Tree allometry describes the relationships between tree biometric variables, such as tree diameter, height and crown width. Understanding of these relationships helps urban foresters to assess many of the economic and ecological benefits (e.g. carbon storage, rainwater interception and regulation of temperatures) provided by trees, such as through the use of the urban forest management tools i-Tree Eco and Treezilla

There is however little knowledge, particularly in Great Britain, about how the relationships between these biometric variables changes between trees from different urban areas or according to tree species. This study aimed to fill some of this gap by evaluating the variation in these relationships in more than 2,000 trees from seven tree species growing in eight urban areas across Great Britain. It focused on establishing relationships between: tree height vs Diameter at Breast Height (DBH), crown width vs DBH and crown width vs tree height.

The study found that mean allometric relationships between the DBH, heights and crown widths of urban trees significantly differs from one urban area to another, even when the areas are closely situated. The relationships between these biometrics in urban trees are therefore influenced at least as much by the complex effect of environmental and management factors specific to particular urban areas, as they are by the regional climate. Common patterns of variation were only identifiable for some of the species suggesting that external factors impact on the growth of different species in different ways. The variations in mean allometric relationships were found to be greater in mature trees than in younger trees.

The findings also suggest that crown width can be correctly predicted from DBH measurements. This knowledge will be useful to those conducting citizen science surveys, where the measurement of crown width is required but is challenging.