Coastal Vegetation of Mull and Iona in Relation to Salinity and Soil Reaction

Abstract

Some of the coastal types of vegetation of Mull and Iona are described and an attempt made to relate these to the prevalent soil factors. The principal modifications of the latter on the coast are an increase of soil chlorides and, locally, of soil carbonates and a decrease of soil acidity and organic matter. In the sheltered sea lochs transition from acid moor to salt marsh commonly occurred within a distance of a few meters and study of the plants of the intermediate zone has yielded information relating to their differential tolerances. Moorland species such as Molinia caerulea and Eriophorum angustifolium penetrated below high water mark (H. W. M.) where the soil solution may be as saline as sea water (35 g/l chlorides). Halophytes such as Armeria maritima and Plantago maritima penetrated upshore in places into the marginal Molinietum of the moor zone. Characteristic species of the transitional zone on silted shores were Carex flacca, C. demissa, Juncus articulatus and Leontodon autumnalis, on pebbly shores Potentilla anserina just below H. W. M. and Iris pseudacorus just above. Except on the most sheltered shores where wave action was negligible and the most exposed shores where all silt was scoured away, organic matter was at a minimum at H. W. M., increasing both upshore and down-shore. The decrease appeared to be partly due to erosion, partly to the increased rate of decomposition arising from neutralization of acids and increased base status in the saline soils, this decomposition being hindered downshore by the prevalent waterlogging and lack of aeration. There was a close relationship between pH and salinity, the critical pH value at the junction of saline and non-saline soils lying between 5.7 and 5.9 on all shores examined. The neutralizing effect of salt water extended farther upshore on friable soils than on peats where absorption of salt water and colloidal adsorption of ionized salt were most marked. The moorland dominants most intolerant of sea salt were Calluna vulgaris, Erica spp., Juncus acutifloras and Sphagnum spp. Calcium carbonate contents of the shell sand of dunes and machair were higher than previously recorded, rising to 81% by weight on Iona. PH varied from 8.3 - 8.4, salinity was low, approximately 0.01% by weight of dry sand. Many of the machair plants were non-calciocolous and their distribution apparently controlled by soil reaction rather than calcium content. A high proportion of these occurred also on the non-calcareous lochside soils where mitigation of acidity was achieved by sea salt; others were intolerant of salinity. The mobility of the friable shell and sand under the influence of wind prevented the occurrence of a sharp transition zone between alkaline and acid conditions as on the sea lochs. Thirteen successional stages were traced from calcareous foredune to acid moorland, these showing local retrogession and diversion due to rabbit activity.