Patterns of plant species richness and diversity across treelines in Himachal Pradesh, Western Himalaya

Treelines are ecologically unique, fragile, and rich in natural resources. They harbour high species diversity and at the same time are under threat due to anthropogenic activities. Recognizing this, the present study has been framed to document the patterns of species richness and diversity in the state of Himachal Pradesh, western Himalaya. A total of six treeline sites (three disturbed and three undisturbed) were identified for vegetation sampling. Trees, shrubs, and herbs were sampled at each site using nested plots of 10 m², 5 m², and 1 m², respectively. The study exhibits the rich diversity of treeline communities, the patterns of which varied between treeline sites. Altogether, 221 species of vascular plants belonging to 47 families and 140 genera were recorded from the area. Amongst families, Asteraceae was the dominant family followed by Apiaceae and Ranunculaceae. The study also revealed the presence of threatened species like Aconitum heterophyllum, Angelica glauca, Bergenia stracheyi, Dactylorhiza hatagirea, Picrorhiza kurroa, and Trillium govanianum etc. at treeline. Moreover, species composition revealed high densities of Betula utilis followed by the under canopy of Rhododendron campanulatum and R. anthopogon at treeline sites. Overall, species richness of herbs, shrubs, and trees were higher at undisturbed site as compared to disturbed one. The diversity indices of herbs and shrubs varied significantly between treeline sites while that of trees was non-significant. At the same time, soil properties showed distinct patterns wherein pH and available nitrogen significantly varied between treeline sites. Present study provides detailed insights into the floristic and ecological aspects of treeline communities from the unexplored ecoregion of western Himalaya. The treelines in the area are anthropogenically depressed and continued land-use activities might result in habitat fragmentation and displacement of plant communities in the near future.