There is magic to great streets. We are attracted to the best of them not because we have to go there but because we want to be there. The best are as joyful as they are utilitarian. They are entertaining and they are open to all. They permit anonymity at the same time as individual recognition. They are symbols of a community and of its history; they represent a public memory. They are places for escape and for romance, places to act and to dream. On a great street we are allowed to dream; to remember things that may never have happened and to look forward to things that, maybe, never will.
Trees can do many things for a street and city, not the least of which is the provision of oxygen, and of shade for comfort. Green is a psychologically restful, agreeable color. Trees move and modulate the light. In terms of helping streets to work functionally, when planted in lines along a curb or even in the cartway they can effectively separate pedestrians from machines, machines from machines, and people from people. The trunks and branches create a screen, sometimes like a row of columns that gives a transparent but distinct edge. Between pedestrian and auto paths they can be a safety barrier for the former. Put a line of trees one lane into a street, as has been done on many European streets, to make a parking lane for example, and that lane becomes a part of the pedestrian realm while still functioning as a place to park cars. Even a few trees along the curb of a busy traffic street can have an impact if they are close enough together.
There are different kinds of diversity, including physical and social, and we are interested in both as well as in how the first might impact on the second. The relationship between more buildings and the likelihood of greater diversity should be no mystery. With more buildings there are likely to be more architects, and they will not all design alike. There are more contributors to the street, more and different participants, all of whom add interest. More buildings are likely to mean more building owners, each with an economic stake in and responsibility for the street. They will do things differently in the first place and maintain and modify them differently as time passes.