Stejarul roșu (Quercus rubra L. syn. Q. borealis F. Michx.) la Zăbala-Covasna: studiu de caz [ Northern red oak (Quercus rubra L. syn. Q. borealis F. Michx.) in Zăbala-Covasna: A case-study]

A pure, 2 x 1 m (5,000 plants ha-1) plantation of northern red oak was established in the park of Mikes Castle (Zăbala, Covasna County) in the spring of 2012. The local ecological conditions (mean annual temperature 7.6 oC, mean annual rainfall 616 mm, brown acid soil) are of moderate fertility for northern red oak as well as for native forest vegetation (sessile oak-European beech mixed stands). A cleaning-respacing was carried out in the two R&D plots established in April-May 2019, which h ave shown some interesting characteristics prior to the intervention at age 7: (i) stocking: 4,692 trees ha-1 (plot 1) and 3,692 trees ha-1 (plot 2); (ii) mean arithmetic diameter: 4.84 cm (plot 1) and 5.01 cm (plot 2); over 40% of trees in both plots show diameters over 5 cm (mean diameter increment over 7 mm yr-1); (iii) quadratic mean diameter: 5.05 cm (plot 1) and 5.12 cm (plot 2); (iv) mean height: 6.48 m (plot 1) and 6.77 m (plot 2); over 40% of trees in both plots are over 7 m tall (mean height increment over 1 m yr-1); (v) height of mean diameter of basal area: 6.75 m (plot 1) and 6.88 m (plot 2). The cleaning-respacing intervention was mostly from below and of high (plot 1) and moderate (plot 2) intensity by number of trees as well as of moderate intensity (both plots) by basal area. The residual stand is still dense in both plots (e.g., stocking over 3,200 trees ha-1; basal area over 7 sq.m ha-1), with higher-than-initially mean diameters and mean heights and lower stability indices in both plots. The number of ”potential” final crop trees selected prior to cleaning-respacing based on the vigour-quality-spacing criteria was higher (538 trees ha-1 in plot 1,461 trees ha-1 in plot 2) than in other European countries (maximum 400 trees ha-1). As a part of these trees show forking at heights lower than 4 m, only 150-200 trees ha-1 of such trees will be selected during the pole stage, when the application of crop tree silviculture, targeting their free-growth state, is desired.