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ŠUMARSKI LIST 1-2/2020 str. 49     <-- 49 -->        PDF

RESULTS AND DISCUSSION
REZULTATI I DISKUSIJA
Poplars are the main species used in the afforestation of the lands around Vidin, as the afforestation with poplars here dates back to 1890 (Mitsov, 1963).
General characteristics of the plantation of all SPs are given in table 1.
The calculated values of the average diameter DBH (for SP1 = 2.7 cm and for SP3 = 1.6 cm) and average height Hav (for SP1 = 2.67 m and for SP3 = 2.55 m) show that, although one year younger (at 2 years of age) the plantation in SP1 in which maize is currently sown has better base dendrometrical indicators. Growth characteristics depend heavily on the clone of poplar (Vassev, 2013), but definitely the hoeing care used in crop cultivation as well as the residual green mass in the system have a beneficial effect on the growth of poplars in these typical habitats. This defines the advantages of implementing agroforestry system from the very beginning of the planting life. At this early age, there is still no differentiation with respect to the average diameter and all trees of Agathe poplar clone in both studied plantations (SP1 and SP3) fall into one degree of thickness (2 cm). The comparison between the older plantations SP2 and SP4 (at 10 years of age) shows that the plantation at which agroforestry was applied (SP2) by sowing corn and bostan at ages 2 and 3 is already significantly superior in terms of average diameters and heights (DBH = 9.9 cm; Hm = 7.44 m) compared to those in which no implementation were performed (SP4) (DBH = 8.7 cm; Hm=7.04) m. This is also confirmed by caliperring of the trees in the two sample plots, where in SP2 there is a concentration of 0.8960 m³ (89.6 m³ / ha), and in SP4 0.7145 m³ (71.45 m³ / ha) of the standing stock. At this age in SP2 (with applied agroforestry), there is already a differentiation of trees from “mnBL” popular clone according to diameter into two degrees of thickness (8 cm-4 and 10 cm-26). On the other hand, in the area without agroforestry (SP4), the trees are located in a broader range with respect to the average diameter (for a degree of thickness of 2 cm – 1 number, for 6 cm – 1 number, for 8 cm-14 numbers and for 10 cm – 14 numbers).
From the statistical analysis, the values of some statistical parameters characterizing the diameters and heights are obtained (Table 2). The values of the arithmetic mean for diameters are almost no different from the calculated weighted averages ones (+0.04, 0.00, +0.03 and -0.15 for SP1 to 4 respectively). The small variation in diameters at this early age is also confirmed by the variance values. The largest number of diameters are centered on the arithmetic mean of the young SP1 and SP3 plantations where the values of the standard deviation are the smallest (0.15 cm). Forestry science and practice also need values of the coefficient of variation of the diameter (Ustabashiev and Ferezliev 2013). Values of the variation coefficients in SP1, SP2 and SP3 fallen in the interval up to 10% show a slight alteration of the diameters, while in SP4 is defined an average alteration of the indicator (19.6% in the interval of 11 to 25%) (Lakin, 1990). In all four cases, the variation of diameters is characterized by negative (left) asymmetry. In SP1, SP2, and SP3 empirical variations deviate insignificantly from the normal curve, which classifies asymmetry as „small“ (with Kurtosis values below 0.5). In the first two cases, the distribution is performed on a curve exceeding minimally the highest part of the normal curve, and in the third case by a curve decreasing to a very small extent the curve of the normal distribution. In the control (SP4) the distribution of the diameters is performed on a curve exceeded relatively more significantly the normal distribution curves in its highest part (the variation curve has a sharp Kurtosis values +12.40).
In a statistical check of height growth, we ascertained that the average values are almost no different from the weighted averages calculated by the Lorey formula (Lorey, 1878) - Table 2.