Modified Logarithm Averaging Method for Clumping Index Calculation Using Tracing Radiation and Architecture of Canopy (TRAC) Measurements

The clumping index (CI) is a key parameter for modeling radiative transfer in plant canopies, but its retrieval leads to significantly reduced accuracy in highly heterogeneous environments. To improve the accuracy of CI quantification and to assess the performance of various CI algorithms under ecologically heterogeneous conditions, five conventional methods based on the Tracing Radiation and Architecture of Canopies (TRAC) instrument, namely the logarithm methods (LX), the gap size distribution methods (CC and P), the combination of gap size distribution and logarithm methods (CLX), and the modified gap size distribution algorithm (CMN), along with a novel LX+ method developed from the LX approach in this study, are evaluated in three 100 m × 100 m natural secondary forests stands (hardwood, coniferous, and mixed broadleaf coniferous) located at Maoer Mountain Forest Farm in Heilongjiang Province, China. The key findings include: (a) The LX+ method demonstrates enhanced sensitivity and accuracy in highly heterogeneous stands. (b) The P and LX methods show no significant variation trends with increasing view zenith angle. In contrast, the CC, CLX, and LX+ methods exhibit a pattern in which the CI values first increase rapidly and then stabilize as the zenith angle increases. In particular, the LX+ method yields the highest precision for CI estimates among all methods. (c) The LX+ method achieves optimal accuracy with varying segment lengths (Coefficient of variation=11.5%) and has strong explanatory ability for heterogeneous canopy structures with different segment lengths. The results demonstrate that the LX+ method significantly enhances characterization of multiscale canopy heterogeneity CI while maintaining the highest accuracy across all experimental scenarios.