I do think it is important to use native species or at least species suitable to the climate and intended use. I think one of the biggest problems with street trees is the "concrete jungle" efffect: pavement gets extremely hot and is not water (or air) permeable. The heat rising off the pavement can be a serious stressor for the tree and the pavement can strangle the roots. Some trees need for their roots to "breathe", which is why some of them break the sidewalk: it isn't mere happenstance that the roots of certain species make a "spiderweb" effect at ground level but other trees roots are more underground. The various root structures are purposeful, both "architecturally" in creating a stable anchor to the earth for a very tall, skinny plant but also biologically for uptake of necessary nutrients.
I am not real knowledgeable about plant biology, but I decided to get my undergrad degree in some kind of environmental studies program because I do think that the tendency for planners (of all types, not just those with that word in their job title) to be ignorant of or oblivious to such details is a real source of problems. Such things should be taken into consideration when choosing the type of tree to plant and also when designing the way in which it will be planted. I have seen too many times where plants are put into what amounts to a concrete box, surrrunded by concrete, concrete, more concrete and asphalt for varation. :-D Is it any wonder that such plants are burned and shriveled and die rapidgly thus having to be replanted frequently?
The lack of permeable ground cover has other serious consequences. In Europe, there are still cobblestone roads and some sidewalks are made with "pavers" that are not solid -- it creates a kind of lattice effect and grass can grow up in the open sections and the ground is permeable to water. This can make a huge difference to things like groundwater recharge, concentration of pollutants in nearby waterbodies, "heat island" effect, and so on. In a downtown area that is intended to be pedestrian-friendly, all of those benefits make it more attractive -- or even simply more tolerable -- to be on foot. Walking across hot pavement will dehydrate people just like it dehydrates the trees that it is baking. A downtown that is tree-friendly will also be more people-friendly.
If you want to successfully plant street trees, it needs to be done with more care and thought than I see typically practiced. You cannot simply cut a hole in the concrete and stick in a tree. Just as trees thrive or die in nature depending upon the overall environment and whether or not their niche is supported, the streetscape must be viewed from a wholistic perspective for creating a viable environment for their survival. These are living creatures, not "inanimate objects", and cannot simply be placed willy-nilly wherever you decide, with no regard for their needs -- at least not if you want them to work well.
But I have recently experienced the frustration of being unable to read the signs of the businesses in a downtown area, in part because of the tree cover. I am not sure what to do about that but I am thinking that what works well is for the trees to be tall, the merchants and their signs to be at street level, and anything on the second floor and above can be offices and residences. If the foliage is high enough up, the trunks will not be a big problem in terms of visibility. Also, I am thinking of the old fashioned practice of putting a sign out that is perpendicular to the store front and can be read from both sides. The practice of putting the name of the store only on the building face makes it difficult to see on approach from either direction of the street, whether you are in a car or on foot.
