Relationships between environmental factors and vegetation in nutrient-enriched fens at fishpond margins

Abstract

Vegetation-environment relationships were investigated in fens of the Třeboň basin (Czech Republic), which are enriched by nutrients and calcium from intensively managed and limed fishponds to test the hypothesis of altered gradient structure after long-term nutrient enrichment in fens. Water-table depth, pH, conductivity, N-NH 4 + , N-NO 3 − , PO 4 3− , total P, SO 4 2− , K+, Ca2+, Mg2+ and Fe were measured four times in 30 vegetation plots of 16 m2 during the 2004 vegetation season. Both constrained and unconstrained ordination (DCA, CCA) were used to relate environmental factors to the species composition of the vegetation. The relationships among particular factors were revealed using PCA. Four fen vegetation types obtained by TWINSPAN classification were compared with measured factors using repeated measures ANOVA. Vegetation types differed significantly in water-table depth, water pH and Ca2+, Mg2+, K+, and N-NO 3 − content. The concentration of major nutrients fluctuated noticeably during vegetation season and displayed large variation within vegetation types. Temporarily the concentration of different nutrients reached extremely high values. However, high nutrient supply has not altered the gradient structure of the vegetation. Water pH and water-table depths were found to be two major determinants of species variation in fishpond-margin fens, as in the majority of other environments studied throughout the Northern Hemisphere. Species richness of both vascular plants and bryophytes was partly explained by pH. However, the extent of variation in total mineral richness and potassium concentration were the next two most important variables determining bryophyte species richness. Water in flooded poor-fen vegetation, directly affected by water from limed fishponds, had calcium and magnesium concentration similar to fishpond water. The calcium concentrations of about 20 mg l−1 in typical poor-fen vegetation have no analogy in the ecological literature. High phosphorus level presumably buffers the effect of calcium by enhancing bryophyte biomass depositing superfluous calcium. In conclusion, extremely high long-term nutrient supply to fishpond-margin fens have not altered gradient structure, but shifted chemical limits of plant communities.