MAJOR FACTORS IN THE DEVELOPMENT OF SIBERIAN STONE PINE UNDERGROWTH UNDER THE CANOPY OF DERIVATIVE PINE STAND
Introduction. The abundance of Siberian stone pine undergrowth (Pinus sibirica) under the canopy of derivative pine stands and small-leaved stands, that are considered to be potential Siberian stone pine forests, is observed rangewide. At the same time, the influence of competition of these stands on the regeneration and development of Siberian stone pine undergrowth under their canopy has not been sufficiently studied. The goal of this research is to analyze the competition of the cowberry-bilberry-green moss pine forest stand against the Siberian stone pine undergrowth. Objects and methods. The research was carried out on the basis of the micro-ecosystem approach using the earlier proposed and tested set of light, root and integral competition indices of stands under the canopy of a 75-year-old cowberry-bilberry-green moss pine forest in the northern part of eastern macro-slope of the Middle Urals after a selective care felling. The record of parameters of the stand, environment and Siberian stone pine undergrowth was made in 106 circular plots with a radius of 7 m. The undergrowth age was determined to within one year by the number of annual terminal shoots increase. The dynamics of annual generation of one-year seedlings was reconstructed according to the age structure of the undergrowth and the empirical coefficient of the survival curve. Results. Against the background of almost continuous annual Siberian stone pine regeneration of different intensity (0.1-0.8 ths. trees/ha) under the canopy of pine forest, an abundant emergence of Siberian stone pine seedlings is mainly observed in the first two years (1.7 and 1.3 ths. trees/ha) after the care felling for the stand (thinning). In 28 years after felling, the parameters of 23-35-year-old Siberian stone pine undergrowth growing in the gaps of the stand (in place of felled single trees) exceed the parameters of the trees growing in the areas under a closed canopy, where trees were not cut down by 1.5-1.8 times; in former technological corridors (skidders) - by more than twice. But over the past five years, the averaged vertical growth of terminal shoots of undergrowth is negatively closely related (R2 = 0.68) with light competition of the stand (the level of photo-synthetically active radiation interception by the canopy), and the relationship with its root competition for soil nutrition is expressed to a lesser extent (R<sup>2</sup> = 0.31). The tightness and reliability of negative relationship (R<sup>2</sup> = 0.68) of the average growth of terminal shoots with the complex index of integral competition of the stand, which characterizes the joint influence of these factors, is closely related to the private index of light competition. Conclusion. The thinning of pine forest by selective felling helps to reduce the competition of the stand, contributes to the successful subsequent regeneration of Siberian stone pine and development of its undergrowth. With the combined influence of factors of light and root competition of the stand, light competition is the decisive one. Less expressed root competition suggests a certain adaptation of Siberian stone pine undergrowth to the conditions of pine forest.