A few months ago, a few paving slabs in a town centre were lifted up in order for the rooting environment beneath to be inspected. What was seen was certainly incredible, though of course not unexpected. The highly compacted surface just beneath the tiles and lack of any sort of planting pit has lead to these plane trees (Platanus x hispanica) being near-exclusively comprised of fine and fibrous roots. No anchorage roots were identified in the slabs lifted up, and all were within around 2m of the tree. Additionally, the fact none of these plane trees have significantly uplifted any of the slabs – even right around their base – is telling. Usually, plane roots with poor pit sizes will tear absolutely everything apart (from paths to roadways), though not here.
Granted, the planes (of which there are about 25) all shelter one another, and the tall buildings on the eastern and western sides also shelter them very significantly. Therefore, perhaps anchorage root growth is not a major requirement, though the abundance of roots directly beneath the slabs was interesting. I suspect that the moisture held just below the surface has attracted the tree roots, as even during a very dry period (when these slabs were lifted up) the sand was very damp. Frankly, it is amazing at how these planes exhibit so signs of stress during the summer, even where their rooting environment is so incredibly harsh.
Lessons to take from this? Have sizeable planting pits, and install underground stormwater retention and irrigation systems directing water into the planting pits (particularly necessary during drought periods).