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.

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Summary1. Theory suggests that there should be a trade-off between nitrogen productivity ( A ) and mean residence time of nitrogen in plant tissues (MRT), and that this trade-off depends on the N availability of the habitats. To test this, we studied A , MRT and nitrogen-use efficiency (NUE, reciprocal of N concentration in the dead shoot) in wetland Carex species sampled from seven lowland mires in Japan. 2. The NUE and MRT of unproductive species that usually grow in acidic and N-poor habitats exceeded those of productive species that are abundant in weakly acidic and N-rich habitats. Except for the most productive species, C. lyngbyei , interspecific differences in A were small, irrespective of productivity. 3. Intraspecific variation in A was relatively large in contrast to those in NUE and MRT. Within each species, no parameter correlated with the concentration of total dissolved nitrogen (TDN) in the soil water. 4. Although NUE and MRT of the plants growing in the weakly acidic habitats correlated negatively with TDN, they were constantly high in the acidic habitats regardless of the TDN concentration. Responses of A did not correlate with TDN concentration of soil water in either acidic or weakly acidic habitats. 5. A trade-off between A and MRT in the five Carex species was not detected in relation to habitat N richness. We suggest that soil water acidity dominates the ecological performance of the Carex species, and that the trade-off between A and MRT does not necessarily apply to wetland vegetation.

Regional variation in size and shape of Sphagnum hummocks of eight lowland mires throughout Hokkaido Island, northern Japan, was examined in relation to climatic factors. The size and shape of hummocks significantly differed among the regions: low and flat hummocks occurred in the northern region and next to the Sea of Japan; moderately raised and conical hummocks occurred in the western part of the Pacific coast; and extremely raised and cylindrical hummocks occurred in the eastern region. Multiple regression analysis revealed that the mean and maximum heights and height/basal area of hummocks were negatively correlated with evapotranspiration rate (E0) in summer, duration of sunshine, and air temperature. Of these, evapotranspiration rate in summer positively correlated with radiation and thermal factors. Mean height of hummocks within a mire was increased with precipitation during the growing season. Because the regional difference of the hummock form was not related to the difference in nutrient conditions, the effects of evapotranspiration and precipitation prevailed over edaphic conditions. Higher E0 in summer was considered to cause water stress for hummock species in the northern region and next to the Sea of Japan, whereas heavy rainfall relieved it in the western region of the Pacific coast. Consequently, lower and flat hummocks were a result of the higher E0 in summer, moderately raised and conical hummocks of the medium E0 and high precipitation, and extremely raised and cylindrical hummocks of a lower E0 due to prolonged foggy days.Key words: evapotranspiration, duration of sunshine, minerotrophy, ombrotrophy, precipitation.

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.