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In order to fully understand the objective law of height and DBH growth of loblolly pine trees and exploring the best DBH (Diameter at Breast Height) Range for loblolly pine tree height growth, 13 340 loblolly pines with initial DBH between 1 inch and 7 inch were selected from Alabama as research objects, and statistics on its growth from 2000 to 2015. Because particle swarm optimization (PSO) is suitable for solving non-linear problems, the optimal DBH of loblolly pine is transformed into the optimization problem of PSO, which quantifies the optimal DBH range of loblolly pine at different scales by mapping strategy. The experimental results show that the range of the breast diameter suitable for the high growth of the pine tree is concentrated between 3.7 inch and 7.3 inch. The height of the pine tree begins to enter a period of rapid growth from a breast diameter of 3.9 inch (±0.2 inch ). The tree height growth rate reached a maximum at a breast diameter of 6.4 inch (±0.6 inch ), and the tree height entered a slow growth period after the breast diameter of 11.92 inch (±0.3 inch). In general, when the breast diameter exceeds 15.26 inch (±0.3 inch), the height of the pine tree stops growing. [ABSTRACT FROM AUTHOR]