Crown width models for woody plant species growing in urban areas of the U.S.

Crown widths of woody plant species growing in urban areas are of considerable importance as an overall indicator of health and also serve as an important factor for assessing leaf area and associated ecosystem services, such as carbon sequestration, air pollution removal, air temperature cooling, and rainfall interception. Unfortunately, assessing crown widths in urban environments is often challenging and time consuming. To help reduce data collection costs and provide consistency over time, models to predict crown widths for urban-grown species were developed using data from 49 cities across the U.S. and Southern Canada. The effort consisted of fitting mixed models for 29 species groups that encompassed 964 species. Cities were considered a random effect and were statistically significant for 22 of the 29 groups. The need for urban-specific crown width models was demonstrated via examination of prediction biases found when applying crown width models based on forest grown trees, where under-prediction up to about 20% was found for the same species growing in urban areas. Application of the models was evaluated by using crown width predictions instead of observed values for calculations of crown leaf area. Mean percent differences in leaf area were about ±10% across most species groups. Further improvements to national-scale urban crown width models should be pursued as additional data become available via i-Tree, Urban FIA, and possibly other sources where data collection protocols are compatible.