Authors: Dr. Paul Ries, Oregon State University, Corvallis, OR & Joshua Petter, Tree Solutions Inc, Seattle, WA
Editor’s note: This article is a follow-up to one published by the same authors in the Winter 2020 edition of PNW Trees.
Urban sprawl and development can lead to a reduction in the canopy volume of urban trees, which reduces the benefits provided by those trees. When selecting tree species for urban areas, carefully considering the site criteria can lead to greater benefits and longer-lived trees. Selecting a diverse array of trees contributes to urban forest diversity which can help avoid catastrophic tree losses due to pests and climate change.
Oregon State University researchers surveyed tree managers in Tree City USA designated cities throughout Oregon and Washington to help understand the municipal tree species selection process. We received responses from 79 out of 151 municipalities for a 52.3% response rate. Both open-ended questions and descriptive statistics were used to understand the answers.
We asked managers to provide us with the five most planted tree species within their municipality in 2016, and the five most planted tree species prior to 2016. Our responses for 2016 included 236 different species, 49 genera, and 23 families. The top five most common tree species planted prior to 2016 included 77 species, 33 genera, and 15 families. While we observed a marked increase in the overall diversity of tree selection in 2016, there was similar tree species selection across municipalities.
Ten managers reported ash (Fraxinus spp.) in their top five most planted genera, which is concerning if (or when) Emerald Ash borer (Agrilus planipennis) reaches the PNW. Our results also showed a propensity towards planting large quantities of the maple genus (Acer spp.), which is already abundant in municipalities across the PNW.
In another portion of the survey, we asked respondents to rank their top three tree species selection criteria, which we used to create a hierarchical ranking called a Borda count. We assigned three points for the first choice, two points for the second choice, and one point for the third choice (i.e. 1=3, 2=2, 3=1).
Our responses indicated that mature size was the most important criterion overall, followed by aesthetics, and proximity of infrastructure. The three criteria at the bottom of the Borda count were genetic diversity, citizen preference, and hours of sun. Of the respondents, 63% ranked mature size as very important and it was also first in the Borda count. Proximity to infrastructure was ranked as very important by 67.9% of respondents, but only nine respondents selected this criterion as their most important criterion.
We also analyzed open-ended questions on tree species selection using qualitative analysis software to illicit greater detail surrounding our quantitative results. We found several interesting responses that contributed greater detail to the challenges faced when selecting tree species. Among the most common themes we found were aesthetics, diversity, “right tree, right place”, maintenance, and hardiness.