STRUCTURE DEVELOPMENT AND POSSIBILITY OF NATURAL REFORESTATION IN THE "MEDVJEĐAK" FOREST RESERVATION WITHIN THE PLITVICE LAKES NATIONAL PARK

Abstract

UDK 630*231:712.2(1-751.2)(497.5)

Beech forests are spread over an area of 1 100 000 ha, which amounts to 56.5% of the area of forests in Croatia. Pure beech forests are spread over 250 000 ha, beech forests with mixed Sessile-flowered oak over 700 000 ha, and mixed forests of beech and fir cover an area of 165 000 ha. Of the total area of beech forests 85% are high silvicultural form forests, developed from seed, and the remaining 15% are forests of low silvicultural type or coppice forests. Natural regeneration by seed tree cuttings on small areas with a longer regeneration period is the optimal method of reforestation for these forests, where, together with appropriate tending of the forest, general benefits and management functions are fully expressed (Matić et al, 1996). The latest investigation (Krejči and Dubravac 2003) indicates the justifiability  of applying even-aged method of management by seed tree cuttings in pure beech stands. The paper presents the results of investigations of the condition of the structure and possibility of natural regeneration of pure beech stands in a secondary virgin forest of beech in the forest reservation "Medveđak". The reservation is located within a large forest complex of beech forests in the northeastern part of the Plitvice Lakes National Park. The investigations were carried out in a natural stand of mountain beech forest with dead nettle (Lamio orvale-Fagetum sylvaticae Ht. 1938). The reservation was established by the Department of Forest Typology, Forest Research Institute Jastrebarsko in 1976, on an area of 156.3 ha, covering three compartments (26, 27 and 61). The investigations were performed on two permanent experimental plots, 1 ha in size. The first plot was established in 1980 by the Faculty of Forestry University of Zagreb within the framework of the Project "One hundred permanent plots in the Republic of Croatia", and the second by the Forest Research Institute Jastrebarsko in 1998, within the framework of the Project "Growth and development of the forests for specific assignment".

On the plot established by the Forest Research Institute Jastrebarsko all trees larger than 7.5 cm in diameter were numbered and basic estimation data recorded (d b h, tree height and length of stems) on the basis of which basic structural indicators were calculated (number of trees, basal area and volume). A sub-plot of 60 x 60 m was established in the most homogeneous part of the plot. Horizontal projections of crowns were recorded for all trees on the basis of which calculations were made of the sum of crown projections per ha in absolute and relative size, soil not covered by crowns, mean projection and mean crown diameter. A digital dynamic model of crown projections and vertical stand profile were produced. For the purpose of monitoring the possibility of natural reforestation on the sub-plot, the structure and number of young growth was recorded, including the shrub layer, on an area of 360 m² (three strips of 2 x 60 m), and classified into height-age classes. Estimation of crown damage was performed by the method prescribed by the International Programme for Estimation and Monitoring of Air Pollution for Forests (ICP Forests).

Results indicate an excessive volume of wood of 647.38 m³, basal area of 42.67 m² per ha accumulated on a small number of trees (301), which reflects the natural development course of these stands. The number, quality and vitality of beech young growth on the observed experimental plot of 3 528 plants per hectare, do not guarantee further development of the expected normal process of natural reforestation. Namely, the majority of young growth is in the height class up to 30 cm (43 %) and up to 60 cm (39 %) amounting to 82 % of the overall number of plants, and due to the heavy density of canopy cover over soil of 96 %, survival is questionable. Somewhat increasingly more present sycamore young growth, in particular shrubs, is unfavourable for beech natural reforestation. According to the measurements of crown widths from the digital model of horizontal crown projections, on the assumption that the tree with the widest crown dies, which amounts to 14.2 m, a canopy of 158.36 m² would be opened. (Table 1), which is insufficient for the development of high-quality new growth of beech, which according to its biology requires reforestation on larger areas.

The results of the tree-crown damage show that in 1998 there were 5.6 % considerably damaged trees (damage of over 25 %), while there were three times more (17.1 %) trees with such damage during the second observation (2003). The increase in considerably damaged trees in 2003 can be attributed to the great share of trees in the third layer which, during their natural development, were isolated in the stand. Results show that when estimating damage in tree crowns the biological position of the trees in the stand should be taken into account, and that monitoring on such permanent experimental plots should be of a continuous character. The average state of significant damage (crown defoliation more than 25%) for Common beech in Croatia during the last 9 years of monitoring has not changed essentially; significant damage ranged from 4.4 - 11.9%, and for beech it was 9.5% in 2003 and continued to retain the name of the most resistant tree species in Croatia.

Measurement results of zero state (1980) and repeated measurements (1988) on the plot in Compartment 27, established by the Faculty of Forestry University of Zagreb, indicate that the distribution of d b h   continues to be of uni-model character of right symmetry. Namely, distribution of heights in both measurements indicates a negative "skošenost" ( leaning, tilting) (β₁ = - 1.108 and - 1.511) and positive "spljoštenost" (damping off, flatness), which also shows that it is an even-aged stand of Common beech. The number of trees decreased by 29 trees (6.5%). The volume increment of beech trees (V₁₉₈₈ - V₁₉₈₀) for the same period amounted to 60.60 m³. Namely, current annual increment in volume (i_v) amounted 7.58 m³ (1.56%).

Pedologically the region is inhomogeneous because it includes various types of soil. The soils are of different productive potentials, which primarily depend on their depth, mechanical composition, nutrients and content of moisture in the soil. The most productive soils are in the region of sink-holes (luvisols), followed by calcicambisols and finally calcimelanosols.

Investigations which are carried out on both plots are the basis for long-term multidisciplinary monitoring of the progression of the natural course of development of structural changes in Common beech stands of virgin type, in accordance with the scientific research plans of the Forest Research Institute Jastrebarsko and the Faculty of Forestry, University of Zagreb.