Height Growth and Site Index Models for Main Forest Forming Tree Species in Poland

The potential forest site productivity is a fundamental information for strategic decisions in fo est management. Site productivity is commonly estimated using the site index, which is determined based on the height at a given age estimated using site index models. Site index models are also used in forest growth predictions. A proper assessment of the site index is important for management, as it is the primary criterion considered when making site- and species-specific economic decisions on silvicultural treatments, selection of species composition, setting the cutting plan and rotation age. Recently, site index models have been developed for the main forest- forming tree species of Poland. However, in developing the parameters of the models, autocor- relation was not fully taken into account and, in addition, there were inaccuracies in the parameter values of the models. In connection with the planned implementation of the models into the State Forests IT system and their application to determine the volume increment of forests in Poland, it was considered desirable to improve the existing models. The aim of this study is to develop unified site index models for the main forest-forming species of Poland using the most up-to-date approach. Using data from 2 636 single tree growth series, we developed new site index models for eight forest tree species in Poland. The use of the dynamic GADA function and taking into account the autocorrelation of the residuals allowed a very good fitting of the models to the empirical data. Furthermore, the models developed meet the main requirements for modern site index models: (a) polymorphism, which would allow for possible differences in growth patterns due to variability in site conditions; (b) variable asymptotes for different sites; (c) the equality of site index and height at a given base age; and (d) the possibility of using the same function to model height growth and site index. The determination of site productivity is the main purpose of the developed models. However, thanks to the properties of the functions used for the development of the models, they can also be used as models for the prediction of the height growth of stands. This research is a response to the demand for the development of new yield tables (growth models) for the main forest forming tree species in Poland from the State Forests in Poland, the Forest Management Office and other stakeholders. The research presented in this paper is the first stage of the construction of new empirical growth models.