Occurrence of distinctive plant communities, rich fens, in specific hydrogeologic settings with high fluxes of calcium-rich ground water has been observed but not fully explained. In fens, ground-water discharge patterns induce spatial gradients in water chemistry that may determine vegetation patterns through effects on nutrient availability. However, linkages among these components are poorly characterized. I hypothesized that transformations in carbonate (CO32?) chemistry along ground-water flowpaths (GWFs) would enhance carbonate precipitation, primarily due to redox reactions and carbon dioxide degassing. I also expected phosphorus (P) co-precipitation would lead to P-limitation of plant growth and differences in species composition along the GWF. I compared vegetation and hydrochemical gradients among four fens during 2002 with nested piezometers set parallel to GWFs. I found that local ground water influenced soil and water chemistry of the wetland edge, whereas GWF from larger-scaled systems influenced interior areas. Topography of the underlying mineral substrate and hydraulic conductivity of the peat controlled spatial distribution of ground-water effects on chemistry of the plant rooting zone. Spatial distribution of redox-sensitive ions (e.g., nitrate, iron, sulfate) and alkalinity conformed to GWFs. Equilibrium conditions prevailed with respect to calcium minerals except where ground-water inputs of sulfate induced SO42- -reduction and net dissolution predominated. Iron minerals, including siderite and iron-sulfides, also strongly influenced pore-water chemistry. Sulfur content and bicarbonate-dithionite extractable P increased in areas with elevated alkalinity and evidence of SO42- reduction, suggesting that iron-sulfur reactions rather than CO32? chemistry regulate P dynamics. HCl-extractable P decreased in these areas, showing that P-co-precipitation with CO32?-minerals does not substantially affect P availability. Further, CO32?- minerals comprised less than 2% of the soil, except in marl fen (25%).
Non-parametric analyses of environmental and species data showed that position along GWFs significantly affected soil characteristics and plant communities. Spearman?s rank correlations revealed that multiple environmental variables, all associated with changes in pore water chemistry along the GWF, were highly correlated with plant species composition, indicating that GWF strongly influences plant species distribution. Autocorrelation among the predictor variables suggested that GWFs control distribution of plant communities through short-term effects on pore-water chemistry and long-term effects on soil chemistry.