> There really are no trees that are "great" for where you live, because without human intervention like what you described, there really are no trees that sustain themselves in the Lower Sonoran area. It is essentially a desert.
The Lower Sonoron us actually a steppe biome. There are plenty of native trees and shrubs that thrive all on its own with the given rainfall. People see "desert" and picture wastelands like the Sahara or the Gobi. Many people who never been to the area are surprised at how lush both the lower and upper Sonoran are.
As far as human intervention goes, there is a nuance here: there is a big difference between acting upon the world as apart from the ecosystem, and acting within the world as part of the ecosystem.
My neighbor's Pakistani white mulberry is very well suited for growing here in the Sonoron, with some help. Jujube trees (one of the buckthorne family) is very well suited here. So are moringas, with people growing dense, backyard moringa groves here in Phoenix.
There are people who figured out how to grow the classic fruit-bearing trees here. Using dwarf varieties, careful pruning, use of deep mulch basins, chop-and-drop, companion planting, they can do very well here without having to rely on flood irrigation.
> Competing for sun isn't really a thing in a desert, because canopies in that landscape are a liability rather than an advantage. They contribute to water loss, which is extremely scarce.
In the park environment I am talking about, despite it being in the Sonoran, it is actually designed like a park you would see in temperate biomes. That means the overstory trees there are large ponderosa pines (which has high water requirements for the area, so they flood irrigate the park during the summer). The tree I was talking about in that park s going to be starved for sunlight is surrounded by the shadows of several of the overstory trees
> If anything, having partial shade is good protection for young trees, which are more susceptible to variations in drought and heat.
That's partially correct. There are other ways to work with this. For example, the people who sold my black mission fig to me told me that it doesn't shade itself. Well, it does shade itself, as long as you don't trim the lower branches that forces you to then put the white reflective paint on its trunks.
In the Sonoran, the mesquites and the ironwood act as nursery trees, providing partial shade to support a variety of plants. The Suguoro cactus, for example, requires a nursery tree to help it along when it is young. Then it kills off the nursery tree when it gets mature enough. Ironwood, despite growing extremely slowly, because it acts as a nursery tree for a variety of plants, is a keystone species.