Spruce population under the canopy of overmature birch forests in the mixed forest zone of the european part of Russia

The results of studies of the under-canopy population of spruce (Picea abies L.) in over-mature birch forests (Betuletum oxalidosum) of the Moscow region are presented. The purpose of the research was to study of the renewal process periodicity, age, vertical structure and condition of the spruce population. The work was carried out on a permanent trial plot in the period 2008–2018. The undulating feature of the spruce renewal process under the canopy of birch forest with a frequency of 30–40 years within the age of the birch trees under consideration has been established. It is determined that the basis of spruce forests formed under the canopy are trees with a renewal period of less than 30 years, which by the onset of the decay of birch forests form the first and second layers with an average age approaching the age of quantitative maturity. The age structure of the spruce population in birch forests aged 115 years is characterized by the dominance of trees of two age groups — 21–40 and 91–120 years, which account for 29 % and 37 % of the population, respectively. The emerging spruce forests belong to conditionally even-aged stands. Trees with a height of 2,0 m dominate (about 40 %) in the vertical structure of the population. The age dynamics of the stand is aimed at a slight increase in undergrowth trees and the first storey, and a decrease in the number of the second storey trees occurs. This is due to the emergence of new spruce specimens and the transition of part of the trees of the second storey to the first one. It was found that weakened trees predominate in the under-canopy population of spruce in overmature birch forests. Trees of the first layer are in the best condition (status category CS = 1). The state of undergrowth trees during the observation period did not change, remaining weakened (CS = 2,3–2,4). The set of trees of the second layer was characterized by positive dynamics — from the category of strongly weakened (CS = 2,7), due to the mortality, it moved to the category of weakened (CS = 2,0). Trees with a renewal period of up to 30 years are in the best condition. As a result of the succession process, after the decay of birch forests, low-density spruce forests with a productivity corresponding to the III site class will be formed.