DIGITALNA ARHIVA ŠUMARSKOG LISTA
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ŠUMARSKI LIST 3-4/1969 str. 8 <-- 8 --> PDF |
used during the previous inventories, because here we were concerned with even- aged stands. Experiments and calculations demonstrated that in this case it would be of no use changing the tariff for computation of the current increment in individual periods between two inventories. Hence I computed the increment on the basis of tariffs chosen at the beginning of the measurements. Consequently I calculated the current volume increment for the following time intervals: 1) period 1950—1955 (years: 1951, 1952, 1953, 1954, and 1955); 2) period 1955—1961 (vears: 1956, 1957, 1958, 1959, 1960, and 1961); 3) period 1961—1967 (years: 1962, 1963, 1964, 1965, 1966, and 1967). The first time interval or control period lasted 5 years, the second and third 6 years each, which totals 17 years from the time I had started these measurements and investigations. The calculation of increment was carried out also for the whole time interval 1950—1967 taking into consideration only the first and fourth inventories. This computation too was performed on the basis of the same tariffs, although for the calculation of the fourth inventory it would have been convenient to use a little higher tariff. But as the differences in the results are not significant, I decided to retain also in thisheight curve« in the stands case the same invest tariffs, becauseigated. of weak »displacement of the Results The results concerning the current annual volume increment are classified in Table 1- From Table 2 the fluctuation of the annual volume increment of Pedunculate Oak in the periods investigated may be seen. Table 2 Period lrst plot 3rd plot 4th plot 6th plot investigated (Compt. 157) (Compt. 155) (Compt. 155) (Compt. 165) Years Current annual volume increment, cu.m. per ha 1950—1955 8,4 8,6 10,9 9,3 1955—1961 6,3 8,5 8,9 6,8 1961—1967 4,9 4,2 5,2 5,5 1950—1967 (Average) 6,4 7,0 8,2 7,0 It should be stated that the average value lor the time interval 1950—1967 was not computed as an arithmetical mean of the measured increments in individual periods, but was obtained as the difference between the fourht and first inventories. Discussion The results obtained show clearly that the current annual volume increment of Oak stands in the time interval 1961—1967 had dropped by about 50 V« if compared with the same increment ten years ago. In order to verify this, I made according to the control method a special calculation of the diameter increment and came to the same conclusion: The annual diameter increment in the period 1961—1967, viz. in the last six years, also experienced a drop of about 50"/» in comparison with the diameter increment in the preceding period 1950—1967. For the sake of safety I carried out supplementary tests, so that by means of increment borer I took increment cores from the neighbouring compartments of the same management unit. An analysis of these increment cores showed more or less the same tendency to increment drop. What is this rapid drop of increment in our best Oako forests to be accounted for? Among the various factors which have caused this loss in increment I should point in the first place to the Gipsy Moth and other insect pests which for quite a time have been attacking more or less severily these forests, and in association with certain fungi in definite circumstances have caused the die-back of Oak stands. Second, I should mention the climatic, edaphic and silvicultural factors which most probably produced favourable preconditions for various atacks on Oak forests and thus brought about a serious reduction of the increment. |