Use of the Tree Diversity Reporting Index (TDRI) to assess variability in urban tree diversity and related reporting practices: A Michigan, USA pilot study.

Plans for resilient urban forests are rooted in principles of urban forest biodiversity, begin with comprehensive inventories of urban tree cover, and are often laid out in urban forest management plans. However, there are no universal standards with which tree inventory data are presented in urban forest management plans, making it challenging to assess and compare urban forest biodiversity and resilience within and among municipalities. Using Michigan, USA as a pilot study, we developed the Tree Diversity Reporting Index (TDRI) as a tool to determine which urban forest diversity metrics were reported in urban forest management plans, the comprehensiveness with which these metrics were reported, and emerging trends in urban forest composition across the state. Findings indicate that the most frequently reported metrics were species composition, size distribution, and condition rating. However, there was significant variability in how comprehensively these data were reported, and in the provision of rationale behind some of the classification systems used to report these data. Regarding urban forest composition, there was an overabundance of Acer (maple genus) across the state. In most municipalities, the majority of trees are in fair or better condition, with age structure generally varying between predominantly juvenile or predominantly maturing trees. Use of the TDRI may be beneficial in providing a baseline standard for which tree diversity metrics are reported in urban forest management plans. This may be helpful as a growing number of new stakeholders are engaging in efforts to establish, manage, and benefit from resilient urban forests.