The above- and below-ground root-to-shoot ratio in urban trees – a ground penetrating radar assessment

Abstract

Abstract Urban trees are renowned for providing fundamental ecosystem services that stem from their proper development in cities. However, the urban environment poses significant challenges for trees, including impairments from the above- and below-ground grey infrastructure. Important steps have been taken in understanding the conflicts between trees and the above-ground infrastructure but assessing the root systems has lagged behind. New technologies like ground penetrating radar (GPR) have opened new avenues to assess the root system of trees. In this study we probe the impact of curbs and pavement on the development of the root system of 30 Cenostigma pluviosum trees growing along sidewalks, a parking lot, and a green area. Based on the GPR data, we estimated the minimum and maximum depths of root occurrence, maximum diameters, and root volume distribution. The results point to a 54% higher root volume in the green area’s trees compared to sidewalks and the parking lot, in addition to an even distribution of the root system around the tree. The ratio between root volume and investigated soil volume increases as trees grow bigger in the park, following a proportion that likely guarantees tree stability. This positive association collapses in the street and parking lot trees, where the pavement, curb, and underground infrastructure constrain the development of roots as trees grow bigger. Thus, trees do not maintain the root-to-shoot proportion putting at risk the stability of trees and their services in the cities.