ply not the case. All growth increases resulting from porous pavement only occurred in the absence of a compacted subgrade. In other words, if soil com- paction wasn't sufficiently high to negatively impact root growth, then porous pavements could positively impact root growth.
The knowledge that improved tree growth only occurred in the absence of soil compaction can explain why porous pavements have generally been unsuccessful in urban settings. Soil in urban areas, and especially in streetscapes, is highly compacted. This is no mis- take. Soils in paved areas are engineered to support heavy loads. So the use of porous pavement in these heavily compacted settings isn't likely to provide any benefits to street trees.
Still, with increased understanding of the important role played by street trees, some streetscapes are employing other techniques for structural stability and moving away from com- pacted soils. Perhaps there's a role for porous pavements in these settings. In the past decade we've seen increased prevalence of structural soils like Amsterdam tree soil and CU-Structural Soil, as well as suspended pavements like Silva
Cells. These alternatives allow soil beneath a paved surface to be relatively uncompacted. This means that trees grown in these systems will most certainly benefit from overlaying porous, rather than impervious, pavement.
There's an important lesson to be learned here. If we want to improve tree growth with porous pavement, we have to stop thinking about it as a surface treatment and start thinking about it as a complete pavement profile or section includ- ing the subbase and subgrade. Covering a highly compacted soil with a layer of porous pavement and expecting improved tree growth is analogous to painting over dents on a car before bringing it to the body shop. It's just a superficial fix that doesn't address the root of the problem. Any suc- cessful use of porous pavement to improve street tree growth relies on having underlying soil that is conducive to root growth.
For more information or questions, please contact the author at: justin.morgenroth@canterbury.ac.nz. Complete details of this study can be found in three peer reviewed articles in Arboriculture & Urban Forestry.
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