Diameter distribution models and height-diameter equations for Estonian forests

To study the structure and growth of Estonian forest stands, a network of 501 permanent sample plots was established in 1995-2001. The plots were placed randomly using the grid of the European forest monitoring prograrn ICP FOREST. The network of permanent plots covers the main forest types and actual age interval of commercial forests. The plots are to be re-measured at 5-year intervals. The plots are circles holding a minimum of 100 trees each. On the plots, the polar coordinates and breast height diameters of all trees are measured. Additionally, the total height and crown length of selected sample trees are also measured. To date, the measurement data for 90,209 trees have been recorded in the database. The aim of the permanent plot network is to create and recalibrate Estonian forest-growth models. However, to achieve this, long-term measurement series on the permanent plots are still required. Nevertheless, on the basis of the existing material, some forest structure models have been tested and developed for Estonia, such as: (i) diameter distribution models and (ii) height-diameter models. The Johnson's SB distribution was flexible enough to describe the diameter distributions of Estonian forests. Regression methods of parameter estimation represented a better fit than percentile methods. A diameter distribution model, the parameters of which were predicted by stand variables, has been developed for Estonian forests. A standardized height-diameter equation has been created for Estonian forests. Depending on the number of height-diameter measurements, the equation can be used as a one-parameter or two-parameter model. Model parameters can be estimated by solving a system of linear equations.