Height to Crown Base Modelling for the Main Tree Species in an Even-Aged Pedunculate Oak Forest

Abstract

The height to crown base (hcb) is a critical measure used in many investigations as an input variable to investigate the vigour of the stands, the social position of the trees, and to evaluate the behaviour of forest fires, among other uses. Though measuring height-related variables in the field is always time-consuming, the foremost benefits offered by modelling hcb are that it permits to generalize and average a very uneven attribute and, furthermore, provides insights about which tree and stand variables have a significant impact on hcb. However, there are many species in which models of the crown base height have not been developed in Croatia. The objective of this research was to develop a height to base crown model for each of the main species present in the two-layered mixed stands of this study. According to previous investigations, logistic models provide the highest precision and require the lowest inventory cost owing to less frequent measurements. Tree- and plot-level variables with distance-independent competition indexes were studied in the fitting model. In this research, we obtained models for the main stand species: Acer campestre (root mean squared error (RMSE) = 2.28 m, R2 = 82.80%); Alnus glutinosa (RMSE = 1.78 m, R2 = 85.36%); Carpinus betulus (RMSE = 2.47 m, R2 = 67.55%); Fraxinus angustifolia (RMSE = 2.46 m, R2 = 82.45%); Quercus robur (RMSE = 2.60 m, R2 = 80.57%); Tilia sp. (RMSE = 2.01 m, R 2 = 89.07%); and Ulmus laevis (RMSE = 1.71 m, R2 = 92.42%). The combination of the total height, tree, and plot-level variables with distance-independent competition indexes contributed to the prediction accuracy of proposed model significantly.

What Is the Most Suitable Height Range of ALS Point Cloud and LiDAR Metric for Understorey Analysis? A Study Case in a Mixed Deciduous Forest, Pokupsko Basin, Croatia

“…In addition to the tree height, the height from the bottom of the tree to the first branch (crown base) was measured for 63.9% of trees. Martín-García et al [44] hcb equations were applied to those trees without field measurements.…”

“…The tree height was measured for 65% of trees using a Vertex III hypsometer, while heights of the rest of the trees were estimated using the species-specific DBH-height models fitted with Michailloff's function [43]. More details of this process can be found in Martín-García et al [44]. In addition to the tree height, the height from the bottom of the tree to the first branch (crown base) was measured for 63.9% of trees.…”

“…In addition to the tree height, the height from the bottom of the tree to the first branch (crown base) was measured for 63.9% of trees. Martín-García et al [44] HCB equations were applied to those trees without field measurements. The second step, returning to the field to take understorey measurements, was conducted in November 2019 (yellow circles with an orange dot in Figure 1).…”