Knowledge on forest productivity is crucial for forest management planning. Related to that, it is important to understand the functioning of forest ecosystems, as changes due to global warming are expected to change the productivity of forest stands. Therefore, the impact of forest mixing on tree and stand volume growth in pure and mixed Sessile oak (Quercus petraea)-European beech (Fagus sylvatica) forests in the forest management units Kras II and Brkini I was analysed. The analysis of volume growth was based on the database of 66 permanent sample plots of the Slovenian Forest Service. The highest average annual tree volume increment was found in pure beech stands, the lowest in pure sessile oak stands, while trees in mixed beech-sessile oak stands exhibited higher volume increments than trees in pure sessile oak stands. On a stand level, the volume increment of pure beech stands amounted higher than that of pure sessile oak stands (7,90 and 3,65 m3/ha/y, respectively). Against expectations mixed Sessile oak-European beech stands experiences lower stand volume increments than pure beech stands, but higher than pure sessile oak stands. The mutual effect of mixing of the two tree species considered was found to be positive, as the best-fitting regression function was a positive quadratic function. Mixing with sessile oak exhibited a negative effect on the volume increment of beech trees, whereas beech exhibited a positive effect on the volume increment of oak trees.
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