Science1 publisher2 min readPublished
UBC model finds just over half of Kelowna's potential tree shade in a fifth of the city
UBC researchers testing two heat-planning tools in Kelowna found that 20% of the city's analyzed area holds 50.8% of its potential new tree shade. Paired with heat maps sharpened to 10-meter pixels, the rankings reach single blocks, though they rest on surface heat.
The Scientist · Science desk
What happened
- UBC researchers used an open-source machine-learning method to sharpen Landsat and Sentinel-2 heat imagery of Kelowna, B.C., to 10-meter resolution, reporting the work in Remote Sensing.
- Hot spots on 30-meter maps averaged 18,000 square meters, while the 10-meter maps picked out hot spots as small as 700 square meters, about the footprint of a large residential lot.
- A second study used the CanopyFit model to remove buildings, sports fields, underground utilities, wildfire buffers and sensitive ecosystems, then scored shade potential against heat and need.
- CanopyFit found room across Kelowna for 248 hectares of potential new mature canopy, spread very unevenly across the city.
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Why it matters
- decision A planting budget spread evenly across Kelowna would send four-fifths of its trees to land holding under half the potential shade gain, so ranking blocks changes where the money goes.
- constraint Where heat and need are high but open ground is scarce, planting alone cannot meet a canopy goal, and the remedy moves to paving removal, development rules or built shade.
- capability Because both methods run on widely available data, another city can build block-level heat and planting maps from its own local layers without inventing a new method.
A standard satellite thermal pixel is 30 meters on a side, or 900 square meters of ground [1]. Rooftops, asphalt, lawns and tree canopy inside it come back as one averaged temperature [3]. The smallest Kelowna hot spots, at 700 square meters, are smaller than one such pixel. On the 10-meter grid each spans about seven [1]. Across the city, the sharper maps showed nearly five times as much temperature variation [4].
The size comparison needs care. The 18,000-square-meter figure is an average for the coarse maps, while 700 is the smallest hot spot on the fine ones [5]. Their ratio, about 26 [5], sets a mean beside a floor, so it overstates how much smaller a typical fine-scale hot spot is.
Sharpening an image can invent detail, and the team tested for that. Tree canopy, paving and other landscape features explained more than 65% of the added temperature detail [6]. The authors treat that as evidence of real landscape differences and not digital noise [6]. Up to about 35% is left unaccounted for by those features [4]. The maps also measure surface temperature, and the researchers note it is not the same as air temperature [7].
For a planting budget, CanopyFit's most useful result is how concentrated the opportunity is. Twenty percent of the analyzed area held 50.8% of the potential shade gains [11], and the other 80% held 49.2% [2]. Per hectare, the top fifth averages about four times the potential shade of the rest [3].
"Cities have limited space and resources for planting, so we need to be strategic," said Melissa McHale, a professor in UBC's Faculty of Forestry and Environmental Stewardship and senior author of both studies [3][14]. "Together, these approaches move us from broad citywide targets toward decisions at the scale of the streets and neighborhoods where people actually experience heat," she said [2].
The 248 hectares is modeled room for mature canopy [10]. McHale said the goal is "to establish and sustain large, healthy trees where their cooling and other benefits are needed most" [4]. The model was developed with her municipal partners [8].
The thing this doesn't tell you is how much cooler a street gets once those trees have grown. Both studies rank places by surface heat and modeled shade [7][9]. Neither, as described, measures the air under a newly planted tree [9].
What to watch
- Air-temperature readings under new trees in CanopyFit's top-ranked Kelowna blocks, compared with lower-ranked blocks, would test whether modeled shade turns into cooler streets.
- A check of the 10-meter surface maps against street-level thermometers would show how much of the finer detail people actually feel in the air.
- A second city running CanopyFit with its own exclusion layers would show whether the 20%-holds-half concentration is particular to Kelowna.