@ECEC Good lord the number of replies here from people whose brains have been destroyed by "planners"...
Trees lower the urban heat island effect.
There's plenty of room for trees in dense places, so long as "density" means efficient housing and efficient transportation rather than parking lots and stroads and single-family homes.
Someone said "trees require maintenance", as if asphalt & pretty much everything doesn't require maintenance?
Dendrocnide moroides, commonly known in Australia as the stinging tree, stinging bush, or gympie-gympie, is a plant in the nettle family Urticaceae found in rainforest areas of Malesia and Australia.[3] It is notorious for its extremely painful and long-lasting sting.
Looking at the trees brings most of the relief, so I expect even those are just as calming as long as you don't touch them and don't know what they are.
There's plenty of room for trees in new dense places, sure. But if you're in the parts of London with old ass-narrow ass-streets, this may be less true.