Rit (Vísindafélag Íslendinga) - 01.06.1975, Page 12

Rit (Vísindafélag Íslendinga) - 01.06.1975, Page 12
which are never under water and on the other hand those which are continuously flooded. The intermediate types, however, are more difficult to contend with. This is not to be denied, hut yet it seems to be that the intermediate types may be classified in the same manner, i.e. that changing, seasonal water levels cause spe- cial vegetation forms, although this may be difficult to determine on the basis of phanerogams exclusively. Although the ground water level is an important factor in the development of the mire vegetation, it does not tell the whole story. The water in the mires is traceable to two sources, on the one hand pure rain water, which falls directly on to the mire surface itself, and on the other to adfluvial water. The nature of these two kinds of water differs and the ratios are subject to seasonal change. Rain water is poor in minerals and hence it affords no nourishment to the mire. Adfluvial water which has been strained during a longer or shorter course through the soil contains varying amounts of nutritional materials, minerals and even nitrogen, which the water has dissolved in its course. As far as Icelandic mires are concerned, it should also be remembered, that they are enriched to a varying degree in minerals by wind-blown deposits, which results in in- creased mineral content of the soil, but it is dissolved in the ground- water regardless of its origins. Thus it is possible that the character difference between rain water and adfluvial water is not as im- portant since the mire receives minerals by other means than water alone. This could for example be the reason for the prevalence of Sphagnmn were blown deposits are scarce as will be shown later. All these factors considered, it is apparent that there are in fact three definitions of mire which emerge in writings: a geological definition, where the peat formation is exclusively taken into ac- count, a botanical definition, where the vegetation alone is con- sidered, i.e. the wet ground is a community of hygrophile plants, and finally a topographical definition, i.e. one which relies on the local geography and accordingly on the possibility of adfluvial water. Here the topographical definition, which is both clear and simple, will mainly be adhered to. The level of the ground water and its fluctuation jointly determine the type of vegetation, both of which are in tum determined by the landscape. The fluctua- tion of the ground water therefore constitutes in every respect the 12
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