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.
