Biologichni Studii / Studia Biologica

Background. The yellow azalea (Rhododendron luteum Sweet) is a relict species, fragmentally distributed in Ukraine, particularly in the Zhytomyr Polissia, covering an area of approximately 62.000 ha. The central part of the species’ subrange is located in the Korosten’ district of Zhytomyr region, where it forms an understory with a canopy closure of 0.8–0.9 (up to 1.0) in the moist fairly infertile oak-pine site type (B₃) and moist fairly fertile oak-pine site type (C₃). These forest communities are rare and have been included in the Green Book of Ukraine (2009). Additionally, the species is listed in the Bern Convention (Convention..., 1979). Excluding such large forest areas from forest use is impractical, therefore it is necessary to analyze their impact on the species’ population.
The aim of this study was to: (1) characterize the communities dominated by R. luteum in the understory of mature parent forests within the moist fairly infertile oak-pine site type (B₃) based on field studies; (2) investigate the dynamics of R. luteum understory regeneration in forest plantations of different ages (1–65 years) following main clear-cutting, assessing canopy closure and species viability; (3) evaluate the success of R. luteum understory regeneration after clear-cutting in the B₃ site type; (4) analyze the long-term dynamics of forest areas with an understory dominated by R. luteum in the moist fairly infertile pine site type (B₃).
Materials and Methods. The distribution of R. luteum was assessed based on herbarium collections from the Herbarium of M.G. Kholodny Institute of Botany, NAS of Ukraine (KW), Herbarium of M.M. Gryshko National Botanical Garden, NAS of Ukraine (KWHA), and Herbarium of Taras Shevchenko National University of Kyiv (KWU). Additionally, floristic databases iNaturalist and UkrBin, standard forest enterprise taxation descriptions, and our own field observations (2000–2025) were utilized.
The primary sources of data on the regeneration of R. luteum understory were field observations conducted in 2025 in the Korosten’ district of Zhytomyr region, particularly in Branch “Emil’chynske Forestry” and Branch “Luhynske Forestry” of the State Enterprise “Forests of Ukraine” as well as Emil’chynske Forestry of the municipal enterprise “Zhytomyroblagrolis”. The long-term dynamics of R. luteum understory area were studied based on standard taxation descriptions of Branch “Luhynske Forestry” of SE “Forests of Ukraine” for the period of 1978–2018.
Statistical data processing was performed using standard methods of variation statistics (Horkavyi, 2009). The calculation of arithmetic means and their errors, as well as the construction of diagrams, was carried out using Excel software.
Results and Discussion. In mature parent pine forests aged 97–107 years, the condition of the R. luteum understory was good, with a canopy closure of 0.8–0.9 (up to 1.0) and a uniform distribution. After main clear-cutting in the B₃ edaphotope, the condition of the R. luteum understory in 1-year-old forest plantations was satisfactory, the species was primarily preserved in inter-row spaces, with a projective cover of 10–15 %. In 2–4-year-old forest plantations, the projective cover slightly increased to 17–20 %, while in the 8–16-year-old group, it reached 20–25(30) %. The limiting factor for its development was the high canopy closure of young tree stands. In 21–26-year-old plantations, the condition of the R. luteum understory remained satisfactory, with the continuing formation of this layer, its canopy closure slowly increasing to 25–30 %, and distribution transitioning from clumped to uniform. As the plantations aged up to 65 years, the projective cover of the species gradually increased to 65–85 %, maintaining a uniform distribution. In 60–65 year-old plantations, R. luteum reached a quasi-equilibrium state, forming a specific lower layer of phytocenoses. Physiognomically, these communities were nearly indistinguishable from parent stands.
It was found that in the Branch “Luhynske Forestry” of SE “Forests of Ukraine”, within the most common edaphotope for R. luteum in the region – the moist fairly infertile pine site type (B₃) – the area of forests with an R. luteum understory significantly decreased from 798 ha in 1978 to 527.2 ha in 2018.
Conclusions. After main clear-cutting and the establishment of forest plantations, a relatively closed R. luteum understory is restored within the period of about 40 years in most sites. By the age of 60–65 years, the understory reaches a quasi-equilibrium state. However, in about 15 % of the plots, the R. luteum understory does not regene­rate after clear-cutting.
Between 1978 and 2018, the area of R. luteum in the B₃ edaphotope within the Branch “Luhynske Forestry” of SE “Forests of Ukraine” significantly decreased. In the moist fairly infertile pine site type (B₃), the forest area with the R. luteum understory decreased from 798 ha in 1978 to 527.2 ha in 2018, a 1.5-fold reduction.
When forest plantations are established using the 2.5×0.7 m planting scheme, the yellow azalea regenerates satisfactorily in the inter-row spaces, but it may suppress seedlings in the rows. It is recommended to maintain 2-meter-wide corridors (1 meter on each side of the row) in young forest plantations by mowing R. luteum shrubs in these areas.
A limiting factor for the regeneration of R. luteum populations in young stands is the high density of young tree plantations, including both planted forests and natural regene­ration, especially of birch and aspen. To support R. luteum understory regeneration, we recommend to conduct intermediate cuttings in young tree canopies.