Predicting the growth in tree height and crown size of three street tree species in the City of Tshwane, South Africa

Abstract

The aim of the study was to determine the relationships between tree age and various parameters of tree size in order to develop models to predict the growth of street trees. Three indigenous species, Combretum erythrophyllum, Searsia lancea and Searsia pendulina commonly used as street trees in South Africa were the focus of this study. The relationship between tree height and age, crown height and age, as well as crown diameter and age for all three species were investigated. The logarithmic function provided a good fit to the data on age against tree height ( r 2=0.73, 0.66 and 0.67 for the three species) and age against crown diameter ( r 2=0.74, 0.75 and 0.69 for the three species) whereas the fit for age against crown height was slightly poorer ( r 2=0.74, 0.54 and 0.59 for the three species). The logarithmic equations derived for the three species were applied to predict tree height at different ages and growth rates for different time intervals. A comparison between the species revealed that C. erythrophyllum starts with a mean annual tree height growth rate of 912 mm yr −1 for the first 5 years of growth while that of S. lancea is 786 mm yr −1 and S. pendulina 894 mm yr −1. However, S. pendulina and C. erythrophyllum have the same mean annual growth rate for the 5–10 years (467 mm yr −1) and 10–15 years periods (315–317 mm yr −1). Both mean annual crown height and crown diameter growth rate consistently ranked the species in the order of C. erythrophyllum > S. lancea > S. pendulina.