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Evergreens are not planted on City boulevards for a few reasons.
1) Already mentioned above is concern about public safety. Young evergreens maintain a high level of needle density near the ground, for several years. This has the effect of obstructing sightlines and views and thus making it possible for someone to hide behind one ( a potential thief etc.), but also poses a traffic sightline issue in the same way, someone walking out from behind a Spruce tree would be invisible till in the road way.
2) A second reason evergreens are not used, is that for the most part they are not salt-tolerant, in particular, Cedar, Pine and Hemlock would all suffer greatly the harsh environment of a narrow urban boulevard or a tree pit.
Spruce is slightly more tolerant of urban conditions, but would still show a lower survival rate than the typical Silver Maple or Honey Locust (the 2 most common street trees). Also keep in mind many evergreens, such as pine also require a different Ph level in the soil than deciduous trees which would man amended the soil before planting, hemlocks tend to grow much better in damp and shade.
3) The last reason they are not planted is that they do very little to meet the City's tree canopy or (shade) goals. A big part of tree planting in the City is done with reducing the heat from paved surfaces such as roads and sidewalks. Evergreens typically have a far narrower profile, especially as they get older, and don't cast nearly as much shade as an oak or a maple.
All of the above does not preclude some planting of evergreens, but most likely on sites like the north side of Lakeshore Blvd. or large boulevard widths in suburban areas, mixed with deciduous trees. In those conditions the above concerns could be more easily addressed.
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As to the overall poor condition of trees. There are a host of reasons, and the City is aware of them all; its really a question of resources.
1) Urban conditions can be rough, more so on some trees than others. The City already tends to plant more urban-hardy trees.
2) Size of area for root growth. Probably the single biggest reason for tree failure. As a rule of thumb, the roots of trees should be as wide as the widest branches of the crown. In the case of a big maple, that means the roots need about 4-6m (12-18ft) of growing room across. That means tiny little concrete boxes or tree pits are terrible for the trees.
The City knows this as is moving where possible to tree trenches (one long pit, covered with concrete plates at some points); as well as larger planters. An example would be the new planters on the west side of Yonge Street just south of Lakeshore (by Pinnacle Centre).
However, this is generally only done with major streetscape work or redevelopment of a whole block.
3) Water! The trees need lots of it. They don't tend to get enough, in part because of their constrained roots, but also because concrete sidewalks and roads let little or no water in below their surface.
Here the City is moving in all new developments to require automatic irrigation of the trees. Its also testing a new design for semi-permeable sidewalks this summer as it does reconstruction on the Queensway.
Follow this link for info:
http://www.toronto.ca/involved/projects/queensway_rehab-mimico/index.htm
Hope that helps; and gives you some hope!
