Vascular plant species richness and rarity across a minerotrophic gradient in wetlands of St. Lawrence County,New York,USA

Thirteen wetlands in St. Lawrence County, NY were sampled to examine the effect of a minerotrophic gradient on vascular plant species richness and rarity. Wetlands ranged from organic soil based poor fens (average conductivity 46.40 microsemens, average Ca 3.55 ppm) to mineral soil based rich fens (average conductivity 342.10 microsemens, Ca 23.00 ppm). Vascular plant species richness was sampled during 1990 in randomly located 1.0 m² quadrats. Specific conductivity, presence or absence of hummocks, and water depth predicted 62% of the variation in richness. Richness increased as conductivity increased until 413 microsemens at which a down trend became obvious. The negative curvilinear relation between conductivity and richness is in accordance with the hump-backed model of Grime but occurs at high rather than intermediate conductivity values. State-listed rare species were found in species-rich wetlands only and had a mean associated richness value of 14.50 species m^-2. This relationship should be taken into consideration when selecting wetlands for protection or managing wetlands for maximum plant diversity.