AGE STRUCTURE OF THE BIRCH STANDS IN THE LEFT-BANK FOREST-STEPPE OF UKRAINE

The aim of this work was to define the features of the age structure of the silver birch stands in the Left-Bank ForestSteppe considering forest site conditions, origin, site index and proportion of this species in the forest composition. Survival of silver birch in almost all age classes was the lowest in the relatively poor forest site conditions. The mean weighted age classes of silver birch are IV.8 in vegetative stands and IV.6 – in artificial seed stands. For all origins, the mean weighted age classes are V.1 for Sumy Region and IV.6 – for Kharkiv and Poltava Regions. The stands of the lower productivity have the least longevity. Survival of pure and almost pure stands (silver birch make 90 % of all the trees) is the lowest. At that, it is the highest in Sumy Region.

49°57' 33°36' 1,6 I-IX (IV.7) * Latitude and longitude for each forest enterprise were evaluated as centroids of respective contours of the territory using MapInfo Mapping Package. **Proportion of birch stands from area of forest covered lands, %.
We studied the data for subcompartments with silver birch as the main forest-forming species according to 10-year age classes for different forest site conditions, origin (natural and artificial seed origin, vegetative origin), site index, and proportion of silver birch in the stand composition.
The probability of stand survival up to the mentioned age class was modeled by means of the method of Yu. P. Demakov (Demakov 2000), which was successfully tested in the analysis of the pine plantations survival in Sumy Region (Tovstukha 2012). According to this method, the area proportion of each 10-year age class was evaluated for each sample of subcompartments with respective forest site condition, origin, site index, and proportion of silver birch in the stand composition. Then we evaluated the proportion of stands' area which survives up to a certain age.
To analyze the data, we applied MS Excel. Coordinates for forests of each forest enterprise were evaluated as centroids of respective contours of the territory using MapInfo Mapping Package.
Results and Discussion. We analysed silver birch stands from subcompartments with all forest site conditions, origin, site index and birch proportion in the stand and found out that in the IV age class from 30.5 % (Chuguyevo-Babchanske FE) to 74.3% (Skrypayivske TEFE) of the trees survived (Table 2). We also found that the part of the stands preserved in the V age class is from 19.7 % in Chuguyevo-Babchanske FE to 48 % in Skrypayivske TEFE and 41.3% in Vovchanske FE. Up to the VIII age class which is suggested as the age of maturity, 5.4 and 5.0 % of silver birch stands survive in Vovchanske FE and Okhtyrske FE respectively. There are no such stands in Zmiyivske FE, and they cover less than 1% of the area in Poltavske FE and Gutianske FE (Table 2).
Grouping data showed that the survival of silver birch in almost all age classes is the lowest in the relatively poor forest site conditions, while it is the highest in the age before the V age classes and after the VII age classes in fresh fertile forest site conditions (Fig. 1). It happens due to the fact that fertile sites are more favourable for many tree species. However, the silver birch is often removed from pine-birch stands in fresh relatively poor forest site conditions during thinning operations.

Fig. 1 -Survival of birch stands in the particular forest site conditions in the forest fund of analyzed forest enterprises of the Left-Bank Forest-Steppe (all origins, and site indices) (B 2fresh relatively poor forest site conditions (fsc); B 3 -moist relatively poor fsc; C 2 -fresh relatively fertile fsc; C 3 -fresh relatively fertile fsc; D 2 -fresh fertile fsc)
We compared the results of evaluation by administrative regions and found out that the silver birch survival was the lowest in the Forest-Steppe part of Kharkiv Region and the highest in the Forest-Steppe part of Sumy Region almost in all forest site conditions (Table 3). However, in the fresh fertile forest site conditions, such relations are actual up to the IV age class only and in higher age classes of silver birch stands in Kharkiv Region.
On calculating the survival of silver birch stands of different origin by plots (Fig. 2), we noted that the mean weighted age class of this tree species in the Left-Bank Forest-Steppe is IV.7. It is slightly higher in vegetative stands (IV.8) and little less in artificial seed stands (IV.6). For all origins, the mean weighted age class is the highest for Sumy Region (V.1) and is IV.6 for Kharkiv and Poltava Regions. The mean weighted age class for Kharkiv Region is the highest for vegetative stands (V.6), for Poltava Regionfor natural seed stands (V.0), and for Sumy Region it is V.1 for vegetative and artificial seed stands and IV.7 for natural seed stands.

