Succession processes and development of the stand structure of a 161-year-old Norway spruce plantation under regime without silvicultural treatment

Background and Purpose: Regeneration and conversion of pure spruce plantations is a very important task for forest management in Europe. This subject is less marked in Croatia in view of the fact that in Croatia there are only 75, 000 ha of pure coniferous plantations, which are raised on unstocked forest land. In Croatia, no investigation has yet been carried out on the development of the stand structure, succession processes of autochthonous species and methods of reforestation and conversion of spruce plantations. This study is one of the first which deals with the aforementioned problems. Materials and Methods: Since 1956 five successive measurements (d b h and tree heights, tree-crown diameters, height of crown base, condition and position of a tree, horizontal crown projections, density and structure of regeneration, comparison of chemical and physical characteristics of soils between the plantation, natural stand and meadow) have been performed in a 161-year-old spruce plantation on a permanent experimental plot (1 ha), in which no silvicultural treatments were carried out during the last 42 years. Results and discussion: The period of the last 42 years, during which there were no management interventions in the stand, is characterised by the specific development of the stand structure. The fact is that in the old Norway spruce plantation there was a considerable potential for succession of autochthonous European beech and silver fir, at the time of the first measurement in 1956. However, in spite of two thinnings performed at the beginning of the 48-year long period (66 Norway spruce trees per ha) and 110 naturally cleared (removed) spruce trees during the last 42 years (self-thinning), the processes of beech and fir succession were very slow. Conclusions: The results obtained of slowed down development of the stand structure of an old spruce plantation, and slowed down succession processes, related to the absence of management interventions and delayed regeneration, are discussed in accordance with the results of spruce plantation management in Central Europe. Further investigations are necessary of the old spruce stand in two directions ; under regime without silvicultural treatment (natural development) and under regime by different silvicultural methods.