Role of tree species in the formation of a complex vertical structure of deciduous old-growth forests in the Niepolomice Forest

The objective of the study was to compare the seven tree species in terms of their role in the formation of a complex vertical structure in mixed broadleaved stands. The data came from 18 multi-storey old-growth forests representing different communities of lowland deciduous forests in the Niepolomice Forest. In each stand, squared sample plot (80x80 m) was established. The height of trees of dbh >= 7 cm was measured. Then, for each studied species, values of the two structural indices were determined. The structural diversity index (ZS) quantifies tree height variation within individual population, whereas the index of structure-forming role (RS) expresses the contribution of the given population into vertical structural diversity of the whole stand. The greatest tendency to form a population with a large height diversity belonged to Tilia cordata, Fraxinus excelsior and Ulmus laevis. Significant opportunities in this regard, however, were exhibited by other tree species, including shade-intolerant Pinus sylvestris and Alnus glutinosa. This phenomenon in the conditions of eutrophic habitats, with the constantly present competitive pressure from shade-tolerant species, should be considered as an exceptional. A major contribution to vertical structural diversity of the stand was often an attribute of the populations with a low internal height diversity, which concerned especially Quercus robur. This means that an important ecological mechanism responsible for the formation of a multi-layer canopy is complementary filling the space above ground by trees of the species with different light requirements. However, the lack of correlation between RS and ZS indices suggests that the ability of particular species to form the population with a high internal diversity is equally important. The results prove the possibility of forming stands with a very complex vertical structure in the conditions of lowland mixed deciduous forests. However, in terms of sustainable maintenance of such stand structure, the issue of the growth rate of trees at different competitive pressure and the nature of their response to the release at different stages of ontogeny remains open.