Variation in size and shape of Sphagnum hummocks in relation to climatic conditions in Hokkaido Island, northern Japan

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 an d next to the Sea of Japan; moderately raised and conical hummocks occurred in the western part of the Pacific coast; and extremely raised and cylindr ical hummocks occurred in the eastern region. Multiple regression analysis revealed that the mean and maximum heights and height/basal area of hummock s were negatively correlated with evapotranspiration rate (E-0) in summer, duration of sunshine, and air temperature. Of these, evapotranspiration rat e in summer positively correlated with radiation and thermal factors. Mean height of hummocks within a mire was increased with precipitation during th e growing season. Because the regional difference of the hummock form was n ot related to the difference in nutrient conditions, the effects of evapotr anspiration and precipitation prevailed over edaphic conditions. Higher E-0 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 reliev ed it in the western region of the Pacific coast. Consequently, lower and f lat hummocks were a result of the higher E-0 in summer, moderately raised a nd conical hummocks of the medium E-0 and high precipitation, and extremely raised and cylindrical hummocks of a lower E-0 due to prolonged foggy days .