Abstract. In the cool‐temperate Bibi Mire, Hokkaido, Japan, valley fens and flood‐plain fens have quite different vegetation. The main variables controlling the vegetation were all hydrological: mean water level, water level fluctuation and surface water flow. Chemical factors such as electrical conductivity, dissolved oxygen and related peat decomposition were less important. The pH was about neutral and has little effect. The flood‐plain fen developed under fluctuating water table conditions. The dominant species are Calamagrostis langsdotffii and Carex pseudocuraica. When temporal inundation occurs in the rainy or typhoon seasons, the submergence stimulates bud germination of the stoloniferous C. pseudocuraica, which can rapidly elongate its stolons upward. Some large floating peat mats occurred in the flood‐plain fen zone. On these mats some Alnus japonica saplings establish and patches of alder forest can arise. Here the water level was higher than in the peripheral alder forest zone. The valley fen is dominated by Carex lasiocarpa var. occultans and/or C. limosa. It is formed under stable water table conditions in the inundated parts of the mire ‐where the non‐inundated wet areas are dominated by alder trees. In the area where the surface water is flowing, these two fen sedges grow in deeper water since the high oxygen content is considered to compensate the flooding stress.

The distribution of two sedge species was studied in two mires which differ in abiotic environments and in distribution ofPhragmites australis. Carex lasiocarpa var. occultans dominated in nutrient-poor valley mire, and Carex thunbergii var. appendiculata dominated in nutrient-rich flood plain subject to water fluctuations. Phragmites australis grew well in nutrient-rich conditions. The distribution of C. lasiocarpa showed a strong negative correlation with P. australis coverage, whereas C. thunbergii coverage was not affected by P. australis. The leaf area per dry leaf mass (specific leaf area: SLA) of C. thunbergii increased with shading by P. australis, but that of C. lasiocarpa was stable. The SLA flexibility of C. thunbergii to light interception might enable this species to invade P. australis patches in nutrient-rich environments. The residual nutrient ratio of nitrogen and phosphorus (the ratio of the residual nutrient content at the end of the growing season to peak nutrient content) in the vegetative ramet of C thunbergii was 1.7 times higher than that of C. lasiocarpa. This low residual ratio may indicate effective nutrient recovery to storage organs. The effective nutrient recovery in C. lasiocarpa might enable this species to grow even in nutrient-poor environments. However, it may be difficult for C. lasiocarpa to expand its habitat to nutrient-rich areas where P. australis dominates as it is not shade tolerant.