How should we measure the DBH of multi-stemmed urban trees?

Foresters use diameter at breast height (DBH) to estimate timber volumes, quantify ecosystem services, and predict other biometrics that would be difficult to directly measure. But DBH has numerous problems, including a range of breast heights and challenges with applying this standard to divergent tree forms. Our study focuses on street trees that fork between 30 and 137 cm of height (hereafter multi-stemmed trees), which researchers have identified as particularly challenging in the ongoing development of urban allometric models, as well as consistency in measurements across space and time. Using a mixed methods approach, we surveyed 25 urban forestry practitioners in twelve cities in the northeastern United States (US) about the measurement and management of multi-stemmed street trees, and intensively measured 569 trees of three frequently planted and commonly multi-stemmed genera (Malus, Prunus, and Zelkova) in Philadelphia, PA, US. Specifically, we measured stem diameter at several distances above the ground: at the root collar, at 30 cm, just below the fork (which occurred between 30 and 137 cm), and at 137 cm (up to six stems following established protocols). Survey responses indicated that current mensuration practices are burdensome, that practitioners employ alternatives to the current protocols for measuring at 137 cm, and that small-statured, frequently multi-stemmed trees are an increasing proportion of street tree populations. Analysis of field data did not find substantial differences between methods of measurement with regard to predictive power for total height and average crown width. Alternatives to the current protocols for measuring at 137 cm have other advantages, including time required, ease of measurement, simplicity, and capacity to compare measurements between trees and over time. For trees that fork between 30 and 137 cm, we recommend taking a single diameter measurement at a lower height-either just below the fork or at 30 cm. Diameter measurements at 30 cm better serve researchers seeking to consistently measure radial growth over time, whereas diameter below the fork may suit practitioners who do not need fine resolution in trunk measurements.