Effects of climatic conditions on morphology, hydrochemical conditions, and plant species composition of Sphagnum hummocks

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In order to clarify the reasons of regional differences in shape and size of Sphagnum hummocks throughout Hokkaido, we examined the growth of 3 Sphagnum species growing on hummocks in 4 mires, with controlling the conditions of snow pressure and shading by other plants. Fuhrengawa has cylindrical and extremely raised type, Sarobetsu and Utasai have flat and low type. Utonai has conical and middle height type, The annual growth was larger in Sarobetu and Utasai. Remarkable suppression of the growth of S. papillosum at Sarobetu was observed in June, where hummock surface was desiccated owing to the highest evapotranspiration and a lack of rainwater. Removal of shading suppressed the growth of Sphagnum in the all mires, probably due to the enhanced evapotranspiration losses from hummock surface. Removal of snow pressure caused the decrease in the growth of next summer at Sarobetu and Utasai in deep snow region, because hummock surface were kept higher enough from underlying water table to … More suffer severe drought.We investigated the water balance of a raised hummock of Sphagnum fuscum in Fuhrengawa mire and consider how water moves interior hummock. In fine days, a surface layer kept a constant water content by re-supplying the same amount of water lost by evapotranspiration from a lower layer, but water content in the lower layer was decreasing because of an insufficient supply of water from deeper parts. When it was raining, rainwater supplied from the hummock surface percolated rapidly through the surface layer without storage, whereas some rainwater was stored into the deeper layer. Intact peat of surface layer has fine structure resulting high water retention and absorption capacity. Therefore, when some water was lost from the surface layer by evapotranspiration, water was able to re-supplied from the deeper layer. Contrary to this, it seemed that moderately decomposed peat with macrostructural collapse in deeper layer has relatively low hydraulic conductivity, resulting in decreasing water content during fine periods and storage of rainwater during rain periods in the deeper layer. The storage mechanism of rainwater would generate an ombrotrophic condition at inner hummocks standing on minerotrophic surface water. Furthermore, the humid climatic conditions throughout a growing season such as frequent rains with short intervals and large negative values of net water loss prevent from declining water content of the deeper layer and decreasing the level of underlying water surface. Less