Stand structure and regeneration processes in high-elevation protection forests of Nízke Tatry Mts., Slovakia | Štruktúra porastov a regeneračné procesy vo vysokohorských ochranných lesoch Nízkych Tatier

The main goal of present study was the description of high-elevation forest structures regarding various levels of their differentiation, the quantification of basic factors that influence the regeneration processes and the proposal of silvicultural measures that would consider the specific features of the forests in high elevations. The object of the investigations were Norway spruce forest stands of upper montane and lower sub alpine zone in north-western part of Nízke Tatry Mts. With respect to main goal following partial tasks were defined: (i) to compare the surveyed stands in terms of the structure differentiation and to analyse the regeneration processes dynamics regarding the ability to secure the next generation; (ii) using the data acquired in investigated stands to propose a model of optimal structure for the high-elevation forest and to quantify its basic parameters; (iii) to assess the quantity and quality of the seed production, to analyse the initial phase (germination and establishment) of regeneration processes and to evaluate the influence of basic factors on its dynamics; and (iv) to quantify the impact of light conditions on height growth of natural regeneration. Investigated stands (compartments 545c and 542a, forest management unit Liptovská Osada) were located in the elevations 1140-1480 m a.s.l., on the slopes of 70-75%. Prevailing group of forets types was Sorbeto-Piceetum and the stands were dominated by Norway spruce (Picea abies (L.) Karst., 100%), with individual admixture of rowan (Sorbus aucuparia L.), mountain pine (Pinus mugo Turra), silver fir (Abies alba Mill.), and common beech (Fagus sylvatica L.). In compartment 545c, we established total eight permanent research plots (PRP) of 30 x 30 m in three altitudinal zones (under 1,300 m a.s.l., 1,300-1400 m a.s.l., above 1,400 m a.s.l.). The plots were chosen in the parts of the stand with the most differentiated structure that was close to the assumed model structure, or at least in the parts with the partly differentiated structure that represents the transition from the homogenous to the differentiated structure. In each PRP we measured all living trees of the dbh above 1 cm, standing and down deadwood as well as natural regeneration (from the seedlings to the saplings with the dbh of 1 cm). For all stems with the dbh above 1 cm we recorded tree species, dbh, height, crown base height, crown radius in four directions, and the location of the stem in the plot (coordinates x, y). For standing deadwood the tree species, dbh, height, and the location of the stem in the plot (coordinates x, y) were registered. Down dead wood was assessed by tree species, length, diameter in the half of the length, and decomposition degree (3 classes). The natural regeneration was distinguished by tree species in following categories: height category under 20 cm according to age - 1-year old, 2-years old, 3-years old, 4-years old, and 5-years old and older; height category above 20 cm according to height - 21-50cm, 51-80cm, 81-130cm, higher than 131 cm up to dbh=1 cm. Besides the basic dendrometric traits we calculated for each PRP the values of following structural indices: stand density index (REINEKE 1933), Gini coefficient (DIXON et al. 1987), coefficient of homogeneity (CAMINO 1976), evenness (SHANNON 1948), aggregation index (CLARK, EVANS 1954), diameter differentiation index (FÚLDNER 1995), structural complexity index (ZENNER, HIBBS 2000). In the altitudinal zones under 1,300 m a.s.l. and 1,300-1400 m a.s.l. we took core samples from 110 randomly selected stems with diameter above 35 cm, that were the objects of dendrochronological analysis. The cores were elaborated using the WinDendroTM software, whereby the age as well as the dynamics of height growth in the past was quantified. Based on these data we reconstructed the stem diameter in every year and we used it for the parametrization of the structure model. The seed production was recorded during three years (2003/04, 2004/05, and 2005/2006) in the compartments 545c (western slope) and 542a (southern slope). In each compartment in the altitude 1,300 m a.s.l., ten seed traps were placed in two parallel rows. The distance of rows was 10 m, the distance between seed traps was 15 m whereby one seed trap was under a canopy opening, one in the stand edge, and three in the stand interior. The size of each seed trap was 0.25 m2. In winter 2003/04 we collected the cones from 16 sample stems, the seeds were weighted and analysed by a germination test. To be able to investigate the impact of environmental factors on the initial phases of natural regeheration, we established seed trials in surveyed compartments 545c and 542a. The experiment was repeated during three vegetation periods (2003, 2004, 2005) in two different aspects (south, west) at four levels of direct radiation (0-0.2 h, 0.2-1 h, 1-2 h, 2 h) and with three replications, i.e. we carried out 72 seed trials in total. The light conditions were measured by the horizontoscope that was originally developed for the needs of silviculture in high-elevation forests and used in several studies so far (FREHNER 1989, 2002, DIACI 1997, BRANG 1996). The amount of light was expressed as the potential daily duration of direct radiation in June. Data for the analysis of growth processes of natural regeneration were acquired from the transects established in three altitudes (1,280 m, 1,350 m, and 1,400 m a.s.l.) in compartment 545c. In the 5 m wide stripes were recorded all Norway spruce saplings with the height from 20 cm to 2 m. For these individuals following characteristics were measured: height, root collar diameter, height increments of last five years, length of two lateral branches in youngest whorl, and the duration of direct radiation for each month. For each sapling we calculated the H factor (apical dominance ratio) as a ratio of height increment to mean length of lateral branches in the uppermost whorl.