Site relationships for Pinus patula in the Eastern Transvaal escarpment area.

Abstract

The largest area of commercial timber plantations in southern Africa is situated along the Eastern Transvaal Drakensberg Escarpment north of Nelspruit. The site requirements of tree species in this area are poorly understood. The purpose of this study was to examine site-tree relationships in the region and the implications of such relationships for the science of forestry. Pinus patula Schiede & Deppe in Schlecht. & Cham. was selected for the study as it is the most widely planted species in the region. In Chapter 1 the geology, geomorphology, climate, soils and vegetation of the study area are described. A geological map was compiled. Soil descriptions were based on 439 soil pits distributed so as to cover the range of site conditions in the area. The regression techniques used to identify key environmental factors and to model their relationships with tree parameters are described in Chapter 2, in which site-growth relationships specifically are investigated. In mature stands of P. patula 159 plots were established in such a way as to cover the widest variation in both site conditions and tree growth. The relationship between site index (mean top height at 20 years) and 100 site plus 10 stand parameters recorded at each plot was modelled by means of best-subsets, multiple and ridge regression. Several candidate models were compared on the basis of coefficient of determination and validation using independent data. The best model predicted the site index of the validation plots within 60 cm of the measured site index. The possible roles of the site variables identified by the models are discussed. In Chapter 3 site-foliar nutrient relationships are described. A close relationship was found between foliar and soil nutrient levels for the six major geological substrates. Site index was more accurately predicted from concentrations of individual foliar nutrients than from ratios of these nutrients. The Diagnosis and Recommendation Integrated System (DRIS), however, appeared to have greater potential for nutrient diagnosis. Provisional ORIS norms for P. patula were computed. In Chapter 4 the excessive accumulation of litter in P. patula stands was examined. Undecomposed litter layers were greater than 15 cm in thickness on nearly 25% of the 159 sites studied. Average litter layers contained greater amounts of nutrients than the underlying topsoil. Due to the colonization of the litter by tree roots, the degree of immobilization of nutrients in litter is not known. Environmental factors associated with variation in litter thickness were identified by models which explained up to 73% of the total variation. These factors are considered to act indirectly by promoting or retarding decay organisms. The possible implications of litter accumulation for the maintenance of site productivity are discussed. In Chapter 5 relationships between site and some wood properties are described. Although between-tree variation was larger than between-site variation, some important relationships with site were identified. 10% of all trees on the 159 plots had severe stem bumps. Most of the variables in a model to predict the severity of bumps could be interpreted as being associated with stem stability or exposure. The conclusion was that wind is probably the major cause of this defect. The findings of the study are summarized in Chapter 6. Particular attention is given to a synthesis of the possible roles of site factors in their relationships with the tree parameters investigated. There were strong relationships between tree parameters and mainly rainfall, altitude, soil wetness, exchangeable bases, effective rooting depth, slope position and geology. The single most deficient nutrient element appeared to be calcium. The implications for both research and management are outlined.