Abstract
The height-diameter curves of natural even-aged Hungarian oak, sessile oak and Turkey oak forests in the regions of Staro Oryahovo, Sherba, Tsonevo and Aytos Forestry Districts have been studied. In order to explore the shape of the curves and, in particular, their steepness, the steepness index SI of Douhovnikov (“the method of natural indicators”) was applied. The curves were divided into three groups according to it: flat curves, medium steep curves and steep curves. Both the traditional height-diameter curves and the corresponding relative curves of Tyurin were studied. Consideration of the traditional height-diameter curves gives a certain ground for investigating to-gether the height-diameter curves of Hungarian oak and sessile oak, disregarding the tree species. This inference was confirmed by comparing the relative height-diameter curves for Hungarian oak, sessile oak and Turkey oak with Tyurin’s uniform curve of relative heights. For these comparisons we used the Willcoxon test. A statistically significant great proximity in their shapes was found. As a uniform average curve of relative heights has been established for all tree species, this proximity indicates a possibility of creating a single fan of height-diameter curves for high Hungarian, sessile and Turkey oak.
Highlights
It is important to perfect permanently the normative-and-reference base of forestry and the models and tables for evaluating the volume of standing timber
According to the values of SI of the height-diameter curves were assigned to three groups, namely: (i) SI≤0.85 corresponded to steep curves; (ii) SI within the range of 0.86 ÷ 1.09 corresponded to medium-steep curves and (iii) SI≥1.1 - to flat curves
Three types of height-structure curves have been identified for the explored natural high forests of Hungarian oak, sessile oak and Turkey oak, and namely: steep, medium steep and flat curves
Summary
It is important to perfect permanently the normative-and-reference base of forestry and the models and tables for evaluating the volume of standing timber. By comparing the distribution of tree numbers by diameter in undisturbed pure simple even-aged stands, Tyurin (1938) came to the conclusion that the shape of these curves did not depend on tree species, site conditions nor stocking rate. What had a certain effect were age and thinning. This gave him the reason for developing a uniform curve of the percentage distribution of tree numbers and basal areas by diameter. Nedyalkov (1955) confirmed that the shape of the diameter distribution of beech trees in high stands depended on the mean diameter and, respectively, the age. While studying growing stocks of uneven-aged spruce trees, this same author (1967) established different curves of distribution for the particular generations. For the generations at different ages, the distribution curves had a bell shape but the general curve of the whole stand was described with an exponential function
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