Ecological gradients of north Japanese mires on the basis of hydrochemical features and nitrogen use traits of Carex species

Abstract Habitat preferences and N use traits of six Carex species characterizing the floristic variation in mire ecosystems were examined in seven lowland mires throughout Hokkaido, northern Japan. Canonical Discriminant Analysis showed that soil water pH was the strongest discriminant between habitats of the Carex species, with total dissolved nitrogen (TDN) concentration also contributing, independently of pH. Two major patterns were apparent in the distribution of Carex species along pH and TDN concentration: pH determines a bimodal distribution into fens with pH > 5.0 and bogs with pH < 5.5, whilst TDN captures the variation in productivity within both fen and bog, but does not differ significantly between them. Although N use efficiency cNUE) and mean residence time of N in plant tissues (MRT) of the sedge plants growing in the weakly acidic habitats (fens) correlated negatively with TDN, they were constantly high in the acidic habitats (bogs) regardless of the TDN concentration. Responses of N productivity (A) did not correlate with TDN in either fens or bogs. Accordingly, the trade-off between A and MRT along habitat N richness theoretically predicted does not necessarily apply to wetland vegetation, because the ecological performances of the Carex species would be dominated by the soil water acidity rather than the N richness. The relationship between N use traits and habitat preferences along pH and N richness of the Carex species derived the ecological rationales of the vegetation pattern through the ecophysiological performance of individual species. These results would much contribute to essential knowledge and practical information for the conservation and restoration of north Japanese lowland mires facing to the serious crisis by direct and indirect impacts.