How Do Urban Tree Managers in the Pacific Northwest Decide What Trees to Select?

Dr. Paul Ries, Oregon State University, Corvallis, OR & Joshua Petter, Tree Solutions Inc, Seattle, WA

Tree selection decisions at the municipal level are made by many different natural resource professionals, including arborists, urban foresters, planners, and landscape architects. A recent study by Oregon State University aimed to explore how managers make tradeoffs and prioritize different tree selection criteria to better understand how trees are selected for public lands. We surveyed primary contacts for Tree City USA designated cities across Oregon and Washington. Of these municipalities, 79 out of 151 responded (52.3% response rate), with six municipalities providing responses from more than one department for a total of 85 responses.  

Our analysis primarily focused on two groups of questions that were organized into matrices. The first matrix was designed to measure tree species selection on a site scale. For these 16 criteria, a total of 81 responses were recorded, and respondents were asked to rank each criterion on a 5-point scale of 1 “not at all important” to 5 “very important”. The second matrix was constructed using seven questions to measure tree species selection on a broader scale across the entire municipality; 78 responses were recorded. Our first objective was to explore how tree species selection differed between International Society of Arboriculture (ISA) Certified Arborists® and those who are not certified. Our second objective was to explore how tree species selection differed between small (≤50,000) and large (>50,000) municipalities.

We compiled the responses and analyzed the results with a statistical test to compare International Society of Arboriculture (ISA) Certified Arborists® to those who are not certified across various tree species selection criteria. We found that ISA Certified Arborists® are more likely to consider “mature size of the tree” and “existing tree species diversity” important criteria. The mature size of the tree is often used to determine the benefits and costs of the tree. Larger trees often provide more benefits; however, they could also incur more expenses in infrastructure damage if sited poorly. ISA Certified Arborists® also differed from those who are not certified on a city-wide scale, particularly in favoring greater tree species diversity.

Another statistical test was used to compare small and large municipalities across the same criteria. We found that larger municipalities are statistically significantly more likely to consider “existing diversity” and “availability” important criteria in tree species selection. If large municipalities aim to increase tree species diversity, tree species availability could be more important.

The differences in urban forest management between ISA Certified Arborists® and non-certified — and between municipality size — can help to influence future educational campaigns targeted towards increasing urban forest health and resiliency for all urban forest managers. Additionally, this study helps to provide a framework for future statistical analysis and greater exploration of how municipalities and managers are selecting tree species for public spaces.