|研究テーマ||“Who killed the reed?Mechanisms of vegitaiton change at Kiritappu Mire,eastern Hokkaido”|
Who killed the reed? Mechanisms of vegetation change at Kiritappu Mire，eastern Hokkaido Stefan Hotes
During the last 50 years drastic vegetation changes have occurred along the middle reaches of Ichibangawa River in the southern part of Kiritappu Mire (eastern Hokkaido，Japan) (Hotes，in press). In an area of ca. 25 ha，reed-sedge vegetation has been replaced by bare peat surfaces，with Zostera japonica growing in shallow water. Patches of alder (Alnus japonica) trees have equally died. Saltmarsh species (e.g. Carex subspathacea，Potentilla egedei，Aster tripolium，Puccinellia kurilensis，Triglochin maritimum，Glaux maritima etc.) are growing around the dead tree stumps. This vegetation change was detected when series of aerial photographs of Kiritappu Mire taken between 1947 and 1990 were compared. Subequent investigations on the ground proved that the different colours seen in the photographs indeed were related to changes in plant cover，and that the mud flats are currently expanding upstream．The water that is supplied to the area through Ichibangawa River has a high electrical conductivity (EC) indicating that the ion concentrations are high (Haraguchi 1995，Saito 1997，Tani 1997). It is probable that an increased influx of sea water is related to the observed vegetation changes. However，reed (Phragmites australis) and sedges like Carex Iyngbyei are known to tolerate high salt contents as well as high water levels，and neither salt stress nor the water level rise alone can explain why most of the area is now bare without any living vegetation．We suggest that the high sulphate concentrations in the floodwater in combination with the wetter conditions are the cause of the reed die-back. Sea water is rich in sulphate，and when this water infiltrates in the soil it becomes anaerobic. Under anaerobic conditions and in the presence of organic material sulphate is reduced to sulphide (S2-) by micro-organisms. Free sulphide is very toxic for higher plant species. In most fens，large quantities of iron (Fe) are present that can bind sulphide in the form of FeS which is harmless to plants. If the iron supply is insufficient，this protective mechanism does not function，and sulphide can accumulate to toxic levels (Smolders and Roelofs 1995). Phragmites can protect itself to a certain extent by leaking oxygen from its roots，which oxidises sulphide in the immediate surroundings of the roots. Algal mats on the ground surface can produce oxygen in the toplayer and thus mitigate the effects of sulphide，but only during day time. We put forward the hypothesis that the deep rooting plant species，such as Phragmites and Carex species suffer from sulphide toxicity during the night. The shallow rooting salt marsh species are not affected or they are better adapted to sulphide stress. The aim of this research was to clarify the mechanisms of vegetation change along the middle reaches of Ichibangawa River. The hypothesis to be tested is: the vegetation change is caused by high sulphide concentrations in the topsoil formed due to sulphate reduction by micro-organisms under anaerobic conditions.
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