Dynamics of the Structure and Growth of Birch Trees of Sour-Blueberry Forests of the Southern Taiga of the Russian Plain

In the subzone of the southern taiga, birch forests are the most widespread plant formation. Under the canopy of birch, undergrowth and the second layer of spruce are often found, which can serve as the basis for the restoration of economically valuable spruce stands. In such stands, birch is the dominant species, which has a significant impact on the development of the spruce population and on the course of succession processes. Without knowledge of the peculiarities of the birch forests formation, it is impossible to assess adequately the prospects of the spruce sub-canopy population. The research has been carried out for 20 years by the Institute of Forestry of the RAS on 18 permanent test plots (PTP) established by the Northern Forest Experiment Station (Yaroslavl region) in the prevailing in the region modal sagebrush-blackberry birch forests with a sub-canopy population of spruce. All growing trees were periodically surveyed at the PTP with measurement of the main biometric characteristics. The studies revealed that vertical structure becomes pronounced in birch forests aged 20 years. The studies revealed that vertical structure becomes pronounced in birch forests aged 20 years. Observed at this age can be the maximums of average periodic growth in height and in the sum of cross-sectional areas for birch trees of the first layer. The growth in the sum of cross-sectional areas of trees of the second layer always remains in the zone of negative values, which can be explained by the transition of some trees to the first layer as well as by them dying off. The age of quantitative maturity of the modal birch forests under consideration is 50 years, which corresponds to the age of birch and spruce stands in the middle taiga, but 10 years more than in normal stands. The stock of birch stem wood of the growing part of the modal birch stands of southern taiga is higher than that of complete birch-spruce stands of middle taiga, but less than that of normal birch stands. Regression relationships between a number of birch forests’ taxation characteristics and their age were established. In the course of successional processes in modal southern taiga birch forests there is a gradual decline of aspen and second-layer birch trees. Renewal under the spruce canopy leads to the formation of birch-spruce stands, in which spruce becomes the dominant species in the second layer of the stand in 120 years, and a small part of trees emerges the first layer. The age of 120 marks the beginning of birch forests’ decay.