Changes in soil properties under experimental tree plots on the Isle of Rum

Abstract

The ecological importance of native woodland in the Scottish Highlands and Islands is widely acknowledged and many calls for its expansion have been made. Much research into the effects of such an expansion on the above-ground ecosystem has been undertaken, but little is known about the impact it has on the underlying soils. Experimental tree plots, containing
species native to western Scotland, were established on the Isle of Rum National Nature Reserve during the 1960's. This thesis seeks to assess the effects of 35 years of birch (Betula spp.), oak (Quercus spp.) and Scots pine (Pinus sylvestris) growth on the relatively fertile, mineral soils at Harris and the less fertile, organic soils at Kilmory Fank.
Significant changes in soil characteristics were identified beneath all three species growing on both soil types. These changes were complex in nature. The effects of oak and birch growth could not uniformly be said to constitute soil improvement, however, the soils beneath Scots pine were significantly improved. The role of antecedent conditions in influencing the direction and type of soil change was demonstrably more important than theinfluence of the species type.
The direction and nature of change was similar under oak and birch growing on the same soils, rates of organic matter decomposition were elevated but some nutrients, particularly potassium, were depleted by up to 50 kghi. This depletion appeared to represent a nutrient redistribution from soil to trees and litter, rather than a loss to the system as a whole. No clear evidence was found to suggest that one species was a more or less effective soil improver than the other Under Scots pine, plant nutrients had accumulated in the soil, particularly calcium which had increased by more than 90 kgha'. At Harris, the acidity had declined by 0.3 pH units. The net nutrient input appears to derive from marine aerosols trapped by the pine canopy and a subsequent translocation of them to the underlying soil via throughfall and stemfiow.
The results are of value to forest managers who perceive the establishment of new native woodlands as a route to soil restoration. They show that many of the commonly held perceptions regarding the influence of tree growth on soils must be treated with caution. This is because the changes that occur appear to be highly site dependent, thus planting of native tree
species can have widely varying effects consequent upon the original soil characteristics and prevailing environmental conditions.