Foliar nutrient levels as indicators of site quality for Pinus patula in the Mpumalanga escarpment area: scientific paper

The South African forest industry is based on intensively managed monoculture systems, and requires diagnostic tools for the development of site-related nutrient management policies. Worldwide, the evaluation of foliar nutrient concentrations is one of the most widely used techniques in forest nutrition research, with many different applications. This paper forms part of a comprehensive study of the differential nutrient supply of forest sites in the study area, as well as the effect thereof on the growth of Pinus patula. The aim of this paper is to investigate the complex interactions between site factors, absolute foliar nutrient levels and tree growth. The levels recorded in this study indicate that most elements are in adequate supply, with possible exceptions being nitrogen and calcium. A range of site factors was found to correlate significantly with foliar nutrient levels, including climatic parameters, topsoil cation levels, N mineralization rate and soil texture. The season of sampling was identified as an important source of variation in foliar nutrient concentrations, partly due to seasonal accumulation of carbohydrates in foliar tissue, causing a dilution effect. Correlations between foliar nutrient levels and tree growth were generally low in this study. Nutrient concentrations recorded during winter were found to be a more accurate indicator of tree growth than summer concentrations. However, the normalization of summer nutrient concentrations after correction for carbohydrate levels, provided an improved correlation with the growth of P. patula. This study can therefore contribute to the interpretation of foliar nutrient concentrations, and has the potential to form part of integrated systems for diagnosing nutrient-supply limitations. Southern African Forestry Journal No.197, 2003: 21-3