Q & A: Tree Stability in Urban Areas

For today’s post, experts L. Peter MacDonagh and James Urban, FASLA tackle one of the most commonly asked questions we hear about whether tree stability is related to an evenly developed root ball. Here is what they have to say. -LM

Without a completely developed root system on all four sides, won’t trees fail? What about trees planted in deep soil – how does that affect stability?

Currently, most trees planted in pavement do not have access to an adequate amount of soil and therefore have poorly developed root systems on all sides, which does negatively affect stability (Johnson et al). Having said that, the most common cause of urban tree trunk failure is actually girdling roots inducing trees to snap off at their base at ground level during wind storms (Johnson et al). This phenomenon is caused by trees being planted too deep – in other words, burying the root ball, which compels the tree roots to move to the surface towards oxygen rich soils. These roots continue circling the trunk and can weaken the stem catastrophically (Watson et al).

Urban trees planted without girdling roots will have a much lower chance of failure, as will trees that have a developed root ball, even if that root ball is not even on all four sides. A deeper, more well-developed root system is an order of magnitude better for tree stability. In fact, the separation of a tree’s root plate from the underlying soil profile is rarely seen with trees planted in paving (Smiley 2009).

Many people I speak to are concerned about the effect high winds have on tree stability, but this is not something we’ve observed many problems with. This is largely because wind speeds within metropolitan areas are dramatically reduced by surrounding buildings (Seely et al). By checking a meteorological website, you should be able to find local wind speeds for your area. Those speeds will usually be well below the failure thresholds for most trees.

L. Peter MacDonagh is the head of science and design at The Kestrel Design Group. James Urban, FASLA, is the author of “Up By Roots: Healthy Trees and Soils in the Urban Environment.”

Top image of a London plane tree felled by Hurricane Irene in 2012. Flickr credit: Flatbush Gardener

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