Hashem Ackbari is an engineering professor at Concordia U who studies heat islands. John Stone was chair of a committee in the IPCC. Eric Davies collects seeds of big ancient trees and grows saplings from them. We discuss ways of using trees in cities to insulate buildings and reduce heating and cooling costs. It is more expensive to raise trees in cities than in natural forests, but their survival is greater where people are around to look after them. Many alien trees species are taking over and replacing native trees, with detrimental effects for the ecological niche, including for the soil and the animals living in that habitat; Norway maples are especially harmful, and have taken over Toronto's ravines. For the video, audio podcast, transcript and public comments: https://tosavetheworld.ca/episode-544-why-trees-in-cities.