Population characteristics, habitat, and conservation status of Rhododendron ferrugineum L. (Ericaceae), a glacial relict new to Poland

The first reliable information on the occurrence of Rhododendron ferrugineum in the Karkonosze Mts (excluding spots of directly acknowledged anthropogenic origin) was provided by A. Boratriski in 1983, but the status and origin of the plants were unknown. A recent phylogeographical study proved the natural character and relict status of the aforementioned population, which makes it the northernmost and most isolated site within the whole distribution of the species. In this study, we characterized the basic aspects of the ecology and conservation status of the population and, more specifically, focused on assessing the size of the population, general health of individuals, generative propagation ability, habitat conditions, and potential threats for the species. The population persists in the Sowia Dolina (east part of the Karkonosze Mts), in a microtopographically controlled, treeless microrefugium. Shrubs of R. ferrugineum are part of an acidophilous dwarf-heath plant community, similar to those occurring in the Alps and the Pyrenees, although less species-diverse. The plant community in the Karkonosze Mts has been preliminarily classified into the Genisto pilosae-Vaccinion affiance. The R. ferrugineum population consists of 68 individuals: 57 fully grown and juvenile and 11 seedlings. In 2017, 10 individuals flowered, seven of which developed fruits, while in 2018, 15 individuals produced flowers and eight developed mature fruits. Seeds collected in 2017 germinated in high numbers. Plants in the Karkonosze population hosted some fungal parasites typically found in Rhododendron species, but no intense disease symptoms strongly influencing plant fitness were observed. A combination of significant isolation, genetic distinctness, and high genetic diversity implies a high conservation priority for the R. ferrugineum population in Karkonosze. Despite the theoretical threats, including stochastic risks, the R. ferrugineum population seems to have been stable for a long time and, importantly, it is composed of individuals of different ages, from large flowering plants to seedlings.