Sulfur And Wetland Plant Diversity: Calcareous Rich Fens As Model Systems

Abstract

Plant diversity in groundwater-fed wetlands is typically extraordinarily high, yet the biogeochemical controls of this diversity are still incompletely understood. I hypothesized that plant community composition could be related to a combination of fine-scale and broad-scale variation in sulfide via direct phytotoxicity and indirect mediation of phosphorus release from iron, coupled with gradients in other chemical constituents such as calcium. I measured porewater chemistry and associated plant species composition at nine groundwater-fed wetlands (rich fens), including one rich fen in which I intensively sampled 400 locations to capture finescale heterogeneity. Porewater sulfate and calcium concentrations were higher at the intensively sampled fen overlying gypsum geology than at other rich fens. Sulfide was highly variable within and across fens, ranging over two orders of magnitude in many fens. Inversely related concentrations of sulfide and ferrous iron in porewater were consistent with tight chemical coupling but were not readily traceable to phosphorus availability. Spatial patterns of sulfide and ferrous iron were conserved across seasons, with sulfide peaking with temperature in summer and ferrous iron peaking in fall at intermediate temperature. I used the corrected Akaike information criterion (AICc) to select among competing models of toxin, nutrient, and mixedchemistry influences on vegetation. In the intensively sampled fen, models with a negative sulfide parameter provided the best explanation of total plant cover, cover of the three most frequently occurring species, dicot species density, and plant height. Calcium and phosphorus combined with sulfide to explain some plant responses, but phosphorus alone did not explain any plant responses at the fine scale. Sulfide had a limited relationship with vegetation at the regional scale, only secondarily explaining total plant cover after first accounting for site-to-site variability. Gamma diversity values for individual sites were a negative power function of within-site sulfide variability values, with average alpha diversity for each site dominating. Overall, results from this work confirmed the relationship of rich fen vegetation to calcium and suggested that direct sulfide toxicity was a persistent but more moderate than expected stress to rich fen plants, while indirect sulfide mobilization of phosphorus was less important to plants than sulfide toxicity.