Aligning design with science : tree grids and rings in the landscape

Abstract

Tree grids and tree rings pose certain challenges to the designer of landscapes especially designing hard landscapes such as pavements, parking areas and roadsides. One challenge is insufficient knowledge of lateral growth space required by tree stems over time. Inappropriate size selection may cause damage to tree rings, tree grids and trees, resulting in costly repairs and unsightly landscape features. Lack of a rigorous knowledge base may prevent the designer or urban forester from choosing appropriate ring or grid sizes. However, these situations may be prevented or avoided should the choices be based on scientific knowledge of tree stem growth rates. This paper presents the results of an investigation of stem growth rates of three indigenous savannah street trees species growing in the City of Pretoria, Gauteng Province, South Africa. The stem diameters of each species were measured at ground level. Random stratified sampling was conducted, with the total sample size being 282, the oldest tree being more than 46 years of age. Stem diameter was regressed on tree age to compute estimated stem diameter growth rates. Thereafter comparisons were made to rings found in the local landscape industry. The differences in growth rates showed that Combretum erytrophyllum, Searsia lancea and Searsia pendulina will outgrow small tree grids in approximately 10.75, 18.50, and 15.50 years, respectively. The paper suggests that the statistical analysis and modelling of field data increase certainty for design parameters, enhancing the excellence of urban landscapes and forests.