The Growth of Schoenus Nigricans on Blanket Bog Peats: I. The Response to pH and the Level of Potassium and Magnesium

Abstract

Over most of Britain and on the continent of Europe Schoenus nigricans grows only in certain types of fen. In the west of Ireland, however, as is well known, it is one of the most important and characteristic constituents of blanket bog. This discrepancy in behaviour has been explained by Tansley in the following terms: 'It has recently been suggested that the abundance or dominance of Schoenus in blanket bog near the western coasts may be favoured by the falling of sea spray, driven by inshore gales on the surface of the bog, thus changing the soil reaction in the direction of its more normal habitat' (Tansley 1939). Gorham discusses the subject in a paper on the acidity and base status of a raised bog in the English Lake District and a blanket bog in western Ireland, but appears to think more in terms of a direct nutritional effect of sea spray. He misinterprets Tansley and Osvald (1949) but provides an alternative explanation when he writes '... it has been claimed that the greater exposure of the blanket bogs to sea spray provides them with more mineral nutrients, thus enabling them to support the growth of plants such as Schoenus' (Gorham 1953a). He goes on to show that for the bogs he studied, both the pH and the base status were considerably higher on the blanket bog. He concludes, however, that because the base status is not so high as that of the waterlogged soils influenced by mineral soil water on which Schoenus is usually found, 'Perhaps the mildness of the climate is the factor allowing Schoenus to grow in habitats less base-rich than it commonly frequents elsewhere.' The present author became interested in this problem in 1954, when working on the relationships between various bog types in Connemara, Co. Galway. It was noticed that Schoenus occupied a variety of waterlogged peats from mineral flushes to ombrogenous blanket bog and that, on the whole, the luxuriance of the plant decreased as the degree of flushing decreased. Thus there was available a range of habitats from something close to the 'normal' to the 'abnormal', all under the same climate. If the mineral salt content of the peat were the factor responsible for the differences in growth observed, it was argued that, since the growth on blanket bog peat was poor, a reduction in nutrient level below that found on Galway bogs might result in failure of the plant altogether and that it may not be found on blanket bogs farther east for this very reason. An attempt to discover what nutrients were involved was made by analysing plants from various habitats in western Ireland and comparing the growth with the relative nutrient content by the technique of partial correlation. The case for this approach has been put at length by Goodall & Gregory (1947) and, in a more refined state, by Leyton