Research was conducted in the experiment plot of euramerican poplar clone I-214, which was founded with one year seedling type 1/1 with spacing 6 x 6 m in the square system on the alluvial soils of river Sava basin in which browning processes is expressed. Eleven years after the plantation establishment 3 blocks with two experimental plot of 0.2016 ha, which are separated by a so-called. protective order were singled out. In the experimental plot all the trees were numbered and measured by two cross-dbh, with an accuracy of 1 mm, and height, with an accuracy of 1 dm. The crown projection radius of 8 positions, each rotated to 45 degrees, were measured. On the three experimental plots (one in each block) selective thinning was carried out (PP-E), at which in each plots 50 % of trees were cut down, or the distance between the trees raised on average 8.5 x 8.5 m. Firstly, a collective called. promising trees set aside, in the number that corresponds to an average distance of 8.5 x 8.5 m, and their main competitors were removed. The trees behind in development, which is mainly due to additional filling in plantations two years after planting, are also removed because they were judged as silvicutural non-perspective. The remaining three experimental plots were control (PP-K). In each repetition trees for dendrometric analysis were sampled, the dominant one (d(g20%)) and a mean tree which has mean quadratic diameter (d(g)). In the 16th years of plantation development diameters at breast height (dbh) and height of each tree were re-measured, sa well as crown projection radius. The development of mean and dominant trees showed that the investigated habitat is the medium favorable for the cultivation of the poplar clone I-214. Current increment of diameter, height and volume in the 11th year was in the intensive phase, where volume increment has not yet culminated (Figure 1-3). As a suitable element of growth to determine the start of thinning operation on a biological point of view, can be ring width along the spindle tree. In the first five years the tree has had a full solitary growth, and from the seventh year there is a competitive relationship between trees in plantation, which is manifested by changing the ring width along the spindle tree (Figure 4 and 5). This means that before the seventh year thinning does not have any biological justification. Models 1 and 2 of volume tables, constructed measuring felled trees from thinning (Table 2), and which are the dependence of the volume of tree from diameter at breast height (model 1) and from the diameter at breast and height (model 2), proved to be equally good at calculating the volume per hectare in the 11th year and more convenient compared to other tables and models (Table 3). To calculate the volume in the 16 (6 year two input volume tables can be successfully used (model 2), while the application of model 1 is limited at the 11th year. In the 11th year at the experimental plot an average of 263-266 trees per hectare are found, which represents a survival rate of about 95 %. Total basal area averaged from 14.70 to 14.97 m(2)/ha, while volume was an average from 156.67 to 157.62 m(3).ha(-1). By applying the statistical t-test significant difference in the number of trees, basal area and volume per hectare was not determined within the experimental plots before applying the silvicultural treatments (Table 4). By applying thinning operation, 122 trees per hectare (46%), 6.45 m(2)/ha (43 %) of the total basal area, 66.08 m(3)/ha (42%) of the total volume and 2645 m(2)/ha (40 %) of the the crown projection area were removed, which represent a strong procedure and is located above the so-called. critical basal area (Table 4, 13). Size of allowable cut in the thinning of 66.08 m(3)/ha was in the limits expected on the basis of previous research fir the density of 278 trees per hectare and the habitat medium favorable for the development of poplar clone I-214. However the assortment structure of allowable cut is more favorable compared to thinning in younger plantations in the more favorable habitats and produces 50 % of technical wood, 30% of pulpwood and 20 % of wastewood (Table 4, 9, 15). Between 11 and 16 years on both series of experimental plot the mortality of trees was not determined. Five years after application of thinning treatment a significant difference in the number of trees, basal area, volume and crown projection areas of trees per hectare were determined between the experimental and control plots (Table 4). The significant difference in the mean diameters, heights and crown projection areas of the mean and the dominant trees (Table 5, 6), as well as between the diameter strucures (Figure 7) were also determined between investigated plots. For a period of 5 years after application of thinning operation at the experimental field current basal area and volume increment per hectare accounted 85 % of basal area increment and 75 % of volume increment of the control plots wich had the number of trees twice higher than experimental plots (Table 14). For five years of positive reactions to increase growing space of remaining trees on the experimental plots the size of the total basal area and volume of trees removed by thinning operation is not compensated: the total basal area and the volume per hectare amounted to 2/3, and the crown projection area amounted 87 % of the size of the control plot. The reaction of trees on experimental plot with thinning treatment for 5 years is great and is reflected in greater mean diameter by 10.6 %, greater mean tree volume by 21.9 % and increased the crown projection area by 59.0 % compared to control plot. In contrast, the mean Lorey's height at the experimental plot with thinning treatment decreased by 4.2 % compared to control plot. This has contributed to the intensification of the current (average periodic) volume increment compared to the period before applying thinning operation. Greater assimilation apparatus of trees in the experimental plots with thinning tretament and higher current diameter and volume increment indicate the need to extend the production cycle at the best assortment structure compared to the control plot. The research results indicate the validity of efforts that the thinning operation in poplar plantations should be treated as a biological and an economic category. As a biological category the thinning operation contributes to accelerating the growth of the remaining phenotypically better established trees and achieve optimum production. As economic categories with thinning operation realized the previous crop, and it being understood that the assortment of allowable cut in the thinning cover cost cutting and the establishment of plantations with more trees. In this framework it is necessary to direct further research, because the poplar plantations, which are aimed to applying thinning operation, are flexible enough to allow adaptation to changing market conditions.
