Can We Stop Drawing Trees on Top of Skyscrapers?
Toronto requires green roofs for new building projects.  Grasses and shrubs are typically grown, and they survive just fine with minimal care.  I suspect the only reason that large trees are not typically used is that there's fear that tree roots will damage the roof of the building.  Oh, and the weight of the soil required for larger plants is an issue.Yep, looking at the bylaw, the growing medium - soil - is only required to be four inches deep.  Weight of soil and drainage starts to be a problem - you have to figure that the soil may be 100% soaked...It's going to be tough to grow trees in four inches of soil.So, nutshell, tree survivability not a problem, but engineering a roof to hold enough soil (and therefore water) to grow a large tree is expensive, and root damage is a problem, and therefore - no large trees on skyscrapers.  Still, there's nothing magic about it, just engineering problems.  I could easily imagine a high-end residential tower with a forest on the roof.