Modelling dimensional growth of three street tree species in the urban forest of the City of Tshwane, South Africa

Tree height, crown height, crown diameter and stem diameter were measured for 282 trees of the indigenous species Combretum erythrophyllum, Searsia lancea and S. pendulina. Growth relationships were modelled using age as explanatory variable for stem diameter and subsequently stem diameter as explanatory variable for tree height, crown height and crown diameter. Coefficients are presented for predicting tree dimensions using a logarithmic function. There were strong correlations for stem diameter and age (r(2) >= 0.75), and crown diameter and stem diameter (r(2) >= 0.74) for all three of the species investigated. Correlations were weaker for tree height and stem diameter (r(2) >= 0.63), and crown height and stem diameter (r(2) >= 0.60) for S. pendulina but stronger for both C. erythrophyllum (r(2) >= 0.83) and S. lancea (r(2) >= 0.70) in both instances. The results can be used in forecasting the physical dimensions of these species as a function of time. The results could also be used in the process of modelling energy use reduction, air pollution uptake, rainfall interception, carbon sequestration and microclimate modification of urban forests such as those found in the City of Tshwane.