Bats in urban areas depend on trees, and bat activity increases with tree cover. To effectively manage bat habitat in cities, it is important to know the distance to which tree cover most strongly influences bats (i.e., the 'scale of effect'). The aim of this study was to estimate the scale of effect of tree cover on bats in Toronto, Canada. I measured bat activity at 52 sampling sites across the city. I then examined the relationships between bat activity and percent tree cover measured within each of 19 landscape scales, 0.025 - 3.5km in radius, surrounding each sampling site. My results suggest that adding or removing urban trees influences bats up to 200m away. Urban tree management decisions should consider the impacts to bats beyond the site of management and within the surrounding landscape of a 200m-radius scale.