Diffusion process analogy-based models can be used to forecast the individual tree and whole stand mean volume, considering the variability and uncertainty associated with regional, operational, and environmental factors. Diffusion processes of the von Bertalanffy-type, Vacicek-type, and Gompertz-type can capture a forest stand pattern, which represents the spatial arrangement and distribution of trees within a specific area of a forest, dynamic fluctuations, and complex uncertainties. The Maple symbolic algebra system was used in implementing the model. Utilizing a sigmoidal tendency for the mean tree size attribute values, this study proposes a bivariate system of the mixed effect parameters Gomprtz-type bivariate diffusion process to quantify the changing patterns of the bivariate distribution of tree size components (diameter at breast height, and height) against the age in a stand. The created model shows that the mean volume evolution of trees in a forest stand can be continuously monitored and described as the trees age over time. A mixed-species, uneven-aged empirical dataset of measurements in Lithuania is used to demonstrate theoretical findings.