Bi-temporal terrestrial laser scanning to measure secondary growth of trees: toward non-destructive assessment of wood properties

The growth process shapes the structure of trees in forest, and the rate the stems thicken over time is linked with wood properties such as ring width and wood density. This study investigated the potential of bi-temporal terrestrial laser scanning (TLS) for measuring the secondary growth of coniferous trees and assessing wood properties derived from core samples using X-ray microdensitometry over a nine-year period in boreal forests. TLS-derived DBH measurements demonstrated high agreement with reference data (r > 0.96). Volumetric secondary growth (Δv) slightly outperformed basal area increment (Δg) in capturing secondary growth dynamics as moderate correlations were found between TLS-derived estimates and mean ring width (RWm; r = 0.60–0.67). In contrast, correlations with ring basal-area-weighted wood density (WDg) were weak but statistically significant (r = –0.16 to –0.18) for both TLS and core-sampled measurements. The findings suggest that TLS can be used to measure secondary growth, but the ability to predict wood density of coniferous species via secondary growth measurements – neither with TLS nor core-sampling – is limited due to internal anatomical factors not captured by external measurements. Overall, these findings support the integration of TLS into forest monitoring frameworks, as it provides equally reliable yet more versatile data than callipers for measuring secondary growth.