Abstract: Rhododendron changii is a threatened plant species. Here, we investigated its population structure, species composition of companion communities, vertical stratification and plant diameter class, and species diversity in tree and shrub layers. Furthermore, the relationships between R. changii population and species composition of tree and shrub layers in community and environmental factors were examined using redundancy analysis (RDA) and canonical correspondence analysis (CCA). The results showed that: (1) The diameter class pattern of R. changii was unimodal, indicating that the population renewability was limited in a discrete way and lack of seedlings. In addition, the sapling period was another peak of death, implying the population was continuing to decline. (2) There were four companion communities around the population, containing a total of 56 woody species belonging to 23 families and 40 genera. The dominant families were Fagaceae, Rosaceae, Aceraceae, and Ericaceae, with a large number of Fargesia spathacea in the understory. The diversity index of tree and shrub layers was relatively low with an underdeveloped herbaceous layer. The companion communities 1 and 2 belonged to shrubs with an average height of 1-1.5 m, the community 3 was an evergreen broadleaf-mossy dwarf forest with an average height of 3-4 m, and the community 4 was an evergreen deciduous mixed forest with an average height of 8-10 m. R. changii grew in the lower layer in all companion communities, while dominated in shrub communities compared to arbor one. There was a large proportion of small-diametric individuals of shade-tolerant species in the community, indicating increasing competition in the understory. (3) The results of the forward selection and Monte Carlo analysis showed that slope position exerted a significant impact on species composition of tree layer, while the canopy closeness and slope exposure controlled that of the shrub layer (P<0.01). Litter thickness and air humidity were two primary environmental factors affecting R. changii seedlings, while canopy coverage and slope exposure influenced the saplings and adults of R. changii. Besides, saplings and adult plants were also affected by the species density and bamboo coverage (P<0.01). In conclusion, it is important to decrease canopy cover, interfere with the growth of the other saplings and F. spathacea in forests and clean up litter layer, to relieve the endangered situation and maintain population development of R. changii.

Key words: forest, carbon storage, carbon density, Shandong Province.