Regeneration dynamics of forest coenoses with Rhododendron luteum Sweet (Ericaceae) undergrowth after clear cutting in moist relatively fertile oak-pine site type in Zhytomyr Polissia, Ukraine

Introduction The yellow azalea (Rhododendron luteum Sweet) is a relict species with a distinct distribution. The lowland populations are primarily located in Ukraine, specifically in Zhytomyr Polissia region. The aim of this study was to investigate the key ecological and silvicultural characteristics of Rh. luteum habitats in moist relatively fertile forest sites and to evaluate the success of regeneration of forest coenoses with undergrowth of this species following clear-cutting in the region. Materials and Methods The study was conducted primarily in 2024. Data on Rh. luteum undergrowth regeneration were derived from field observations in Korosten district, Zhytomyr region. To assess the regeneration dynamics of Rh. luteum undergrowth, 30 experimental plots, each of 0.5–1 ha, were established in different forest age groups, including: mature maternal forests (130–140 years old), young forest plantations (before crown closure): 0–1 year and 4–7 years old; forest plantations after crown closure – 8–10, 11–20, 21–30, 31–40, 41–50, 51–60, 61–70 years old. Within each experimental plot, assessments were conducted to evaluate the development of the floristic composition, coenotic structure, and condition of the Rh. luteum undergrowth. Results The total area of forests with Rh. luteum undergrowth in the moist relatively fertile pine site type (C3) in two forestry enterprises amounts to 2,614.4 ha. Of this total, 88.2% consists of forests of natural origin, while only 11.8% comprises forest plantations. Quercus robur dominates in 53.5 % of the studied stands, and the remaining 46.5 % of the area is represented by secondary stands. Within these, Betula pendula occupies 28.6 %, Pinus sylvestris 14.4 %, Populus tremula 2.1 %, and Alnus glutinosa 1.5 %. The largest proportion of the studied forests belongs to the 71–80-year age group (29.2%) and the 61–70-year age group (26.8%). It was shown that in stands older than 40 years, the forest coenosis reached a fully developed state, characterised by a typical floristic composition and coenotic structure. The Rh. luteum undergrowth exhibited a projective cover of approximately 45% at 41 years, increasing to 60–70% at 58 years and 80–90 % at 66 years. The species displayed high vitality, even distribution, and mass flowering. These findings indicate that after clear-cutting Rh. luteum undergrowth regenerates satisfactorily within 40 years. By the 60–70-year age group, the development of a closed Rh. luteum undergrowth is nearly complete, and the physiognomy of the coenosis closely resembles that of the original (maternal) stands. Conclusions Following clear-cutting, Rh. luteum undergrowth regenerates satisfactorily within 40 years in those forests. However, in 17% of surveyed plots, Rh. luteum failed to regenerate, primarily due to excessive stand density, as well as untimely and insufficient thinning in stands under 30 years old. In the Branch “Luhyny Forestry” of the SFE “Forests of Ukraine”, the area of forest stands with Rh. luteum-dominated undergrowth in moist relatively fertile pine sites (C3) declined by 2.2 times between 1978 and 2018, from 926.0 ha to 423.8 ha. 10 Figs., 2 Tables, 30 Refs.