Soil chemical properties in pine plantations of different ages in the Eastern Escarpement region of South Africa

Master Thesis


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University of Cape Town

It is widely accepted that afforestation adversely affects soil chemical properties. A significant decrease in pH, base cation status and acid neutralising capacity (ANC), coupled with increasing acidity have been observed to occur in acid coniferous forest soils both in the Northern and Southern Hemispheres. International and local studies in Southern Africa have observed declining productivity over successive rotations in pine plantations. Increasing acidity loads in the Northern Hemisphere and their adverse effects on forest health have generated a search for chemical indices that would assist in pursuing thresholds beyond which forest vitality is damaged. A common and widely used chemical index is the Ca/Al ratio. The most common parameter adopted is the Ca/Al molar ratio in soil solutions. Recent studies in Southern Africa (Swaziland. Kwazulu-Natal) and in the Sabie area (Eastern Escarpment) have indicated that some forest compartments (Pinus sp.) may experience Ca/Al molar ratios dropping below unity. The value of unity is widely used and proposed as a threshold beyond which tree vitality may be adversely affected. This study was conducted to determine the state of surface (0-20cm) soils underlying pine plantations of different ages in the Sabie area on the Eastern Escarpment of Southern Africa. The pine plantations were selected in areas overlying a uniform bedrock (shale), high attitude, high rainfall, north facing aspect and confined to 3 pine species (P. elliottii, P. patuia and P. taeda). A composite sample was taken from each of 22 forest compartments varying in age (between 22 and 67 years) and rotation (first, second and third). In addition, the results of previous investigations of forest soils in the Eastern Escarpment region were considered in order to expand the array of forest compartment ages and the size of the overall data set used for statistical analysis, so that comparisons could be made.

Bibliography: leaves 71-78.