SHELTERWOOD REGENERATION OF SCOTS PINE (PINUS SYLVESTRIS L.) WITH RESPECT TO SITE AND STAND CONDITIONS

The aim of the study was to evaluate the impact of micro-site conditions on natural regeneration prosperity and quality of Scots pine (Pinus sylvestris L.) in shelterwood regeneration system. Data were collected in four localities across the Czech Republic in 26 forest stands. In each forest stand with pine natural regeneration a square 0.25 ha research plot was established. We evaluated pine regeneration density, dendrometric characteristics (height, diameter of root collar, increment in last five years, crown diameter), quality characteristics including angle of branching and silvicultural value of each individual. As independent explanatory variables we measured light conditions based on hemispheric image analysis, ground vegetation characteristics such as composition and coverage, thickness of surface humus horizons and competition expressed as distance to the nearest individuals of natural regeneration and to the nearest mature tree. Results showed high variability in natural regeneration density (for seedlings higher than 25 cm), its mean values amounted to 4,285 (Hradec) - 18,570 (Plasy) pc.ha(-1) with 75% quantile of values 5,714 - 30,000 pc.ha(-1). Number of regeneration individuals was significantly influenced by light conditions (the openness and diffuse PPFD (photosynthetic photon flux density) and by ground vegetation characteristics, especially the cover of mosses. The dendrometric and growth characteristics (height, diameter, increment, and crown diameter) were significantly influenced by intraspecific competition, but relative values of both the increment and crown diameter depended also on the diffuse PPFD. Relative height increment between 10% and 15% was reached at values higher than 2 MJ-m(-2) day of diffuse PPFD corresponding to stand basal area below 25 m(2).ha(-1). The best quality individuals (with straight stem and regular crown) occurred mainly in the upper height class of the natural regeneration. In Central Europe, regeneration of Scots pine stands is traditionally achieved after clear-cut, but in some cases there is also a possibility to use shelterwood regeneration method. Data were collected in four localities (Trebori, Plasy, Hradec and Doksy) across the Czech Republic in 26 forest stands covering the altitude of 270-570 m a.s.l. with average yearly temperature 8-8.5 degrees C, and average yearly precipitation 550-670 mm (Tab. 1). Mostly acidic and nutrient poor soils were represented. Selected forest stands were characterised by the age of 90-140 years, canopy 50-80%, and presence of young natural pine regeneration in the understory. In each forest stand a square 0.25 ha research plot was established. Consequently, 5 circular (diameter 3 m) subplots were selected in regular pattern within each research plot. Further we evaluated pine regeneration density, dendrometric characteristics (height, diameter of root collar, increment in last five years, crown diameter), quality characteristics including angle of branching (2nd whorl, branch angle with horizontal level) and silviculture value of each individual (1 - being the highest value, 4 - the lowest). As independent explanatory variables we measured light conditions based on hemis-pheric image analysis (openness, direct, diffuse and total photosynthetic photon flux density /PPFD/), ground vegetation characteristics such as composition and coverage, thickness of surface humus horizons and competition expressed as a distance to the nearest individuals of natural regeneration and to the nearest mature tree. We used Canon Eos camera with fisheye lenses, Photoshop 7 to adjust photographs regime to black and white and WinSCANOPY 2012 software to light characteristics calculation. For statistical analyses we used R software: Kolmogrov-Smirnov was used to test normal distribution; logarithmic and angular data transfor-mation was performed for values without normal distribution. Linear-mixed-effects model with test REML, nlme library, was used for data evaluation (PINHEnto et al. 2012). Basic dendrometric characteristics were evaluated considering not only the locality, but also height class of regeneration (class 1: > 1m; class 2: 1-2 m; class 3: > 2 m). Results showed high variability in natural regeneration density (for seedlings higher than 25 cm), its mean values amounted up to 4285-18570 pc.ha 1 and maximum value 40,000 pc.ha(-1) (Fig. 1). There was significant difference in basic dendrometric characteristics between some of the plots and localities (Fig. 2), also for the three height classes of natural regeneration (Fig. 3). Number of regeneration individuals was significantly influenced by light conditions (the openness and diffuse PPFD (photosynthetic photon flux density) and by ground vegetation characteristics, especially the cover of mosses. Relation between environmental variables and regeneration characteristics is shown in Tab. 2. Dendrometric and growth characteristics (height, diameter, increment and crown diameter) were significantly influenced (Tab. 3) by intraspecific competition, but relative values of both the increment and crown diameter depended also on the diffuse PPFD. Relative height increment between 10% and 15% (Fig. 4), was reached at values higher than 2 MJ.m(-2).day of diffuse PPFD corresponding to stand basal area below 25 m(2).ha(-1). The quality of individuals (Fig. 5) depends also on diffuse PPFD, but can be strongly influenced by competition. Amount of direct radiation also correlates with the angle of branches in the 2nd whorl. The horizontally oriented branches generally indicate low amount of direct PPFD. The best quality individuals (with straight stem and regular crown) occurred mainly in the upper height class of the natural regeneration (Fig. 6).