentration of available P was higher in wet than in drier sites. Natural abundance and survival of introduced Dactylorhiza maculata (L.) Soo plantlets was highest in Juncus acutiflorus-dominated transitions between meadows and Parvocaricetea marsh. These were characterized by the occurrence of a water type of intermediate mineral content and by low soil fertility, limited by N and P. Survival of D. praetermissa (Druce) Soo showed exactly the opposite pattern to that of D. maculata. It had its optimal spontaneous abundance in more fertile parts of the meadow influenced by discharge of groundwater with an intermediate mineral content. D. majalis (Rchb. f.) Hunt & Summerh. also occurred in the more fertile parts of the meadow, but showed an optimum in drier stands associated with discharge of highly calcareous groundwater. The phosphorus-pool of these soils was high, but the availability of phosphate was low. The soil fertility of D. majalis site was limited by N. D. incarnata (L.) Soo occurred mainly in transitions between basiphilous small sedge vegetation and productive hay meadow stands. The species occurred at fertile but waterlogged soils with high concentrations of NH4+. The presence of base-rich groundwater with a high pH apparently prevents the species from ammonium toxicity. None of the species occurred in degenerated (drained) and acidified meadow sites.