Quantitative Structure Modeling of Excavated Stump-Root Systems from Terrestrial Laser Scanning Point Clouds: Formulation and Destructive Validation in Pinus taeda

Belowground biomass (BGB) is an important component of forest carbon stocks but remains poorly quantified owing to the difficulty of measuring it directly. This study evaluated terrestrial laser scanning (TLS) combined with quantitative structure models (QSM) to estimate root volume, biomass and carbon in Pinus taeda. Twelve root systems were excavated, washed, weighed and scanned, then reconstructed using a TreeQSM formulation adapted for stump-root systems. Volume was converted to biomass using mean and diameter class-specific basic density, and to carbon using measured carbon contents. QSM-derived volume showed weak-to-moderate agreement with measured volume (A = 0.6292; CC1 = 0.3916; d = 0.6692), with a systematic underestimation of 43.6%. BGB estimates using mean density showed comparable agreement (A = 0.7339; bias 37.9%), while diameter class-specific densities slightly reduced it (A = 0.7126; bias 40.7%). Basic density was lowest in the finest and highest in the coarsest class (0.224–0.298 g·cm−3), whereas carbon content varied only slightly among classes (43.73–44.62%). Belowground carbon showed agreement comparable to BGB and higher than volume (A = 0.7093–0.7308; bias 37.9–40.7%), with no gain from class-specific carbon contents. A single mean density was sufficient to estimate BGB. This simplification is appropriate as long as the error in volume estimation is greater than the error introduced by using a mean density, but this may change as reconstruction methods improve. In addition, the method requires root systems to be excavated and does not allow the assessment of roots in situ.