Abstract: The correct affiliation of the tree stands to productivity class, particularly the young, between 30-40 years old stands is hindered by difficulties due to highly variable height growth dynamics which is dependent upon a multitude of factors such as stand characteristics and site conditions. The determination of productivity class according to the average stand height in old growth managed stands was performed with certain amount of approximation causing the affiliation of the same stand to different productivity classes during its lifetime. Hence, stands of 30-40 years are affiliated with approximation to productivity classes due to multiple causes, all circumscribed to the accuracy of tree heights measurement. The combined influence of factors determining height growth leads to misclassifications within productivity class system. There are situations when stands, during their life cycle transit to different productivity classes, an inexplicable situation from the perspective of the growth dynamics under long-term more or less stable site conditions. Therefore, the present paper proposes indices based on the ratio between average tree height and basal area which enable the correct determination of the productivity class. The ratios were calculated for same tree age separately for the five consecrated productivity classes and regression equations were obtained for the corresponding productivity class for spruce (Picea abies) and fir (Abies alba). The paper aims to establish an index covering the link between growth and site conditions for several important and valuable tree species from the forestry fund. Data from tree and stand biometry tables were employed for heights and basal areas ratios which were calculated in accordance to productivity classes and age. That is why there are some inconvenient to establish very rigorous the productivity classes in the old system. The approach presented in the paper can be used as a starting point for a new and reliable system permitting the determination of the productivity classes .

Average tree height, basal area, productivity class, regression equations

Presentation: oral