Bothell planners release tree canopy data, flag species concentration and potential planting areas

Cameron Colvin, long-range planner in the City of Bothell Community Development Department, presented preliminary findings May 7 from a consultant-led urban forest management plan that includes a near-complete street-tree inventory and a 2023-based tree canopy assessment.

The work matters because the data will guide decisions on where to focus tree preservation and new plantings as Bothell grows: the citywide canopy is 44% inside city limits and 35% in the Urban Growth Area, and patterns of canopy loss and gain differ by neighborhood and zoning district, Colvin told the planning commission.

Colvin said the city retained Planet Geo to lead the project and that Planet Geo’s field crews — certified arborists — collected street-tree data in a GIS application, recording species, size, condition, maintenance needs and conflicts with infrastructure. He said the street-tree inventory focused on trees in the public right of way and maintained strips, planters and medians and is intended as phase one; expansion to parks and other city properties could follow if funding permits.

The inventory shows species concentration and size-class imbalances. "About a third of the street trees inventoried are red maples," Colvin said, with Douglas fir making up about 16% and other species at lower shares. He noted the commonly used diversity guideline — the “10-20-30 rule” that recommends no more than 10% of trees be the same species, 20% the same genus and 30% the same family — is exceeded at the species and genus levels because of the high share of red maples, though the family-level threshold was met.

Colvin said the size distribution of street trees skews toward small and medium trees and that the inventory found relatively few large, mature street trees. He also said most street trees were rated in "good" or "fair" condition and that only a very small number of street trees were identified as high-priority removals: "priority 1 removals" represented five trees in the inventory, he reported.

The canopy assessment, which used 2023 aerial imagery and LiDAR-derived analysis, maps current canopy, potential planting space and environmental constraints (for example, utility lines or unsuitable soils). Colvin described spatial analysis by census block, subarea and zoning district; residential and conservation zones and parks unsurprisingly retain higher canopy percentages, while downtown and Canyon Park have lower canopy cover. Comparing the 2015 baseline to 2023, Colvin said the citywide canopy changed by less than 1% (a small net decrease) when losses and gains are aggregated.

Commissioners pressed for details on age and the value of mature trees. Commissioner Sarah Gustafson asked about retention of older trees and whether a goal to protect "heritage" or very large trees would be part of the plan; Colvin said council had flagged a heritage-tree program as a possible direction and staff could return with examples from other jurisdictions. Commissioner Robson and others emphasized concern that counts based on aerial imagery can miss qualitative loss when large mature trees are replaced by many small plantings, and commissioners urged attention to species selection and "right tree, right place" guidance for developers and homeowners.

Commissioners suggested making the inventory and canopy maps publicly available and using demographic overlays so future plantings can target equity priorities. Colvin said Planet Geo overlaid canopy with indicators such as age, income and education to identify areas where plantings would create equitable benefit and that public engagement will begin this summer with a draft expected for review in late summer.

Staff noted no formal action was required at the May 7 meeting; the project will return later for additional public engagement and draft review.

For now, Colvin recommended next steps: complete the field inventory, publish maps and data for public use, develop "right tree, right place" guidance, and explore a heritage-tree program. Commissioners broadly supported those directions while repeatedly urging caution about overplanting single species and attention to retaining large, mature trees.