I can, however, easily explain the division in Europe: In Italy (for example, in Palermo), the vigorous growth of many species very often leads to significant damage to infrastructure.
Here in Vienna, there’s a directory of trees[1] where you can see, among other things, the species and age.
That's like explaining frost-jacking of a wall in terms of temperature instead of hydrology.
The inner center and hot south can be more dusty and discouraging, but you can still be surprised by a few, not well known, jewels like Cabañeros, Valsain's pine forest, Alto Tajo, or Grazalema and the last relict Mediterranean -humid- forests in Cadiz. Plenty also of lagoons, marshes and aquatic ecosystems to visit, like Doñana or Daimiel. The biosphere reserve Hayedo de Montejo is located in Don Quixote's land.
I've seen suburban development that would easily satisfy the three tree test from any window on any property, but they still come off as desolate wastes. The age of the trees seems to be a non trivial factor.
For this reason, I'd prefer to have compact cities with a good amount of high-rise buildings and city parks dotted in between. As opposed to large sprawling suburban zones.
That leaves more space for natural areas outside cities where people are few & far between.
Alas, doesn't work very well outside of britain, but it's a good metric :)
Now I am curious if there is a dataset for the location of every tree in every city in the world? https://overpass-turbo.eu?
[0] https://www.london.gov.uk/programmes-and-strategies/environm...
Here we go, correlation does not equal causation. Simple as that. Planting 3 trees will not give you a better mental health nor will planting 10 trees. But moving in to an environment where many trees grow in front of your window will probably change a lot more than just putting trees in your view.