[riotnrrd]

Those suburbanites were probably doing some variation of coppicing their trees, to keep them stunted and bushy for aesthetics.

I lived in San Francisco, too, and helped to plant dozens of trees with FUF (https://www.fuf.net/). You can't just randomly plant a tree along a sidewalk; some trees do badly in cities, attract pests, or drop inedible fruit that attracts vermin.

[adhesive_wombat]

Pollarding (not the same as topping) is very common in cities to prevent trees becoming monsters that block out all light (that 40m oak might look nice from far off, but nothing else grows under that canopy, and someone might want to see sky from their garden), can't be maintained easily and eventually become safety issues with huge windage and heavy, old, brittle branches that can break off and fall (or the whole thing goes down in a storm and takes a house with it).

A pollarded tree looks stark and misshapen when it's done, but the foliage will return bushier and denser in fairly short order.

Pollarded trees can also live longer because not only do they not blow over so often, they stay in a juvenile state of young growth longer.