Calcium intolerance of fen mosses: Physiological evidence, effects of nutrient availability and successional drivers

Species composition of peatlands is determined by the dominance of either Sphagnum or non-sphagnaceous (brown) mosses. Sphagnum species are more or less intolerant to alkaline waters rich in calcium bicarbonate, but the physiological background of this intolerance is poorly understood. Recently, sphagna have been widening their realized niches, expanding to alkaline brown-moss fens and altering their functioning. One possible reason is increased nutrient availability, but existing evidence is equivocal. We approached this problem by a series of laboratory experiments with 15 fen moss species cultivated submerged in solutions corresponding to natural poor- to rich-fen waters. We tested basic ecophysiological mechanisms of calcium tolerance (ion compartmentalization, cell-wall cation-binding properties, phosphorus and iron uptake), the breadth of fundamental pH/calcium niches for protonemata and adult plants, interspecific competition, and relationships between nutrient availability and pH/calcium tolerance. Our results suggest that calcium toxicity in calcifugous bryophytes is caused by insufficient control over the balance of intracellular Ca2+ uptake/efflux. Cell-wall cation-exchange sites of living mosses remain unsaturated with Ca2+ even in calcareous solutions, contradicting the proposed inhibitory effect of Ca2+-oversaturation on cell-wall expansion and monovalent cation uptake. Growth and biomass accumulation of brown mosses was highest in alkaline fen waters, but they could also survive and germinate in poor-fen waters. Calcium-tolerant sphagna survived along the entire poor–rich gradient, but their growth was inhibited by calcium bicarbonate. The three most obviously expanding sphagna produced protonemata even under calcareous conditions. Flowing but not stagnant alkaline fen waters were toxic for calcifugous sphagna, the strongest competitors in poor-fen waters. Increased potassium availability facilitated the survival of calcifugous sphagna in alkaline fens, corroborating field observations that potassium facilitates sphagnum expansion. Surprisingly, the rare and declining moss Hamatocaulis vernicosus was supported by nitrogen and phosphorus more than its competitors. Our comparison of fundamental and realized niches suggests that the dominance of particular moss functional groups in fens is governed by a competitive hierarchy altered by different calcium levels. The expansion of calcium-tolerant sphagna into brown-moss fens therefore requires perturbation that weakens competition. Additionally, expansion of calcifugous sphagna to alkaline environments may be stimulated by potassium availability.