South-East European Forestry (Jun 2013)
Spatial Structure Indices of Mature Pedunculate Oak Stands in NW Croatia
Abstract
Background and Purpose: In order to potentiate a valid comparison of forest stands, numerous indices were developed to express forest structure numerically. Each of those indices described a specific measured or calculated value. In the present study, three of the stand structure indicators, dependent on tree distance, were used: the aggregation index of Clark and Evans, the species mingling index and the diameter differentiation index. The objectives of this study were: to obtain further information about forest structure using the selected indices and to discover any limitations that the implemented indices might display. Materials and Methods: Mature pedunculate oak stands were selected as objects of the study, all located within the “Repaš – Gabajeva Greda” forest management unit, the Forest Administration of Koprivnica. The stands were aged 75 to 132 years. A systematic 500 m grid of 45 circle sample plots was established. The sample plot radius was 15, 25 or 30 meters, depending on the stand’s age. In 2001, the DBH (diameter at breast height) and tree positions in regard to the centre of a plot were measured on each sample plot. The mutual distances between trees were calculated, as well as the values of the three selected stand structure indices. The two procedures of the aggregation index of Clark and Evans were calculated for all 45 sample plots. In the first case only the pedunculate oak trees were observed, and in the other all trees on the plot. The species mingling index and the diameter differentiation index were calculated for each tree in two procedures: in relatio to three and four nearest neighbouring trees. The plot/stand totals were managed as the average index of individual trees. Results: Values of the aggregation index of Clark and Evans after all trees have been observed were from 0.89 to 1.28, which indicated a random distribution of trees. In case of considering only pedunculate oak trees, the index of the plots was 1.02 to 1.51. This indicated a tendency to a regular distribution of trees. The species mingling index that considered three nearest neighbouring trees was 0.395 to 0.620. When observing four nearest neighbouring trees, the index values showed slight decreases in all age classes. The diameter differentiation index (0.407-0.424) when three or four nearest neighbour trees were taken into account, showed almost the same differentiation in the 5th, 6th and 7th age class. Conclusion: Though they contained the component of distance between the trees and were based on individual trees parameters, the three analysed stand structure indicators were a useful tool in further categorizing of forest stands.
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