A Mixed Effects Height-Diameter Model Based on Univariate Diffusion Process

Statistical models using stochastic differential equations to describe dynamical evolution of natural systems are appearing in the scientific literature with some regularity in recent years. In this paper, our aim is to describe how stochastic differential equations can be used to aid our understanding of the height-diameter process of an individual tree. The tree height distribution was examined by using a Vasicek type stochastic differential equation and mixed effects parameters. The drift function depends on random effects and a diffusion term without random effects. The parameters were estimated by considering discrete samples of the diameter and height of individual trees and by using a maximum likelihood procedure. Used dataset was provided by the Lithuanian National Forest Inventory (LNFI) (2006-2010) from Scots pine trees. All results are implemented in a symbolic algebra system MAPLEVytauto Didžiojo universitetasŽemės ūkio akademij