Phytocenotic conditions and structure of the local population for Betula humilis Schrank near Zhornyska village (Ukrainian Roztochchia)

hytocoenotic conditions and structure of the local population for Betula humilis in the north-eastern environs of the village Zhornyska, Yavoriv district, Lviv region, on the territory of the Roztochchia International Biosphere Reserve have been studied. Four coenopopulations (CP) of the species are confined to secondary plant communities of sicaciogenic and pyrogenic successions with different sod levels of the peat soil, as well as missing or unequally expressed tree layer. The least transformed communities with the dominance of B. humilis (40-80% p.c.) in the shrub layer and no stands in terms of floristic composition and spatial structure correspond to the derived mesohygrophytic and hygromesophytic variants of the association Betuletum humilis Steffen 1931. The most transformed areas with sparse tree layer are represented by pioneer forest communities - birch woodlands. Seventy-five species of higher plants (seventy vascular plants and five bryophytes) were found in the floristic composition of phytocenoses. The minimal participation of helophytes and the presence of invasive species indicate a high level of mesophytization and synanthropization of modern phytocenoses. Within the locality B. humilis forms different in area and density of formation shoots (turions) of the colony-curtain mainly from semi-autonomous subpartial bushes 0.3-2.2 m tall. The largest curtains (up to 300 m2) with a high density of living formation shoots (up to 293 pcs/m2) are confined to open well-lit areas. The age of old skeletal axis, not exceeding 9 years was determined and the annual growth of shoots has been calculated. The parameters of seed productivity (SP) of the local population of B. humilis vary within: potential SP - 89.48–109.24 seed germs, actual SP - 80.84–100.4 seeds. The coefficient of seed productivity is quite high (86.69-92.16%) for all coenopopulations. Based on the results of research and analysis of the literature the life form of B. humilis, which belongs to vegetatively mobile hypogeogenic-geoxyl deciduous shrubs that form colonies or clones of (semi) autonomous (sub) partial (daughter) shrubs and are characterized by monopodial-sympodial shoots growth has been clarified. The big life cycle of the species characteristic of shrubs consists of development cycles for systems shoots formation (main skeletal axes), which are successively replaced, and lasts for decades. The development cycle of the system formation shoot covers the periods of intensive growth (appearance of formation shoot from an additional or dormant bud in the first year), relative stabilization of growth processes with the formation of first and higher-order branching shoots (from second to tenth years) and ageing and death of shoots (after about ten years of age).