Status of Juglans mandshurica (Juglandaceae) coenopopulation introduced to the Volga-Kama Nature Reserve (Republic of Tatarstan).
The aim of the work was to assess the population stability of Juglans mandshurica Maxim., introduced to the Middle Volga region. Four cenopopulations of introduced J. mandshurica growing mixed with Tilia cordata Mill. in the Volga-Kama Nature Reserve (Republic of Tatarstan) were studied. The dynamics of abundance, density, age and spatial structure of the cenopopulations was monitored for five years. Evaluation of the annual shoot growth suggested the following duration of life stages: seedling - 1 year, juvenile - 2 years, immature - 4-5 years, virginile - 5-8 years. Morphometric and dendrochronological analysis were used for identifying young and adult individuals and their generations (planting time). The first generation was planted in 1949 and has a calendar age of 50-56 years, the life stage is classified as "g2, mature adult", tree height is 20-21 m, trunk diameter - 48-54 cm. The second generation of 30-37 years of age has tree height of 12-18 m, and trunk diameter - of 22-39 cm. The age spectra in various ecological and phytocenotic conditions include all life stages, which confirms the stability of all four J. mandshurica populations. Young non-generative plants predominate in the spectrum. The average annual increase in the number of individuals calculated over five-year seedling monitoring period was: for seedling - 73%, for juvenile - 27%. Dying-off of J. mandshurica begins in the immature and virginile life stages, and is caused by periodic hard winters, lack of light, trunk splitting, and other factors. The overgrowth of the T. cordata promotes shading and, consequently, reduces the germination and development of J. mandshurica undergrowth. The pioneering, for this type of habitat, research of the radial increment and sap flow in J. mandshurica revealed the main climatic factors determining growth of annual rings and water flow in xylem vessels. Correlation analysis of J. mandshurica tree-ring growth indices and meteorological factors showed the positive correlations of radial increment with January and, especially, May temperatures (P<0.01) and with precipitation in June. In contrast, for T. cordata, a relatively high correlation of radial increment with temperature and precipitation in July was observed (P<0.05). A significant correlation of sap flow with solar radiation (r=0.8) and air temperature (r=0.9) was found. Sap flow data of photophilous J. mandshurica and Betula pendula Roth, forming mixed stands with shade-tolerant T. cordata were compared. T. cordata demonstrated low values of sap ascend as compared to J. mandshurica. For example, during the hot days of 2016, the sap flow in T. cordata did not exceed 0.11 kg.hr-1.cm-1, while in J. mandshurica it was up to 0.22 kg.h-1.cm-1, and in B. pendula its value was intermediate. The observed variations in sap flow in deciduous trees are associated not only with different irradiation requirements, but with differences in leaf areas and tree height. Thus, in hot dry days, J. mandshurica needs more water than the aboriginal shade-tolerant species due to the higher transpiration. The obtained results indicate that J. mandshurica has naturalized in the forests of the nature reserve, and now is characterized by good germination capacity and high productivity of generative (adult, reproductive) individuals. At the same time, natural forest regeneration processes reduce J. mandshurica undergrowth development, as T. cordata is its successful competitor.