The Botany of Iceland - 01.12.1945, Page 152

The Botany of Iceland - 01.12.1945, Page 152
494 STEINDÓR STEINDÓRSSON with this a difference in the formation of ice in the soil. I regard this assumption as highly probable as far as the rústs are concerned, but I believe, also, that differences in the vegetation of the mýr may exert a similar influence to that of different snow-covering. In Illugaver I made an observation which in my opinion points in the same direction. When I visited this place on July 25th, 1937, I noticed that in the wettest parts of the svamp Eriophorum polystachyum occurred ex- clusively on the higher-lying places, while a Carex rariflora association was found in the intermediate depressions. As the development of leaves is always very abundant in Eriophorum, a great deal of withered material from the previous year was found in the Eriophorum growths, and this material formed a thick mat which covered the ground; no such mat was found in the C. rariflora growths. On closer inspec- tion it tumed out that while the soil in the C. rariflora growths had thawed entirely, only a layer some 20-30 cm thick had thawed in the Eriophorum growths. There is hardly anv doubt that the thick mat of withered material considerably delays the thawing of the soil; but this renders it obvious, as was also assumed by F r i e s, that it is the different mode of thawing of the two places, situated near each other, which exerts the greatest influence on the Palsen formation. If thus, as assumed by F r i e s, the elevation constantly increases, it is quite obvious that its vegetation will change. The hydrophilous vegeta- tion disappears, giving room for another, more xerophilous vegetation. When once the rúst has attained its maximum size (how great this is will be determined by the external factors in each locality), its destruc- tion will commence. The xerophilous vegetation is unable to form a closed carpet on top of the high, snow-free tops of the rústs. The frost will form cracks and fissures in the rúst which will make the soil even more unfavourable for the vegetation, and in connection with the bare spaces they will give the wind free play to sweep away the dry soil. At the same time the evaporation from the rúst increases, the water is, so to speak, sucked up through the porous soil, possibly due to capillary action, and the desiccation will therefore not only affect the rúst itself but also the surrounding depressions, so that the flá as a whole will begin to dry up. The farther this development proceeds, the more easily will the whole flá fall a prey to the wind erosion, so that at last it will perhaps be swept away entirely; in several places there is evidence of this. Possibly, however, so much moisture will remain that the erosion does not developed beyond the flag stage, and the plants growing there will be able to prevent further destruction, perhaps even
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