It all depends on what you want for your city. My city is in a semiarid part
of the Northwest. We only have 3 native large trees, and a number of smaller
straggly decidous trees - ponderosa pine, douglas fir, and in moist
locations, thuja (western red cedar)are the large native conifers, which, on
their own, would make a continuous but fairly widely spaced canopy.
However, if well-watered for 3 or 4 seasons, our climate can sustain many
kinds of deciduous trees from other areas - london plane tree, maples of
many kinds, (especially norway maple), lindens, oaks, honey locusts, black
locusts, catalpas, hawthornes, spruces, firs, dogwoods, apples and
crabapples, flowering cherries, magnolias, tulip poplars, horse chestnuts -
in short, nearly every kind of tree which will grow in any zone 5/6 part of
the US. However, without supplemental watering, in 100 years or so the land
would return to the 3 native trees, because the others cannot reproduce
successfully here because of our extremely dry summers. From an ecological
viewpoint, it would be most logical to only plant the native species here to
line the streets, etc - but from an esthetic point of view, it is nice to
have the diversity of species. Birds and wildlife will find food and shelter
in the non-natives - but it is food and shelter that ends at the outskirts
of the city...after that, they have to rely solely on native species. There
have been newspaper articles recently imploring homeowners to retain and
plant ponderosa pines - because if large ones die or are taken out they are
seldom replaced. The point of the articles is that the character of the city
is defined by the 100-125 foot canopy of pines that dominates over all the
introduced deciduous trees - but once their lifespan is reached and they
start coming down in large numbers, there will not be an instant similar
forest to replace them - and none of the introduced species is as well
suited to the climate, soil, and insect life as the native pines.