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.
Highlights
The objectives of this study were (i) to develop species-specific hcb logistic models for the main tree species in the mixed, lowland pedunculate oak (Quercus robur L.) forests in Central Croatia; and (ii) to study which variables best explain the variability of hcb for each species and how each group of variables affects the accuracy of the models
The forests of the study area consist of evenaged pedunculate oak stands mixed with other tree species: common hornbeam (Carpinus betulus L.), black alder (Alnus glutinosa (L.) Gaertn.), and narrow-leaved ash (Fraxinus angustifolia Vahl.)
The species-specific hcb models (Table 3) were developed for seven tree species (A. campestre, A. glutinosa, C. betulus, F. angustifolia, Q. robur, Tilia sp., and U. laevis) that had at least 20 trees measured in the field
Summary
Tree crowns are involved in many key physiological processes, such as photosynthesis (Ma et al 2017, Liu et al 2019), respiration (Plain et al 2009, Drake et al 2016), and transpiration (Köstner et al 1992, Gupta et al 2018), and they influence soil water storage (Meinzer et al 1999, Zhang et al 2007) and the interception of radiation (Stenberg et al 1994, Chen et al 2017), being the most important part of the tree.The crown structure affects the biomass productivity (Valentine et al 1994, Russell and Weiskittel 2011), stem form, and wood quality (Maguire et al 1991, Hein et al 2008) and individual tree competition status (Kershaw et al 1990, Cameron et al 2020). The crown structure includes decisive parameters to assess the crown fuel characteristics that influence potential crown fire behaviour (Cruz et al 2003, Ruiz-González and Álvarez-González 2011, Jiménez et al 2013). These metrics are critically important for informing forest management (Li et al 2020).
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