Shade-tolerant tree species from temperate forests differ in their competitive abilities: A case study from Roztocze, south-eastern Poland

The objective of this study was to compare the relationship between the competition intensity and performance of three shade-tolerant species: European beech, silver fir and European hornbeam. The competition intensity was calculated using two different competition indices, and the tree performance was represented by diameter growth, height growth and mortality. Our research was based on data collected in the years 1993 and 2003 in four permanent sample plots of 0.5 ha each located in old-growth stands of the Roztocze National Park. The competition index, which included the height of a target tree relative to its neighbors, yielded higher correlations with the diameter and basal area increments than Hegyi's competition index for both hornbeam and fir, but the correlation was lower for beech. The importance of the height difference between the target silver fir and its competitors was also confirmed by a strong relationship between the competition intensity and the relative crown length, indicating that the increase was strongly related to the height advantage over neighbors. In contrast, beech appears to be a successful competitor even when shorter than its neighbors. Hornbeam, which was very abundant in a stage of advanced regeneration, was notably less numerous among the shorter trees, suggesting a limited recruitment to the tree layer. The growth rate of hornbeam was much lower than beech and silver fir and was relatively weakly related to the competition intensity: even the hornbeams relatively free of competition did not respond with vigorous growth. No relationship between the indices and height increases was found for the analyzed tree species. In the case of silver fir and hornbeam, the trees that died during the study had higher values of competition indices compared to the trees that survived; the opposite tendency was found for beech, yet the number of dead beeches was very low. The results suggest that, in mixed stands, hornbeam is a weaker competitor than beech, and the most likely explanation for its high abundance in these analyzed stands is the occurrence of natural disturbances, particularly strong winds. (c) 2012 Elsevier B.V. All rights reserved.