Defining the allometry of stem and crown diameter of urban trees

There is a strong allometric relationship between stem diameter at breast height (DBH) and crown diameter in healthy trees in the young to mature stages of their growth. How do geographical position, site conditions and management treatments influence this relationship? This study included only free-standing urban trees, thus providing data on the growth potential of the species included in the survey in typical urban conditions by linking this with estimated tree age. Fieldwork involved recording the dimensions and growing conditions of 400 urban trees in two UK cities; Norwich and Peterborough. Species selected for this study were pedunculate oak (Quercus robur L.), sycamore (Acer pseudoplatanus L.), silver birch (Betula pendula Roth.) and Norway maple (Acer platanoides L.). The mean relationship between DBH and crown diameter exhibited a restricted range (a ratio of 24 to 27) in this large sample. The results indicated that the factor of species did not have a strong impact on the allometric relationship in the case of the four species measured. It is therefore possible to produce good predictions of crown size by combining data from all the species used in this survey. A key finding of this study is that previous tree pruning and external site factors, such as hard surfacing over the rooting area and soil type, had no significant influence on the relationship between DBH and crown diameter.