Utility of Ground-Penetrating Radar as a Root Biomass Survey Tool in Forest Systems

Traditional methods o f measuring tree root biomass are labor intensive and destructive in nature. We studied the utility of ground-penetrating radar (GPR) to measure tree root biomass in situ within a replicated, intensive culture forestry experiment planted with loblolly pine (Pinus tneda L.). The study site was located in Decatur County, Georgia, in an area of the Troup and Lucy (loamy, kaolinitic, thermic Grossarenic Kandiuddts and Arenic Kandiudults, respectively) soils. With the aid of a digital signal processing GPR, estimates of root biomass to a depth of 30 cm were correlated to harvested root samples using soil cores. Significant effects of fertilizer application on signal attenuation were observed and corrected. The correlation coefficient between actual root biomass in soil cores and GPR estimates with corrections for fertilizer application were highly significant ( r = 0.86, n = GO, p < 0.0001). Where site conditions are favorable to radar investigation, GPR can be a powerful cost-effective tool to measure root biomass. Verification with some destructive harvesting is required since universal calibrations for root biomass are unlikely, even across similar soil types. Use of GPR can drastically reduce the number of soil cores needed to assess tree root biomass and biomass distribution. The quality and quantity of information resulting from a detailed GPR survey, combined with soil cores on a subset of plots, mn be used to rapidly estimate root biomass and provide a valuable assessment of lateral root biomass distribution and quantity. T H E CONTRIBUTION OF TREE ROOTS to soil C is significant and difficult to survey accurately. In the southeastern USA, where excessive erosion from past farming practices has depleted the mineral soil of C, tree roots likely represent a greater proportion of belowground C. In a 34-yr-old loblolly pine ecosystem at the CaIhoun Experimental Forest in Union County, South Carolina, root systems comprised 18% of the belowground C (Richter et al., 1995). At the CIemson Experimental Forest, in Oconee County, South Carolina, tree roots comprised 24% of the belowground C in a 55-yr-old loblolly pine plantation (Van Lear et al., 1995). Mineral soil C concentrations are very static in these established forest systems, Richter and others (1999) attributed <1% of current C sequestration to accretion in the mineral soil. Tree roots are the most dynamic pool for belowground C accumulation in these forests. Traditional approaches used for root biomass harvests (e.g., soil cores, pits, and trenches) provide reasonably accurate information, but they are destructive in nature, labor intensive, and limited with respect to soil volume and surface area that can be assessed. Data J.R. Butnor, K.H. Johnsen, and L. Kt-ess. Southern Research Station, USDA Forest Service, 3041 Cornwallis Road. Research Triangle Park, NC 27709; J.A. Doolittle USDA-NRCS, 11 Campus Boulevard. Suite 200 Newtown Square. PA 19073; L. Samuelson and T. Stokes. School of Forestry & Wildlife Sciences. Auburn Univ.. Auburn, A t 36849. Received 7 Jan. 2002. "Corresponding author ([email protected]). Published in Soil Sci. Soc. Am. J. 67:1607-1615 (2003). O Soil Science Society of America 677 S. Segoe Rd., Madison, Wl 5371 1 USA derived from traditional root extraction approaches are also generally limited to root biomass averages across plots or treatments rather than information on root distribution. Sampling needed to detect differences among treatments can be expensive as well as time-consuming for technical personnel. Ground-penetrating radar can be used to detect tree roots and estimate root biomass rapidly and noninvasively (Butnor et al., 2001). While GPR may be an effective tool, its successfuI application is site specific. Ground-penetrating radar is limited by the eIectromagnetic properties of the soil being surveyed (Doolittle et al., 2002). Electrically resistive soils (i-e., high sand content) are more amenable to study then conductive soils. Site factors that limit detection of tree roots in the southeastern USA are considered in detail by Butnor et al. (2001). Without intensive, methodical scanning of grids, separation of roots by size class or depth is not practical (Butnor et al., 2001; Wielopolski et al., 2000). Raw GPR data can be of great interpretive value to a trained technician in the field, but is typically used in a qualitative manner. Tasks such as mapping depth to bedrock or water table, estimating the dimensions of large subsurface objects, and delineating changes in soils are-interpretable in the field. ~uan t i t a t ive analysis of raw data is possible. However, for the investigation of roots, soil features that are not the intended target often reduce the quality of the data. These undesired soil features produce clutter and interfere with data analysis. The goal of post-collection processing of radar data is to reduce clutter and minimize the effects of multiple hyperbolic reflections (Daniels, 1996). The radar image of a buried object often will not be representative of the objects' actual dimensions, and signal processing serves to transform the image to a format that can be more readily interpreted (Daniels, 1996). However, processing cannot replace the need for appropriate experimental design and amenable site conditions to gain meaningful root biomass data. There are numerous factors that can interfere with the resolution of roots. This study explores the potential of GPR and processing techniques on a near-ideal site. The purpose of this research is to: (i) assess GPR as a rapid non-invasive means to augment destructive root biomass harvests, (ii) improve the quality of radar data through advanced processing techniques, (iii) calibrate/ correlate radar estimates of root biomass with those obtained from soil cores, and (iv) determine if calibration is affected by block or treatment effects in a replicated study. The intent was to assess the full potential of GPR and processing techniques on a site that was, a priori, considered to be amenable to radar investigation. Abbreviations:GPR,yound-penetrating radar: SIR, subsurface interface radar. 1608 SOIL SCI. SOC. AM. J.. VOL. 67, SEPTEMBER-OCTOBER 2003 MATERIALS AND METHODS