Hugh G. Miller

Forest growth as a possible cause of soil and water acidification: an examination of the concepts

Acidity consequent on root uptake was calculated as the excess cation accumulation in both stems and all above-ground tree components in a range of forest types, and values were derived for acidity resulting from the accumulated humus. In addition data from one pine stand was used to construct models of rate of accumulation of excess cations in trees and humus throughout the forest rotation. Rates of hydrogen ion production reaches a maximum very early in the life of a forest and the average annual permanent acidification resulting from removal of harvested material covers a wider range than either measured inputs in rainfall, including throughfall and stemflow, or estimated weathering rates. However, it is suggested that because rainwater inputs are episodic, include a mobile anion and may be channelled through the profile, whereas root generated acidity varies only gradually, does not involve the movement of an anion and occurs in intimate contact with the soil surfaces, rainwater acidity is the more likely to lead to streamwater acidification whereas root uptake will primarily acidify the soil.

Nutrient cycles in birchwoods

Birch has a long-established reputation as a soil-improver, a characteristic often presumed to result from some aspect of the nutrient cycle peculiar to this species. However, models for the cycles of nitrogen, phosphorus, potassium, calcium and magnesium by age in slow-growing birch based on data from the literature for biomass development, nutrient levels and rates of decomposition suggest that nutrient cycling in birchwoods is comparable to that in forests of other species with similar rates and patterns of growth.

A comparison of above-ground component weights and element amounts in four forest species at Kirkton Glen

Abstract Trees, selected to cover the basal area size classes, were sampled at Kirkton Glen, part of Strathyre Forest, to determine the weights and element contents of the above-ground components. The main species present in this catchment were Sitka spruce, Norway spruce, European larch and Scots pine, all about 50 years old. Plots were established within representative areas of each species in the catchment, the stem size distribution was determined and a number of trees were selected for intensive sampling. Measurements of ring area and height increment were assembled to study tree growth, and showed that at all sites Sitka spruce was the most productive species. Component weights and nutrient contents from this mature site were compared with results from younger forested stands from other areas which had been determined using similar sampling strategies. Compilation of nutrient budgets also allowed evaluation of the consequences on element budgets of the various harvesting techniques to be assessed at this site, which is due to the progressively clear-felled.

Studies of Proton Flux in Forests and Heaths in Scotland

Models of nutrient uptake by tree roots and subsequent release on litter decomposition show that, where ammonium is the dominant nitrogen source, acidity as a result of excess cation uptake is equivalent to that introduced in acid rain, but because that in acid rain is introduced together with mobile anions this is the more likely to reach streams, provided that drainage over or through the soil is so rapid that neutralization reactions are not completed. In polluted regions the neutralization process within a forest canopy appears to fail in late winter, thus accentuating the episodicity of hydrogen ion input to the soil.