Adjusting to the climate changes of the anthropocene, the US Forest Service is very interested in large, landscape-scale urban forestry. Street trees assume additional value now for their ability to absorb storm water surges and mitigate an intensified urban heat island phenomenon, among other benefits. The architecture of urban tree roots has become an urgent area of inquiry.

Recent research on tree root ecology in the urban environment by arboriculturalists like Susan Day at Virginia Tech, has revealed that “tree roots are primary contributors to the development of soil structure and, in the longer term, soil formation.”

There are many factors in the urban environment that contribute to degradation of soils and in particular, soil structure. Thus, the potential of tree roots to influence soil structure is of considerable interest. This new appreciation of the influence of roots on soil is redefining and enlarging our concept of rhizosphere: the area where soil interacts directly with living roots. Tree root contributions to soil structure not only affect plant growth, but [they also affect] a host of other soil functions that provide ecosystem services such as stormwater runoff mitigation through enhanced soil permeability. via

Again Nina Bassuk, “Urban trees have a foreshortened life compared to rural trees. The dilemma we face is that tree roots need a penetrable soil so that they can grow for long distances to get the water, nutrients and oxygen they need. On the other hand, when we have sidewalks surrounding trees, engineers require that the soil be compacted so that there is a load bearing surface to lay their concrete on.” As a contemporary design and materials solution, Nina and a team of applied researchers designed CU Structural Soil, to meet engineers’ specifications while allowing tree roots to penetrate freely through (structured) soil to get the water, nutrients, and oxygen that they need.

Of tree grates and new ways to protect roots, Nina prefers a simple open soil bed, covered in shredded bark mulch to manage root surfacing / pavement heaving. If that isn’t possible, she points to unmortared granite blocks or concrete pavers—both of which have collars that can be unfussily opened as the tree flare grows.