Surface-based GPR underestimates below-stump root biomass

While lateral root mass is readily detectable with ground penetrating radar (GPR), the roots beneath a tree (below-stump) and overlapping lateral roots near large trees are problematic for surface-based antennas operated in reflection mode. We sought to determine if tree size (DBH) effects GPR root detection proximal to longleaf pine (Pinus palustris Mill) and if corrections for could be applied to stand-level estimates of root mass. GPR (1500 MHz) was used to estimate coarse root mass proximal to 33 longleaf pine trees and compared to the amount of biomass excavated from pits proportional in area to tree basal diameter. Lateral roots were excavated to a depth of 1 m and taproots were excavated in their entirety. GPR underestimated longleaf pine below-stump mass and the magnitude of the underestimation increased with tree DBH. Non-linear regressions between GPR estimated root mass/excavated root mass and tree diameter at breast height (DBH) were highly significant for both below-stump (lateral + taproot) root mass (p < 0.0001, R-2 0.77) and lateral coarse root mass (p < 0.0001, R-2 0.65). GPR underestimates root mass proximal to trees, and this needs to be accounted for to accurately estimate stand-level belowground biomass.