Using terrestrial laser scanning for characterizing tree structural parameters and their changes under different management in a Mediterranean open woodland

Analyzing tree structural features and capturing their temporal dynamic is challenging but crucial for determining key state variables related to plant function, management practices, and aboveground vegetation stocks. Terrestrial laser scanning (TLS) provides a mean for representing those key variables in three-dimensional space and through time. 

Here we further developed and tested two point cloud slice-based methods for estimating diameter at breast height (DBH) based on how the delineation of the slices of the tree stem point cloud was performed: the first method was based on the circumference of a slice (CM), and the second method was based on the average diameter of a slice (PM). Further, we focused on the characterization of three tree structural properties from TLS: maximum tree height (h_max), crown projected area (CA), and DBH, and then on the evaluation of their biometric relationships. Finally, we tested the potential of multitemporal TLS for evaluating the impact of tree management on tree structural properties and growth in a Mediterranean open woodland. In particular, we evaluated the effect of tree pruning and fertilization on h_max and CA. 

The study was conducted in three plots exposed to different fertilization treatments in a Mediterranean open woodland with ≃20% of fractional cover of evergreen holm oak (Quercus ilex L.): a control treatment without fertilization, a Nitrogen addition treatment, and a Nitrogen plus Phosphorus addition treatment. Pruning effects on tree growth were assessed in two plots within the control: an unpruned area and an area pruned in 2005. 

Estimation of the DBH showed a good agreement with field measurements (R² = 0.93 for PM and R² = 0.94 for CM). Despite Quercus ilex L. being characterized as a slow-growing species, a temporal difference of three years among TLS was sufficient to quantify the influence of different management strategies on tree structural properties and growth rates. Our results showed: (i) CA increased relatively more than h_max, independent of the respective management strategy, suggesting a preferential lateral growth in holm oak tree canopies in this open woodland, (ii) pruned trees showed larger changes in CA than control trees (Wilcoxon test p < 0.01), whereas fertilized trees grew more in height compared with control trees (Wilcoxon test p < 0.01), and (iii) biometric relationships among plant properties were affected by nutrient fertilization and pruning. These results show that multitemporal TLS shows promise for monitoring even small changes in crown dimensions, and it is a valuable tool for conducting vegetation dynamics studies and quantifying management effects.