Introduction

Roots make up 5 to 45 percent of tree biomass in upland forests worldwide (Cairns et al., 1997), but estimates from urban settings are limited. Roots of field-grown trees are difficult to assess and delineate without laborious and destructive excavations. For this reason, relative to studies on above-ground biomass, root systems have rarely been studied; often more is assumed about their nature than is really known. Foresters and ecologists are interested to learn how different management techniques (site preparation, cultural practices, and harvesting methods) and natural disturbances affect tree productivity and carbon sequestration. Tree roots play a dynamic role in sustainable forest productivity and serve as a conduit for atmospheric carbon to enter the rhizosphere. After a harvest, fire, or other disturbance, the majority of the recalcitrant carbon is retained in roots for a time and slowly released by oxidative processes, and a small fraction becomes a stable constituent of the soil. Research interests aside, trees play an important role in urban environments, providing shade, reducing temperatures, and providing aesthetics. Municipalities, arborists, and property owners are interested in mapping roots to establish protection zones during construction (Jim, 2003), assessing tree health and taking proactive steps to help urban trees thrive.

Ground-penetrating radar (GPR) can be used to detect and monitor roots if there is sufficient electromagnetic contrast with the surrounding soil matrix. This methodology is commonly used in archeology and civil engineering for non-destructive testing of concrete as well as road and bridge surfaces. GPR is ideal for these applications because the electrical properties of concrete and rebar (steel) are very different. Under amenable conditions (i.e., electrically resistive, sandy soils are ideal), tree roots are detectable and can be quantified. GPR has been used to resolve roots

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