DIGITALNA ARHIVA ŠUMARSKOG LISTA
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Pedunculate oak forest management is complicated by the appearance of tree dieback and nowadays represents the most significant economic and environmental problem in Croatian forestry. The consequences are evident through large economic losses amounting to 40 % of potential market value of the timber, stand value decrease due to minor stocking and weakening of beneficial functions of forests.
Structure violation of the elderly and mature stands consequently causes a significant deviation from optimal, theoretical estimating models of timber quality. A more precise estimation of quality classes and stand structure quality and thus the value of its timber stock, allows efficient management planning towards primarily regeneration of forest (or stands) parts with most disrupted structure. In Croatia timber is traditionally classified by its purpose (e.g. Croatian standards HRN (1995)) as opposed to the newer standards (e.g. Croatian standards HRN EN 1361-1 (1999)) which are based on the Western European practice where timber is classified according to its quality, without prejudice of its purpose.
Previous research into the possibility of forming patterns and finding the best model to evaluate the quality class structure suggest the limited use of certain models due to stratification of the sample according to the level of tree species, management class, silvicultural forest type, age class, cutting type, the revenue type, share of dieback trees, etc. This paper explored the factors influencing the qualitative (quality class) structure of the remaining trees in elderly and mature pedunculate oak stands of central Posavina. Based on results of the qualitative structure variability, habitat and economic variables, structure models were constructed to estimate the volume of quality classes according to Croatian standards EN 1361-1.
Within the pedunculate oak management class in middle Posavina (size of 29 190 ha) randomly was chosen 37 stands older than 100 years with reduced stocking where was placed 146 circular plots with radius of 35 m. On average, the plots covered 5 pedunculate oak trees closest to the plot centre. According to Danhelovski method qualitative structure of selected observed trees was estimated and calculated. On the observed plots has been estimated a wide range of habitat, stand and economic variables (Table 1). Equalizing the volume of quality classes was conducted by quality formula (3) which is in previous studies (suitable function for equalizing the quality class volume depending on DHB) marked as suitable. Breast height diameter of mean tree (d_SPS) and the volume share of quality classes A and B (vABp) were chosen in this study as the dependent variables in the process of multivariable research influence of factor groups; stand, environmental and management factors by multifactorial analysis of variance.
The total volume of sample trees of 3 964 m3 mostly constitutes quality class D (30 %) while the remaining three quality classes are equally represented,(about 20 %) (Table 1). The complex influence of stands and habitat group variables to breast height diameter of mean tree is much less important than the economic variables group (Tables 3 and 4). As expected, site index and canopy density significantly affect the qualitative structure of pedunculate oak trees through the length of the trunk, height of first branch and other factors (Table 3). Identified arithmetic mean tree breast height diameter (SPS) of pedunculate oak (64 cm) and an average content of 35 % volume of A and B quality classes (Table 2) indicates the exceptional quality of the stands. Explanation of the volume variability for each quality class by function (3) is in the range of 92.1 % for quality class DO to 63.4 % for quality class C (Table 5).
Comparison of modelled and estimated quality class structure (Figure 2) indicates the acceptability of the selected models. The volume distribution per quality classes indicates homogeneity above DBH of 67.5 diameter class and pointing the fact that the oldest trees retain their quality (Figure 3) above DBH of 70 cm. The quality and tree value does not necessarily follow the stand value for the fact of constant dieback and sanitary cutting which declines stand stock and the volume proportion of pedunculate oak, and thus the value of the stands. In these circumstances it is necessary to find and isolate structurally maintained and stable forests parts where should be accumulated value increment in order to establish a balance at rotation period and improving the forest age structure.
Key words: Pedunculate oak, mature stands, stand structure, quality class structure