Site index of common oak (Quercus robur L.) under the canopy of Scots pine stands

The aim of the study was to compare the site index of common oak growing under the canopy of Scots pine stands with the values typical for this species on a given habitat. The research was carried out in 20 Scots pine stands with well−developed lower layers of common oak occurring in the fresh mixed forest habitat. The analyzed stands were located in 5 forest districts in central and north−eastern Poland. To determine the real age of oaks and their height, 5 under canopy common oaks from the group of the highest ones were selected and felled. The total sample size was therefore 100 trees. Cut trees were divided into sections of 1 m in length, what gave the greatest accuracy to determine the dendrometric parameters. From the middle of each section a sample for testing was taken. It was found that the oak undergrowth growing about 30−40 years under the pine canopy on a fresh mixed forest habitat, usually achieved II−III site index class, while on a mixed fresh coniferous forest habitat it was III−IV class. These site index classes are typical for the stands growing in full access of light, so no adverse changes in site index class were observed as a result of the pine canopy occurrence for 30−40 years (tab.). Oaks growing for more than 40 years under the canopy of pine obtained worse site index class than the average for a given habitat, so the complete exposure of undergrowth for the light should not be extend for too long in time. Common oaks responded with increased height to full exposure. This reaction was definitely more pronounced for the younger oaks at age of 30 than for the older ones at age of 70. The height growth curves of under−canopy common oaks differed from the model curves according to Bruchwald et al. [1996] and stand volume and increment tables [Szymkiewicz 1962], especially in the initial (juvenile) phase, which indicate the occurrence of differences in growth under the pine canopy and without cover (figs. 2 and 3)