Urban Forester's Column: Soil Volume

December 18, 2023

Tree size decreases with less soil volume

Trees need adequate soil volume to grow and to provide the ecological services they offer. The more soil volume the better.

In this photo, you’ll see three trees, in order of size, all the same species, growing in a triangular lawn between two curbs in the pavement. The tree on the far left is the largest, and most robust, with a symmetrical crown, and good structure. The next tree is smaller, it has been pruned to allow for traffic, and branches have been tipped back into dog legs. The last tree, in the point of the triangle, is the smallest and weakest. It lacks secondary branches, has bad taper, and little to no elongation in its twigs. The tree with the biggest soil volume is the biggest. The one with medium soil volume is medium sized. And the one with the least soil volume is the smallest.

I am confident that those three trees were planted on the same day. Species, geolocation, soil type, weather and age are all well controlled in this unintentional experiment. The main difference between the trees is their available soil volume.

Engineers, review board members and landowners who must have all signed off on the drawings for this strip mall, understood that roads, parking lots and lanes need to be well designed to function. They knew that the speed limits, signage and even the painted markings on the ground had to be finely tuned to the needs of the drivers. The interwoven turning lanes and parking lots and drive-throughs were all engineered for the smooth, almost inevitable, flow of cars off the four-lane highway and into these little eddies of vehicular traffic where they swirled into restaurants and shopping centers without needing to come to a complete stop. What if we put as much expertise into growing trees?

Pavement is just like paint. It hardens into something that looks so permanent, but once you see it scraped up you realize how very thin it is. We can repaint the stripes on the road to create bike lanes and crosswalks. We can rip out pavement to make root zones just the same.

Bigger trees provide bigger benefits. We can use green infrastructure to deal with storm water, heat islands and poor air quality, all side effects of the climate crisis, but the trees can’t provide these benefits without enough soil volume and pervious surface to thrive. We can design adequate soil space for trees using structural soils and silva cells and unpaved swales between rows of parked cars. We are in control of our built environment. It is no more permanent than a coat of paint. This is something we need to celebrate in order to tear out, clean up, and rebuild the future we want to raise our children in.

Arborists can do a whole lot with a climbing rope and a saw. Imagine what we can do with drafting pencils and jack hammers.

Author: Adam McCullough, VT UCF Urban Forester