Fig. 2 -Survival of birch stands of different origin in analyzed forest enterprises of certain regions of the Left-bank Forest-Steppe (all site indices and forest site conditions)
A comparison of the survival for silver birch stands of different site indices showed that the stands of the worse growth have the least longevity (Fig. 3).
At the same time, we figure out that the stands with the best growth (site indices I a -I g ) survive up to the VII age class only, while the stands of I-III site indices survive up to the X age class.

Fig. 3 -Survival of birch stands of particular site indices in the forest fund of analyzed forest enterprises of the Left-Bank Forest-Steppe (all origins, and forest site conditions)
Such a conclusion is also true for certain regions (Table 4). There no silver birch stands of IV-V site indices in Sumy Region, they survive over 30 years in Kharkiv Region and up to V age class in Poltava Region. The stands of I a -I g site index survive up to VII age class in all analyzed regions. The survival of birch stands with the best growth is the highest in Sumy Region.  A comparison of silver birch stands of different species composition showed that survival of pure (100 % of silver birch) and almost pure (90 % of silver birch) stands is the lowest (Fig. 4). The stands with 50-70 % of silver birch have the highest survival level. The stands with a low proportion of silver birch (up to 40 %) take an intermediate position. We detected that the survival of pure and almost pure silver birch stands is the highest in Sumy Region (Table 5). Survival of stands with 50-70 % of silver birch is the highest in Sumy Region up to the IV age class only. Survival of the stands with a low proportion of silver birch (10-40 %) is the highest in Sumy Region after the VI age class. The results of the research showed that the survival rate of silver birch in the forest stands of the Left-Bank Forest-Steppe depends on the region, the type of forest site conditions, the origin, the site index of the stands, as well as on the proportion of silver birch in the composition. At the same time, the manifestation of various factors of the weakening of birch stands also depends on these factors. Therefore, one of the tasks of our subsequent research is to reveal the combination of factors most favourable for the long-term existence of silver birch forests. Furthermore, if there is a threat of mortality to silver birch plantations and/or a risk of timber quality decrease, it is necessary to provide in the regulatory documents the possibility of reducing the age of main felling in case of expectation of the deadline for the main felling in accordance with existing standards. Conclusions. The probability of silver birch survival in the Left-Bank Forest-Steppe was evaluated depending on the region, the type of forest site conditions, the origin, the site index of the stands and the proportion of silver birch in the composition.

Table 4 Survival of birch stands of particular site index in the forest fund of analyzed forest enterprises of the Left-Bank Forest-Steppe grouped by regions (all origins, and forest site conditions)
We came to the conclusion that survival of silver birch in almost all age classes is the lowest in the relatively poor forest site conditions and is the highest in the most of age classes in fresh fertile forest site conditions. Moreover, the mean weighted age class of silver birch in the Left-Bank Forest-Steppe is IV.8 in vegetative stands and IV.6in artificial seed stands. For all origins, the mean weighted age class is V.1 for Sumy Region and IV.6for Kharkiv and Poltava Regions.
The stands of the worse growth have the least longevity. A survival of pure (100 % of silver birch) and almost pure (90 % of silver birch) stands is the lowest. The stands with 50-70 % of silver birch have the highest survival level. The stands with a low silver birch proportion (up to 40 %) take an intermediate position. Survival of pure and almost pure silver birch stands is the highest in Sumy Region.
To conclude, it is necessary to provide the possibility to reduce the age of the main felling if there is a risk of decrease of the timber quality in case of expectation of the deadline for the main felling in accordance with existing standards.