60-YEAR DYNAMICS OF THE UPLAND STANDS OF QUERCUS ROBUR IN THE SOUTHERN FOREST-STEPPE IN CONNECTION WITH THINNING AND MASS DECLINE
Introduction. Current state of oak forests requires an assessment of risks of reduce of their productivity and mass decline and enhancing the sustainability in forestry practice. The aim of the paper is to analyze the influence of thinning and mass decline of oak on stability and productivity of the overstory stands. Three periods in dynamics of upland oak forests of natural origin were studied: thinnings (20 years), mass oak decline (10 years), and restoration (30 years). Results. During the first period, three modes of thinning were performed. In the first mode, both absolute and relative values of average-period increments and stock changes were offered, in the second mode - only relative ones, in the third mode - only relative increment tightly correlated with thinning intensities (r > 0.9). In the period of mass oak decline (1976-1985), the ranging of stands by their productivity was violated. The relative increment negatively depended on intensity of the dying off in 1976-1979. The increment of forest stands did not practically decrease (in contrast to that of oak) for the expense of associate species. The share of oak in the dying off in the overstory was 90% at its 66% share in the stock. The dying off in the overstory for 10 years reached 150 m<sup>3</sup>ha<sup>-1</sup>, and the stock decrease - up to 105 m<sup>3</sup>ha<sup>-1</sup>. The dying off intensity of oak for 10 years correlated with its stock in 1973 (r = 0.92) and did not depend on the stock of forest stand. The oak value G > 17 m<sup>2</sup>ha<sup>-1</sup> can be considered critical, with intensity of its dying off 55-64%. During the recovery period, the relative increment and stock change positively correlated with intensity of dying off in 1975-1985 (r > 0.6). The increment of forest stands increased in comparison with previous periods due to associated species and to a lesser extent due to oak. After 1998, a decrease of stock in control was fixed. Thus, thinning had positive influence on increment in stock and stability of growth of the overstory. By 105-year age, oak stands slightly and moderately thinned and weakly affected by mass decline reached largest stocks (including large-size trees). Mass oak decline can be considered as regulating in its populations density. Aiming to increase the sustainability of mixed forest stands to damages by leaf-eating insects (at high risk) one should support the value of oak basal area not exceeding 16 m<sup>2</sup>ha<sup>-1</sup>.