Population exposure to vegetation is lowest where it is most needed: A multi-city comparison of the “3–30–300” rule

Source abstract

Urban vegetation is widely promoted to mitigate environmental stressors and improve population health, yet comparable assessments of residents’ exposure across cities remain limited. We applied the “ 3-30-300” rule, a multidimensional benchmark requiring visibility of at least three trees, ≥30% neighborhood canopy cover, and residence within 300 m of a park, to five Canadian urban regions (Montreal, Toronto, Ottawa, Quebec City, and Vancouver). We used high-resolution LiDAR-derived tree-structure data and consistent analytical procedures across 10,391 dissemination areas, small Canadian census units generally containing 400-700 residents. Compliance was high for tree visibility (55.5% to 92.6% using trees ≥10 m ), moderate for park proximity (39.8% to 54.7%), and extremely low for canopy cover (0.8% to 7.4%). Across urban regions, few neighborhoods met all three criteria when mature trees (≥10 m ) were considered (0.6% to 3.4%). Sensitivity analyses showed that lowering tree-height thresholds substantially increased visibility estimates, from 55% (using a 10 m height threshold in the least compliant city) to over 90% across all five regions when a 3 m height threshold was applied. However, it had minimal influence on absolute canopy-cover compliance, underscoring the structural importance of mature trees for achieving canopy targets. Compliance with the “ 3-30-300” rule was lowest in four of the five urban regions where exposure to vegetation was most needed i.e ., in neighborhoods with high human population density. However, urban regions with very different population densities exhibited comparable compliance levels. Our results reveal a pronounced shortfall of mature canopy within residential neighborhoods even in urban regions reporting much higher total canopy cover.