Wpływ konkurencji na przyrost grubości drzew w wyżynnych lasach mieszanych o złożonej budowie

The aim of this study was to assess the effect of competition on diameter growth of European beech (Fagus sylvatica L.), silver fir (Abies alba Mill.) and European hornbeam (Carpinus betulus L.) in forests with complex structure. For each tree we calculated set of 20 competition indexes in 4 variants for determining the scale of the competitors influence. The use of such a set of indices, allowed to describe accurately the competition between trees in the specific conditions for the particular tree species. Given species belong to the group of shade−tolerant trees. The species composition of the tree stands and their structure are the results of long−lasting natural processes, which is different than in the majority of Polish forests. The empirical material collected in the years 1993 and 2003 in four sample plots of 0.5 ha each located in natural stands of the Roztocze National Park in south−eastern Poland. The relationships between the competition and the growth were analysed using generalized additive models. This method allows to extend of the standard linear model, by taking into account non−linear function of each variable. One of the option of quantifying competition processes is the use of competition indices. The competition situation of an individual tree was described using distance−independ and distance−depend indices. Each of analysed species reacts differently on competition. Among the three analyzed species, the best competition index CI15 explained 48.3% of the variation diameter growth of European beech (tab. 2). The competition index CI3 explained only 26.1% of the variation diameter growth of European hornbeam (tab. 2). It turned out to be the worst result for the described species. The competition index CI14 explained 31.1% of the variation diameter growth of silver fir (tab. 2). Additional use of DBH in the models, increased values of the coefficient of determination, and extended the possibilities to use the developed models.