FAQs: Silva Cells & Tree Stability

A few weeks ago we tackled Frequently Asked Questions about how the Silva Cell manages stormwater on-site. This week we’ll examine questions about how tree stability is affected by the use of the Silva Cell.

1) The proposed trees on my plans will be rooted to a depth of 900 mm of macropore rich soil within the Silva Cells. Is that adequate? 

Yes. Trees that root to a depth of 900 mm are going 3 times deeper than trees in typical urban (highly compacted) soils, where 90% of tree roots are in the top 300 mm (Watson). A deeper, more well developed root system will be an order of magnitude better for tree stability.

2) In a typical Silva Cell streetscape application, trees will only have space to develop root systems on 3 sides (rather than all 4) — will that adversely affect tree stability?

Due to a lack of available soil volume in areas adjacent to tree openings, most trees planted with traditional techniques in pavement have poorly developed root systems on all sides. These trees are very unstable (Johnson et al). The most common cause of urban tree trunk failure is girdling roots inducing trees to snap off at their base at ground level during wind storms (Johnson et al). This phenomonen is usually caused by trees being planted too deep, compelling the tree roots to move to the surface towards oxygen rich soils. These roots continue circling the trunk and weaken the stem catastrophically (Watson et al).

In urban situations where trees are planted in linear lawn panels, they will develop two-sided root systems parallel to the curb with minimum roots perpendicular to the curb. These trees are usually stable to large sizes. These trees become unstable when the root zone is quite thin and/or some roots are cut due to sidewalk or utility repair.

With the Silva Cell, trees are given the opportunity to root on 3-4 sides, depending on the application and the site design. By increasing the available rooting volume and allowing the tree to root on more sides, use of the Silva Cell in the built environment can substantially reduce tree stability problems.

3) Won’t high winds cause instability? Without a completely developed root system on all 4 sides, won’t trees fail?

Wind speeds within metropolitan areas are dramatically reduced by surrounding buildings (Seely et al). Since the Silva Cell is used in the built environment, winds strong enough to knock over a tree are rare.
By visiting a meterology website you can check typical wind speeds in your area. For most places, these wind speeds will be well below failure thresholds for most trees. Remember that under high enough wind speeds, all trees will fail.

 

Anything you’d like to see answered that we haven’t addressed here? Just leave a note in the comments section and we’ll get to it!

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