Physiological responses of fen mosses along a nitrogen gradient point to competition restricting their fundamental niches

Nutrient availability determines vegetation patterns and ecological
functioning of intact groundwater-fed peatlands (fens). Bryophytes,
commonly referred to as ‘brown mosses’, dominate calcareous fens (rich
fens), are an integral part of their unique biodiversity and contribute
significantly to peat formation and carbon sequestration. Brown mosses are
replaced by vascular plants as nutrient availability increases. The
decline of brown mosses may either be due to their physiological
intolerance of high nutrient levels or to them being outcompeted by
vascular plants. We aimed to distinguish between these two hypotheses by
investigating whether the ecological optima reflect the physiological
optima of brown mosses. Eight brown moss species, common in calcareous
fens of the northern hemisphere, were grown under a gradient of nitrogen
and phosphorus availability. Biomass increment, chlorophyll content and
biomass nutrient concentration were measured. All brown moss species
studied achieved the highest growth rates and chlorophyll contents when
grown under conditions of nitrogen and phosphorus concentrations higher
than those where they occur naturally at the highest frequency. Two of the
species showed no growth saturation even at the highest levels of nutrient
availability, while the others appeared potassium-limited at the highest N
and P levels. Brown mosses dominate natural fens at the lower edge of
their physiological optimum in terms of nutrient availability, i.e., their
realized niche is much narrower than the fundamental one. Based on the
literature, we argue that it is the competition for light with vascular
plants which limits the occurrence of brown mosses in nutrient-rich
habitats and prevents them from occupying their entire fundamental niche.