Monitoring of the growth of Erythrophleum fordii and analysis of the effect of phosphorus application based on binocular vision.

Tree growth monitoring provides a basis for understanding tree nutritional needs, and formulating and optimizing scientific fertilization strategies, and is essential for realizing precision forestry. Computer vision can be used to reconstruct the three-dimensional geometric structure of trees from two-dimensional images, which is highly beneficial for managing and planning tree growth. Based on a binocular vision system, this study proposes an approach for monitoring the growth of Erythrophleum fordii and analyzing the effects of phosphorus application. The MicaSense RedEdge TM3 multispectral camera was used to obtain images of E. fordii, and the initial parameters of the camera were obtained through the Zhang Zhengyou calibration method (ZCM) and optimized using the bundle adjustment method (BA). A multi-directional semi-global stereo matching (MD-SGBM) algorithm is proposed and applied to obtain disparity information, and E. fordii is reconstructed based on the triangulation principle. The DBSCAN algorithm and principal component analysis (PCA) were used to denoise and correct the direction of the acquired three-dimensional point cloud, and then extract the tree height, crown width, and ground diameter growth. On this basis, a one-way analysis of variance was used to compare the growth differences of E. fordii under different phosphorus application levels. The results show that the combination of the beam adjustment method and Zhang Zhengyou calibration method can significantly reduce the camera reprojection error and improve the camera calibration accuracy. Compared with the actual measured values, the MAE of tree height, crown width, and ground diameter growth measured by the system were 1.315 cm, 2.128 cm, and 0.453 mm, respectively, and the RMSE were 1.561 cm, 2.520 cm, and 0.575 mm, respectively. In this experiment, low, medium, and high phosphorus treatments (5, 10, and 20 g/tree, respectively) did not significantly promote the height growth of E. fordii. Medium phosphorus treatment (10 g/tree) significantly promoted crown width and ground diameter growth, while under high phosphorus treatment (20 g/tree), the growth rates of both crown width and ground diameter began to decrease.