DIGITALNA ARHIVA ŠUMARSKOG LISTA
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ŠUMARSKI LIST 7-8/2021 str. 26     <-- 26 -->        PDF

less overshadowing impact of their own and other crowns, resulting in more radiated solar energy and higher intensity of photosynthesis).
Distribution of 137Cs in the annual rings of silver fir – Distribucija 137Cs u godovima obične jele
Figure 4 shows the distribution of 137Cs above the limit of detection in all sampled annual rings of silver fir trees at heights of 0.1, 8 and 16 m for samples from 2003. It is noted that in the annual rings of silver fir at all altitudes present much greater variability of 137Cs activity that is measurable way back in the past, reaching to the very center of the three rings (one on each level) from the fir tree felled for this survey in 2003, which was, as well as two trees felled in 2004 and 2017, more than hundred years old.
These long time series of radioactivity values can be divided into four intervals based on a graphic insight into the measured data (Figure 4):
1) In the period from the beginning of life trees from 1910 to 1943, constant value of 137Cs activity of 28.4 ± 0,9 Bq/kg (both in terms of the radius of the coil and in terms of the height of the tree) was measured.
2) The period 1944 to 1958, almost constant (in terms of the radius of the disc and the height of the tree) 137Cs activity was observed, varying from 22.7 ± 0.8 Bq/kg to 23.3 ± 0.9 Bq/kg,
3) For the period 1959 to approximately 1983, variation of 137Cs activity was also recorded in tree rings (without obvious regularity with regard to the year and the height of sampling), in the interval between 14.3 ± 0.7 Bq/kg, (16 m; 1982) to 36.3 ± 1.2 Bq/kg (8 m; 1974),
4) For the period from approximately 1984 to 2003, in which, a continuous increase in 137Cs activity was recorded in the youngest rings.
Based on the same results, it could even be assumed that the old xylem tissues that no longer participate in the flow from the root to the canopy represent a kind of accumulation reservoir of 137Cs, leaving the question of whether 137Cs in these (mostly dead) tissues is permanently excluded from migration within the plant and “captured” (but can disappear only by radioactive decay) or it is a temporary seasonal accumulation during the winter period (in which case, the constancy of the recorded values could explain the assumed constant capacity of dead wood). Additionally, given the existence of zone 3. in the dendrochronological sequence of fir, it can be assumed that the transition between the physiologically active and physiologically inactive part of the xylem in felled fir was very gradual (extending to more than thirty years of the dendrochronological sequence), and moreover, anatomically non-homogeneous (due to the variability of the recorded disc height sampling).
Figure 5 shows the distribution of 137Cs activity above the detection limit in all sampled annual rings of silver fir trees at heights of 0.1, 8 and 16 m for samples from 2004.
Graphic insight into the measured data shows the following:
1. Measurable 137Cs activity is present up to the oldest rings, as with the 2003 annual rings
2. The variability of 137Cs activity in the dendrochronological sequence from 2004 is lower than in those from 2003, which is mostly due to lower values in the youngest (and physiologically probably the most active) years (2004 compared to 2003), which could be attributed to the fact that 2004 samples were from the vegetation season (as a possible cause of increased migration of 137Cs from conductive tissues to the most physiologically active tissues in the canopy) and
3. Significantly weaker possibility of defining typical zones within the dendrochronological sequence in relation to the series from 2003, although the zone of increased 137Cs values in the youngest rings is also visible here, and,  on the other hand, the zone of almost constant 137Cs values in the oldest rings is well noticeable (on the basis of which it could be assumed that during the vegetation season there is even a migration of 137Cs from the inactive to the physiologically active xylem zone, and in the winter 137Cs is temporarily accumulated even in the physiologically inactive part of the xylem zone; compare above for dendrochronological sequence from 2003).
Divided time sequences can be observed at intervals for: a) the period from the beginning of tree life to 1958, in which a similar level (in terms of circle radius and tree height) of 137Cs activity was recorded, which varied from 15.2 ± 0.8 to 19.4 ± 0.9 Bq/kg at a height of 0.1 m, and from 20.0 ± 1.4 to 23.2 ± 1.5 Bq/kg at a height of 8 m, b) the period of 1959 to approximately 1982, in which a variation in 137Cs activity was recorded, both in terms of ring radius and in terms of tree height (without distinct regularity with respect to the  year formation), ranging between 10.4 ± 0.7 Bq/kg (0.1 m; 1980 - 1984 interval) to 25.1 ± 1.5 Bq/kg (16 m, the interval 1970 - 1974) and c) the period of about 1985 to 2004, in which substantially continuous increase of 137Cs is recorded in the most recent annual rings.
Figure 6 shows a comparison of 137Cs activity in the wood rings of silver fir trees (at a height of 0.1 m) between the average values of samples from 2003 - 2004, on one hand, and recalculated (on July 1, 2003) values from 2017 on the other.
Graphic insight into the measured data shows the following: a) lower values in recalculated samples of 2017 compared to the samples from the period 2003 - 2004, b) still measurable 137Cs activity along the dendrochronological sequence from 2017, and c) low variability in 137Cs activity