Development of Mathematical Models for the Estimation of Dendrometric Variables Based on Unmanned Aerial Vehicle Optical Data: A Romanian Case Study

Abstract

Research highlights: In this study, the possibility of developing predictive models for both individual trees and forest stands, based on information derived from digital surface models (DSMs), was evaluated. Background and objectives: Unmanned aerial systems (UASs) make it possible to obtain digital images with increased spectral and spatial resolution at a lower cost. Based on the variables extracted by means of the digital representation of surfaces, we aimed at generating mathematical models that would allow the prediction of the main biometric features of both individual trees and forest stands. Materials and methods: Forest stands are characterized by various structures. As such, measurements may address upper-level trees, but most often are oriented towards those belonging to the mean tree category, randomly selected from those identifiable from digital models. In the case of grouped trees, it is the best practice to measure the projected area of the entire canopy. Tree and stand volumes can be determined using models based on features measured in UAS-derived digital models. For the current study, 170-year-old mixed sessile oak stands were examined. Results: Mathematical models were developed based on variables (i.e., crown diameter and tree height) extracted from digital models. In this way, we obtained results characterized by root mean square error (RMSE) values of 18.37% for crown diameter, 10.95% for tree height, and 8.70% for volume. The simplified process allowed for the estimates of the stand volume using crown diameter or diameter at breast height, producing results with RMSE values of 9%. Conclusions: The accuracy of the evaluation of the main biometric features depends on the structural complexity of the studied plots, and on the quality of the DSM. In turn, this leads to the necessity to parametrize the used models in such a manner that can explain the variation induced by the stand structure.

Using Mathematical Models Based On Unmanned Aerial Vehicle Optical Imagery to Estimate Tree and Stand Characteristics

“…In the 110-year-old sessile oak stands, the field measurements were carried out in two sampling plots of 1.0 ha each (100x100). On these plots the models that estimate the dendrometric characteristics of the trees (d, h, and v) were applied depending on their cw, and the models were developed for 170year-old sessile oak stands [20]. To generate these models, two sampling plots of 0.5 ha each (50x100) were placed in the respective stands [20].…”

“…On these plots the models that estimate the dendrometric characteristics of the trees (d, h, and v) were applied depending on their cw, and the models were developed for 170year-old sessile oak stands [20]. To generate these models, two sampling plots of 0.5 ha each (50x100) were placed in the respective stands [20]. In these stands, in a representative sample of trees from the plots, the coordinates (X and Y) were determined by surveying methods using the Leica TC 407 total station (the sample was established to ensure a measurement error of the volume of maximum 10% for a 95% probability coverage).…”

“…However, UAV-SfM data could be used as a complement rather than as an alternative to LiDAR data [9]. As a result, stand structure remains one of the factors that can influence the choice of equipment for recording information, as well as the accuracy of the results [20].…”

“…Relations of competition and preference are established among trees, which provide specific features to each structure. Characteristics such as tree diameter, tree height or crown size are variable in relation to the structural conditions of the stands [20]. If structure shapes the size and form of the trees, the mathematical models based on measurements done on digital models (derived from UAVs) best characterize the structures for which they were generated or structures similar to them.…”

“…The aim was to obtain mathematical models based on the tree crown diameter, which can be determined by measurements on the digital model. The starting point for this study is the research conducted in 170-year-old sessile oak stands based on measurements made on a digital model [20]. The current study represents a continuation of that research.…”