Nitrogen:phosphorus ratio as the main ecological explanation of the differences in species composition in brown-moss rich fens of north-eastern Poland.

The species composition of 22 mires (brown-moss rich fens) was surveyed in north-eastern Poland, located in continental boreo-nemoral Europe. Detailed analyses of the water chemistry, biomass N, P, and K content, productivity and water level were made. Two floristically different types of richfen vegetation occur in this area, one with numerous Caricetalia davallianae (calcicole) species such as Scorpidium cossonii, Campylium stellatum, Carex lepidocarpa and Eriophorum latifolium (Cd fens) and the other with a few Caricetalia davallianae species and mainly Hamatocaulis vernicosus, Marchantia polymorpha, Plagiomnium ellipticum, Carex diandra and C. rostrata (non-Cd fens). In these two rich-fen types surface water chemistry and water levels were similar, but the Cd fens characteristically had higher pHs and lower PO43– concentrations. N and P availability revealed by the N:P ratio is the ecological factor that best accounts for the differences in species composition of the two rich-fen types: the non-Cd fens are N-limited while the Cd-fens are usually P-limited. Moreover, the Cd fens differ from the non-Cd fens in a higher productivity of the bryophyte layer. In the case of P and K concentrations, there is a correlation between that in vascular plants and bryophytes, while N concentrations are not correlated. We believe that in the case of low productive ecosystems the use of the broad “nutrient availability gradient” should be replaced by a (from) N-(to) P-limitation gradient with N:P ratio as a useful measure. Natural N-limited, low-productive rich fens and their ecological conditions can be a phenomenon typical of continental-eastern temperate European areas, which have been poorly surveyed and need further research.