Moderate Diversity in Forest Structure and Its Low Dynamics Are Favored by Uneven-Aged Silviculture—The Lesson from Medium-Term Experiment

For several decades, there has been growing interest in highly diversified forests as a result of their management using the so-called uneven-aged silviculture. Such management and forests resulting from its application fulfil the purpose of forest ecosystem protection without significant limitation of the other forest functions and services. The usefulness of such silvicultural system depends largely on the species composition of the stand. In Poland, this system has been used primarily in silver fir (Abies alba Mill.)-dominated forests with an admixture of other shade-tolerant tree species and rather small admixture of light-demanding species. In our study, we analyzed the structure of fir-dominated uneven-aged forest consisting of a certain mix of eight tree species with different light-requirements, in which uneven-aged silviculture was performed for over two decades. The analysis of stand structure concerned three aspects of the forest structure: species diversity, spatial pattern of tree distribution, and tree size diversity. On the basis of three stand inventories, we captured the mid-term dynamics of the stand structure under moderate human pressure conditions. The results showed that the stand was characterized by high growing stock, with stable growth over the entire period. The silver fir was the dominant tree species, but the Scots pine and pedunculate oak also played an important role. However, the silver fir was abundant in the ingrowth class, whereas the pine and oak were absent or very rare in this stand strata. Noteworthy is the increasing importance of the European hornbeam, which is particularly evident in the ingrowth class; its abundance was twice that of the silver fir. The results clearly demonstrated that within 24 years, the share of light-demanding tree species in the forest clearly decreased, while that of shade-tolerant species clearly increased. Structural parameters showed a moderate tree-size diversity, despite the high species richness of the forest. This was mainly the result of selection cuttings, which clearly decreased tree-size variability after the second period of study. However, selection cuttings did not favor regeneration establishment, especially that of light-demanding tree species, whose role in the stand is assumed to be only minor in the future. The relatively high level of volume growth indicated that cutting intensity may be increased. More intense selection cuttings would also improve the stand structure by providing better light conditions for potential regeneration of different tree species, also light-adapted ones.