AN ASSESSMENT OF THE POSSIBLE IMPACT OF EXPANSION OF NATIVE WOODLAND COVER ON THE CHEMISTRY OF SCOTTISH FRESH-WATERS

An assessment has been made of the probable impact upon freshwater solute chemical composition of increasing woodland cover in Scotland using native species appropriate to natural woodland. Mineral soil acidification is likely as a consequence of tree growth where the soil parent material is a rock of low base status such as quartzite, granite or old sandstone, and will, in turn, contribute to freshwater acidification. Additional atmospheric pollutant or sea salt trapping by trees will exacerbate drainage water acidification. Acidity of surface horizons depends upon the underlying mineral soil composition, acid deposition and tree species, and all three factors should be taken into consideration, along with increased water interception loss, when assessing afforestation impact on stormflow water. The interception factor is, however, less important in wetter areas than in drier regions. The spatial distribution of soil characteristics relative to the stream channel is very important, and in areas with acidification-sensitive soils, buffer zones of adequate width may be used to combat water acidification. Ground rocks such as gabbro may be more appropriate than conventional liming materials for the management of such strips, to avoid excessively high surface soil pH values.