DIGITALNA ARHIVA ŠUMARSKOG LISTA
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ŠUMARSKI LIST 7-8/2021 str. 23     <-- 23 -->        PDF

tree silver fir rings were taken at a height of 0.1 m, 8 m and 16 m and then stratified on samples of biological material, that is, samples of bark and sapwood, and the growth rings with the time steps to the center of the trunk were set aside. On the sawn-off reel of a felled tree at a height of 0.1 m, rings were singled out in sections of two years for the period 1974 - 2003, in sections of four years for the period 1944 - 1973 and then one sample for the period from 1909 – 1943. The same collection dynamic is used for tree rings samples at altitudes of 8 and 16 meters, provided that at 4 meters the sample covered the period 1958 - 2003, and at a height of 16 meters the period 1978 - 2003. The main root and lateral roots (bark and wood) were sampled, and the group of samples were split according to diameter dimensions (<1 mm, 1-2 mm, 3-4 mm, 5-8 mm, 9-16 mm, 17 - 24 mm and 25 - 38 mm). Twigs (peak shoots) were sampled as individual annual samples for shoots that grew in the period 1994 - 2003 at heights of 8 m and 16 m, and at a height of 25 m in the period 1997 - 2003.
The second sampling on the silver fir tree was done during vegetation period on September 3, 2004. On the felled tree of 24.5 m height and 54 cm breast diameter, the rings were takes as in the first sampling, and then stratified on samples of biological material. On the reel of a felled tree, at a height of 0.1 m, rings were singled out in sections of five years each for the period 1945 - 2004, and in sections of ten years for the period 1903 - 1944. Samples were collected on the rings of a felled tree at heights of 8 and 16 meters at the same dynamics, with the period 1927 - 2004 being covered at eight meters, and the period 1960 - 2004 at a height of 16 meters. The main root and lateral roots (bark and wood), were sampled and separated in groups of samples as well as in the first sampling. Twigs (peak shoots) trees were sampled as individual annual samples for shoots that have grown in the period 1994 - 2004 at the height 8 m and 16 m, and a height of 25 m for the period 1996 – 2004. In the first and second sampling, along with peak shoots, samples of peak shoots needles were collected at the heights of 8 m, 16 m and 25 m.
Third sampling on the tree silver fir was conducted on March 27, 2017 for the purpose of comparing partial results after a long period since the last sampling (12 years). At Komarnica, the forest management unit, in subdivision 12a, felled the tree silver fir 26 m in height and 59 cm breast diameter, and the taken disc was then stratified on samples of biological material, that is, samples of the bark, bark and cambium, and annual rings with time intervals to the center of the trunk were singled out. The samples for this analysis were selected at 0.1 m height as the lowest level of the tree where the active life cycle takes place, and that is comparable to the first two samplings to determine the activity of 137Cs. In the period 1974 - 2016, the years were separated into sections of two years each, and for the period 1939 - 1973 in sections of five years, and one sample was separated for the period 1887 - 1938. Twigs (peak shoots) trees were sampled as individual annual samples for shoots that have grown at the height of 26 m in the period 2013 - 2016. Along with peak shoots, samples of peak shoots needles were collected. Composite soil samples were collected with felled trees in 2003, 2004 and 2017 for the analysis of 137Cs concentration in soils by gamma spectrometric method. For each analysis, approximately 1 kg of the average sample (taken from multiple points (5), homogenized, disrupted in a state) was collected.
Sample preparation and gamma spectrometric method – Priprema uzoraka i gamaspektrometrijska metoda
The activity of all samples was measured by gamma spectrometric method in the Laboratory for Radioecology, Division for Marine and Environmental Research, of the Ruđer Bošković Institute which is accredited for performed gamma spectrometric measurements since 2008, according to HRN EN ISO/IEC 17025. Before the measurements, all samples were homogenized, dried in an oven to constant mass, and then placed in the measuring geometry 125 cm3 volume, weighed and sealed. Canberra’s HPGe (High Purity Germanium) detector system was used for measurements, where each sample was measured for 80.000 seconds. The Genie 2000 program from the same manufacturer was used to analyze the recorded gamma spectra. The concentration of 137Cs activity in the samples was determined from the photo-peak at an energy of 661.6 keV, and the concentration of 40K activity from the photo-peak at an energy of 1460.7 keV. The reliability of the system was checked during intercomparison measurements regularly.
Since the analyzes showed that the composite uncertainty (U) is predominantly influenced by the uncertainty of the counting rate (Ubb), which also includes the uncertainty of the counting rate of the basic radiation, the uncertainty of determining the efficiency (Uef) and the uncertainty of determining the surface of photo-peak (Ufv - especially in the case of a small number of registered events) these parameters were taken for the calculation of the measurement uncertainty according to the relation:
                U2 = Ubb2 + Uef2 + Ufv2
Extended measurement uncertainties since 2008 were expressed with an overlap factor k = 2 (95% reliability of the results) as opposed to measurements performed until 2008, when measurement uncertainty calculations included only the uncertainty of determining the photo-peak area with an overlap factor k = 1 (68.27% reliability of the results). The measurement uncertainty was calculated as described above and shown in the results. At lower activities, the measurement uncertainty is high, but we consider the obtained results