They don't necessarily do silva cells when the trenches already have (I think) 30 cubic metres or more of soil exposed to the sky in them. That's deemed enough to produce a healthy, mature tree.
@Northern Light know more about this, and could correct me.
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Not quite that simple.
Soil Volume is an important measure, the low end requirement for a large canopy tree is roughly 30 cubic metres of uncompacted soil. This can be achieved thorugh planting in open trenches, lawns/forests, and/or using Silva Cells.
However, there's another part to the story.
The tree would prefer to put roots out equally in all direction (think of the tree as being in the heart of circle with roots radiating out).
Given a chance, trees will send out roots quite far. A common guideline is 12 inches from the tree for each inch of diameter of the trunk. (think of how big you want the tree to be at maturity)
That means a tree as little as six inches (2ocm around) could send routes out six feet from the tree, but the high end is even higher, studies have shown a six inch diameter tree can throw out roots to 19 FEET.
To put this in perspective, the City of Toronto tree by-law only protects trees that are 30cm (1 foot) around or larger, which would indicate roots at a distance between 12 feet to 38 feet from the tree, if the conditions permit.
Of course there are variations by species, and there's a difference between 'ideal' and necessary.
From a soil volume perspective, even Silva Cell planted trees are recommended to receive minimum volume of 1000m3 of uncompacted soil volume per tree for a canopy sized (large maple/oak) tree, with volumes being less, if a smaller growing tree is desired (600m3)
How does the apply in the context of an open-trench planting?
You can almost certainly hit the soil volume easily enough without use of a Silva Cell.
However, unless the planter is quite wide there is likely still some advantage to using the cells under the adjacent sidewalk or road bed.
You really don't need to protect 20ft every direction, much as that would be nice.
But you really do want at least 5 feet of protected width (2ft 6 inches on each side of the tree).
Put another way, if your planter/trench is 1.55m wide or more, and sufficiently long and deep to get you 30m3 of soil volume, per tree, you're in pretty good shape. Most urban trees deal with far worse.
But even then, if you're able to increase that width by another 1.5m or more per side, your potential for a much larger tree increases substantially.
The Silva Cell and/or similar concepts are really about protection from compaction and opportunity for water infiltration.
Exposed soil carries with it additional benefits in terms of easier infiltration of water without intervention (permeable paving, or even conveyment of water via pipes or irrigation) as well as some opportunity for natural nutrient acquisition via dead leaves and insects and the like.
In general, the more the better.
Of course, the need to protect against excess salt is critical as well, and options for this vary, but typically involve a curb or seatwall around your trench/planter in a climate where heavy salt use is common.
But remembering the root reach we discussed, salt may be an issue even beyond the planter/trench area.
However, once a tree reaches a certain size its much better able to handle some saline intake. Salt is a more critical issue when the tree is smaller.
Worth adding here is that the cost of Silva Cells will vary by design choice/layout, but you're looking at $10,000 plus per tree. So the tendency will be to avoid using them if you've created really good conditions otherwise.
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I'm just eyeballing the pics, I don't have the schematics in front of me. I would say the trees going in the wider pits on the north side can probably do pretty well without additional Silva Cell protection.
The ones in the narrow planters on the south side would likely benefit more from such protection.
aily