  Urban trees play a crucial role in urban climate in many aspects. However,
existing research has not adequately explored the impact from a time-evolving
perspective, that is, tree growth over time. To bridge this research gap, this
study investigates in a wind tunnel the effects of tree-to-canyon foliage cover
and relative height (0.32-1.1 times canyon height), mimicking growth of trees,
on conditions in street canyons during moderate and extreme heat. The results
reveal that trees may affect canyon-wide ventilation and heat removal in two
different scenarios. First, when canyons are in isothermal conditions, medium
and large trees, that fill half the canyon height or reach slightly above the
canyon, decelerate the shear layer and weaken the vortical flow, as a result
reducing the canyon-wide ventilation. Second, in extreme heat conditions,
medium and large trees trap heat at the pedestrian level due to the blockage of
air entrainment and the suppression of upward buoyancy-driven flow from the
ground surface. An air temperature rise that corresponds to 1.5 degree Celsius
in a full-scale urban setting is observed in measurements. These observations
suggest that urban trees' foliage cover must be managed for a canyon's optimal
ventilation and heat removal during nighttime.
