Current Applications of GPR in Forest Research

Abstract

Forests, both naturally regenerated stands and plantations are complex, long-lived systems, which can be difficult to assess and monitor over time. This is especially true of belowground biomass and internal features of trees which are inaccessible except by destructive sampling. Traditional methods are expensive, destructive, time-consuming, usually yield a small sample size and are not conducive to long-term monitoring. Since GPR was first used to map tree roots ten years ago, a variety of new applications have been introduced. On soils suitable for radar studies, root biomass surveys have been valuable means to quantify belowground biomass, spatial distribution of roots, measure root diameters and even map individual roots. Methods include collecting linear transects in reflection mode, interlacing grids of transects in order to create 3D reconstructions of roots and applying high frequency borehole antennas used in transmission mode to model vertically oriented roots. One of the more difficult problems we are currently considering is how to analyze permanently marked transects over time to monitor root development while soil moisture, temperature and surface conditions change seasonally. In a departure from subsurface analysis, we have recently employed a method using GPR to detect defects and moisture gradients in stems. Introduction Forests are complex, long-lived systems which present many challenges to analysis and inventory. Over the past hundred years, numerous methods of inventorying aboveground biomass have been developed ranging from allometric equations used to extrapolate wood volume from tree diameter to satellite-based sensors which can measure canopy height and density over a wide area. Methodology to quantify belowground biomass has lagged behind, until recently destructive excavations were the only option. Unlike annual crops; trees live for decades or longer, so destructive sampling is usually undesirable and not repeatable. The investment trees make in roots is considerable, up to half of the biomass in trees may be hidden belowground. Ground-penetrating radar (GPR) has been demonstrated to be a rapid means of detecting tree roots and measuring lateral root mass in well-drained, electrically resistive soils (Butnor et al. 2001; Butnor et al. 2003; Barton and Montagu 2004; Cox et al. 2005; Stover et al. 2007; Samuelson et al. 2008). Today, tree root biomass studies provide valuable insight into belowground productivity in forest systems and are used to test the effect of tree species, genetic selection, and subsequent management on carbon (C) allocation. In this paper we discuss several successful applications where GPR was used to quantify root mass, map root distribution, assess vertically oriented tap roots with borehole radar and detect defects in stems. Our purpose is to present how the technology is currently being used and highlight areas where the research is headed.