Study on Species Diversity of Rhododendron-Abies Forest in the Typical Timberline Area in the Zhegu Mountains in Western Sichuan

The community species diversity of Rhododendron-Abies old-forest in the eastern Qinghai-Tibet Plateau was investigated by plot method. A total of 24 families, 31 genera and 32 species of plants were recorded, including 2 families, 3 genera and 3 species of arbor; 7 families, 8 genera, 8 species of shrubs; 21 species, 21 genera, 18 families of herbs. The dominant tree species were Abies faxoniana and Rhododendron trichogynum; the dominant shrub species were Sorbus koehneana and Ribes glaciale; the dominant herb species were Oxalis griffithii and Deyeuxia scabrescens. The species with the occurrence frequency of 81%~100% in the tree layer were the most, and the species with the occurrence frequency of 1%~20% in shrub and herb layer were the most.. The species diversity was at a low level. The Shannon-Wiener index, Species richness, and Simpson dominance index were the highest in herb species (H′ = 1.9932, S = 21, D = 0.8109), followed by shrub species (S = 8, H′ = 1.3488, D = 0.621) and lowest in tree species (S = 3, H′ = 0.8157, D = 0.5312). The Pielou evenness index of the tree layer was the highest (J = 0.743), while that of the herb layer (J = 0.655) was similar to that of the shrub layer (J = 0.649).